CLIMATIC CHANGE
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15.1 MEANING AND CONCEPT
Variability, in both time and space, is an inherent feature of climate, as the atmosphere is always in the state of turmoil and instability leading to variations in weather and climatic conditions. The climatic change, thus, is defined as variations and shifts in weather conditions over space and time of different scales and magnitude resulting into change of climatic type for example, from warm and moist climate to warm and dry climate, from warm and moist climate to cool and moist climate (as happened during Carboniferous period in India) etc. Infact, climatic change refers to drastic or secular changes in heat balance of the earth-atmosphere system, moisture, cloudiness and precipitation caused by either external factors such as variations in orbital characteristics of the earth, solar variability (fluctuations in radiation from the photosphere of the sun), tectonic proc- esses (mainly plate tectonics and displacement of continents and ocean basins), vulcanicity, changes in atmospheric composition in terms of concentra- tion of atmospheric aerosols and carbon dioxide contents etc. or by internal factors such as
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exchanges of energy between the atmosphere, hydrosphere, lithosphere and cryosphere (ice cov- ered surfaces of both lithosphere and hydrosphere) or by both, at local, regional and global levels. The climochronology (history of palaeoclimates) re- veals the fact that climates have changed in the geological past and hence it is opined that 'the world's climates have changed in the past, are changing now, and there is every reason to expect that they will change in future' (J.E. Hobbs, 1980). If the change is law of nature, the change in climates is a reality because the climate of a region is not fixed and static rather it goes on changing. Some times, the changes are cyclic and rhythmic, such change is called climatic cycle. It may be mentioned that James Hutton (a Scottish geolo- gist) while propounding the concept of uniformitarianism, postulated the concept of cyclic nature of earth's history. The example of occurrences of ice ages during (1) pre-Cambrian period (850-600 million years before present, mbp = million years before present), (2) Ordovician period (450-430 mbp), (3) Carboniferous-Permian periods (300 mbp), and (4) Pleistocene period (2-3 mbp) validates the concept of cyclic nature of climatic changes.
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The climatic changes are supposed to be quick and rapid rather than slow and gradual but this may not be always true as climate changes both gradually and rapidly, partly and drastically. For example, the climatic change, which occurred during Jurassic period leading to mass extinction of dinasaurs due to sudden onset of cold climate, was rapid and instantaneous. In fact, the rate of climatic changes depends on the nature of causal factors. The rapid Jurassic climatic change is related to sudden collision of the earth and a giant meteors and consequent release of enormous amount of dusts in the atmosphere. One cannot infer the nature of long-period climate on the basis of present-day climatic conditions. It is also an observed fact that the 'cool periods of earth history are periods of greater than normal climatic instability' (J. E. Hobbs, 1980).
The human society with present-day weather conditions is seized with the problems of possible climatic changes in near future. The most signifi- cant global environmental problem faced by the world community is global environmental changes (GEC) leading to probable climatic changes consequent upon global warming resulting from a host of causal factors, namely ozone depletion, increase in the emission of green house gases at alarming rate, deforestation etc. The probable net result of global warming would be climatic changes at local, regional, and global levels. The international communities are scared of cata- strophic adverse effects of future climatic changes on different spheres of man and nature, e.g. deglaciation and sea level changes, submer- gence of island nations and major coastal low- lands, atmospheric dynamics including evapora- tion and precipitation, global radiation balance, photosynthesis and ecological productivity, plant and animal communities, human health and many more. It may be summarized that climatic change is a reality, it has changed in the past, it is changing at the present, and it will change in future. The change of climate may be slow and gradual, rapid and catastrophic, periodic, semi-periodic or non-peri- odic, short-term or long-term, may be at local, regional and global scales, it may be due to natural factors or anthropogenic factors. It is, thus, necessary to discuss various aspects of climatic
CLIMATOLOGY
changes, namely scales of climatic changes (both spatial and temporal scales), indicators of climatic changes (i.e. evidences of climatic changes), reconstruction of palaecolimates (climochronology), causes of climatic changes, theories of climatic changes, and effects of climatic changes on both nature and biological communities including human beings.
15.2 SCALE DIMENSION
The climatic variations and changes are viewed in terms of temporal and spatial scales depending on the purpose of studies. The temporal scales of climatic changes range from a very micro-scale involving 10-day period to macro- temporal scale involving thousands to millions of years. 'The variability may be periodic (A), quasi- periodic (B), or non-periodic (C), or alternatively it may show a progressive trend' (Barry and Chorley, 2002). It may be mentioned that spatial and temporal scales of climatic changes are correlated e.g. as the temporal scale of change becomes shorter, the area also becomes smaller. In other words, the changes may be more perceptible and pronounced in localized area during short period of recorded climatic data, but if we consider climatic data at regional level involving large countries or continents, and at global level, the local level changes are overshadowed. Thus, the climatic changes may be viewed in terms of three temporal scales, namely (i) macro-temporal scale (millions of years), (ii) meso-temporal scale (thousands of years), and (iii) micro-temporal scale (hundreds of years). These three temporal scales correspond to global, regional, and local spatial scales. At regional and local spatial scales the climatic variability can be looked upon at even shorter temporal scales which may range from very micro-temporal scale to increasing time span e.g. (1) 10 days time scale, (e) 10-100 days time scale, (3) 100-1000 days time scale, (4) 1000-10000 days time scale etc. It may be remembered that such changes may be detected only if the instrumented data are very accurate and regular. The climate changes which occurred in the past, say before the industrial revolution, cannot be viewed in such a micro- temporal scales.
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CLIMATIC CHANGE
Generally, climatic changes are considered
at two levels e.g. (1) short-term changes, and (2) long-term changes. Short-term climatic changes involve the consideration of changes in the energy balance of the earth-atmosphere system leading to periodic changes in weather and climate. This temporal scale involves time span ranging from few years to thousands of years. Short-term climatic changes are either inter-annual or they may persist for many years and are generally caused by anthropogenic factors. On the other hand, long-term climatic changes persist for thousands to millions of years and are exceedingly slow. These are always caused by natural factors.
15.3 INDICATORS OF CLIMATIC CHANGES
The evidences of climatic changes in the past, which are utilized in the reconstruction of climochronology (history of palaeoclimates) are called indicators of past climatic changes. The riddle of reconstruction of palaeoclimates is a fascinating puzzle, the solution of which basically depends on proxy data (which include alternative sources of data as substitutes for real data) and logical deductions by the investigators. It may be mentioned that recorded data of weather and climate are available only since industrial revolu- tion, more precisely for the last 100 years or so, and hence proxy data for recorded data are the only indicators to decipher the pages of past climatic history and changes on the basis of Huttonian theory of uniformitiarianism (postulated by Scot- tish geologist, James Hutton in the year 1785). This theory was based on two concepts of Hutton, namely (i) 'present is key to the past', and (ii) 'no vestige of a beginning, no prospect of an end.' The principle of uniformitarianism states that 'the same physical processes and laws that operate today, operated throughout geological time, although not necessarily always with the same intensity as now' (Hutton postulated same intensity of processes throughout geological time).
The indicators (clues) of palaeoclimates may be classified on the basis of basic sources as
follows:
(1) Biological Indicators
(A) Floral indicators
(i) fossil remains of plants (ii) fossilized pollens
(iii) oxygen isotopes
(iv) tree ring growth
(dendrochronology))
(B) Faunal indicators (evidences)
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(i) fossilized animal remains lol (ii) distribution and dispersals of ani-
mals
(2) Geological Indicators
(A) Terrigeneous ancient deposits
(i) lacustrine deposits
(varves)
(ii) evaporite deposits
(iii) sedimentary deposits (coals) (B) Marine deposits japt pin
(i) sea floor deposits (C) Pedogenic indicators (3) Cryogenic Indicators ((A) Glaciation
(i) ice ages
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(ii) glaciers and glaciation
(iii) ice sheets and ice cores (B) Periglacial evidences (4) Tectonic Indicators
(A) Plate tectonics
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(i) pole wandering and continental
drift
(ii) palaeomagnetism and sea floor
spreading
(B) Sea level changes
(5) Geomorphological Indicators (A) Morphological features
(i) erratics (ii) sand dunes (iii) river terraces
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(iv) duricrusts
(v) pediments
(iv) tors
(B) Geomorphological processes (6) Historical Records
(i) flood records
(ii) drought records
(iii) migration of man and animals
1. Biological Indicators
The fossilized organic remains of plants and animals (i.e. organic residues) provide signifi- cant evidences and proxy data for contemporary climatic conditions mainly temperatures and moisture. It may be mentioned that the nature and types of organisms and their habitats are deter- mined by a climatic type. After their deaths the plants and animals are preserved in the forms of fossilized organic remains in the contemporary and subsequent geological formations. The analy- sis of such organic fossils through the application of proper techniques (e.g. carbon dating, oxygen isotope analysis, pollen analysis, dendrochronology etc.) reveals important clues to decipher contem- porary past climates. The biological indicators fall under two broad categories e.g. (1) plant (floral) indicators, and (2) animal indicators (faunal indicators).
(1) Plant (floral) Indicators
Presently, there is very close correspond- ence between the distribution of climate types and vegetation types. Based on the principle of uniformitarianism, as stated above, and the con- cept of 'present is key to the past' it may be opined that similar relationships and conditions might have prevailed throughout geological history of the earth. Thus, on the basis of fossilized plant remains, the vegetation type is inferred and such inferred information (proxy data) provide clue to determine the nature of climate. For example, the existence of coals having remains of tropical forests, in the north-west Europe and Great Britain remind us hot and humid climate of these areas during Jurassic period. The plant physiology gives sufficient information about climatic conditions.
CLIMATOLOGY
For example, the fossil remains of plants having long roots, thick barks, waxy leaves, thorns, little leaves etc. indicate warm and dry climate (warm desert climate), while plants having drip leaves are indicators of warm and very moist conditions (tropical rainforest climate). Now question arises, why plant physiology indicates a particular climate? The answer is simple, the physiological characteristics of plants are indicative of their adaptation to particular type of climatic condi tions. A few more examples may be cited. The vegetation community having sclerophyllous char- acteristics (stiff and hard leaves and stems with thick barks) can withstand extreme aridity of summer season. The plant remains of such sclerophyllous vegetation indicate dry summer and wet winter climate (Mediterranean type of climate). The cryophyte vegetations are well adapted to severe cold conditions as they develop such unique features which enable them to withstand extreme cold conditions. Thus, the fossil remains of cryophyte plants indicate cold climatic conditions (tundra climate).
The remains of vegetation in the thick coal seams of Carboniferous period worldover are related to horsetails and club mosses which develop in marsh and swamp environment. The fossil remains of trees in Carboniferous coal seams do not indicate annual ring growth which reveal the fact that such trees thrived in warm and moist climate with no appreciable seasonal
contrasts.
Pollen Analysis
The pollen analysis involves the inference of climatic conditions on the basis of dating of fossilized pollen grains preserved in the inorganic sediments layers. It may be mentioned that pollen grains of flowering plants are very durable organic substances which are duly preserved by the nature. The pollen grains are distributed by wind in wide areas having similar type of vegetation. The science of palynology, which is the study of pollen grains and spores of plants, helps in identifying the types of vegetation on the basis of pollen grains, and the vegetation types give clues for deciphering the climate in which that vegetation type might have developed. The pollen analysis
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inv pol dat tio
det
ve
po po lo
CLIMATIC CHANGE
involves the identification of layered sequence of pollen grains in the terrigenous sediments, the dating of pollen grains of each layer, determina- tion of vegetation type of each layer and finally the determination of climatic condition of each vegetation type (fig. 15.1). Fig. 15.1 depicts the pollen analysis technique wherein the fossilized pollen grains preserved in lacustrine deposits of lower, middle and upper layers denote dominance
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of pine in the lower layer, prevalence of pine and spruce in the middle layer, and oak, beech and spruce species of trees in the upper layer under the modern plant covers. On the basis of the principle of uniformitarianism, the lower and middle layers having pollen grains of pine and spruce indicate cool climate while the upper layer containing pollen grains of oak, beech and spruce trees reveals the prevalence of warm and moist climate.
Wind
Pollen
Modern plants
Climate
Warm and
Lake
Upper layers
Oak, beech,
moist
spruce
Mud
Middle layers
Pine, spruce
Cool
Land
Lower layers
Mostly pine
Cool
Core through lake deposits
Fig. 15.1: Presentation of simplified method of pollen analysis for the reconstruction of palacoelimates. After J.E.
Oliver and J.J. Hindore, 2003.
It may be remembered that pollens of flowers of plants are blown away by winds, they settle down on land surface as well as water surfaces of lakes, ponds etc. The pollens deposited on land surfaces may decay after lapse of time but pollen grains falling on water surfaces sink down and are deposited in the layers of sediments. The pollens so fossilized in different sedimentary layers are identified and their plant species are determined. The identification of plant species on the basis of their pollen grains then helps in determining the climatic conditions on the basis of present relationship between plant types and
associated climatic conditions.
Inspite of wide use of pollen analysis technique in determining vegetation types and associated climatic conditions this method suffers from the following shortcomings.
(i) In the case of mixed vegetation area having several plant species, the pollen grains are mixed, and in such circumstances it becomes difficult to separate pollen grains of dominant and co-dominant plant species and hence this technique fails to give desirable result.
(ii) The winds can carry the pollen grains and deposit them in distant places away from the area of the vegetation of which the pollens have been blown away.
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(iii) Not all the pollens of vegetation of a region are deposited, rather they are deposited selectively and randomly, and hence they may not be taken as representatives of particular vegetation type.
(iv) The distribution of vegetation has been tempered and modified by human actions, both advertent and inadvertent and hence pollen analysis would not be useful for the reconstruction of climates of recent past mainly since the neolithic times.
(v) 'A vegetation cover only attains matu- rity after fairly a lengthy period of time, and it is quite feasible that the vegetation established through pollen analysis represents successional stage that is not totally representative of the prevailing climate' (Oliver and Hidore, 2003).
Dendrochronology
Dendrochronology is the science that deals with 'the study of the annual rings of trees in determining the dates and chronological orders of past climatic events'. The ring growth of the trunk and stem of a tree provides significant clues for detecting seasonal rhythms of climate during the life time of the concerned tree. The thickness and spacing, and colour of growth rings indicate the climatic conditions and rate of annual growth of trees. The widely spaced thick growth rings denote warm and moist climate with sufficient rainfall to support luxurious and quick growth of trees, while narrow and closely spaced tree growth rings denote arid climate and poor growth of trees. The colours of growth rings also denote seasonal weather conditions. The abrupt change from light to dark- coloured rings (growth rings) delineates the annual increments of growth' (Oliver and Hidore, 2003). It may be mentioned that the study and interpreta- tion of different aspects of tree rings such as, the size (thick or narrow), number, colour shades, symmetry or asymmetry etc. provide information about climatic and environmental variations sus- tained by the concerned tree during its life time and hence the climatic events which happened in the recent past, say 3000-4000 years before present may be reconstructed on the basis of dendrochronological characteristics.
CLIMATOLOGY
The study of tree ring growth, known as dendrochronology, was initiated by A. E. Douglas and his team at the University of Arizona, USA. After the analysis of growth rings of trees in the southwestern USA he found close relationship between annual amount of rainfall and growth rings in the adverse climatic conditions of the S. W. United States. The scientists have become successful in reconstructing the climatic condi- tions and environmental changes for the last 3000 years on the basis of analysis of growth rings of living trees, and for the last 5000 years on the basis of fossils of dead trees. The scientists at Labora- tory of Tree Ring Research, University of Arizona, USA, have also derived logical and fruitful inferences about the relationship between weather elements (e.g. temperature, air pressure, and atmospheric circulation patterns) and widths and spacing of tree rings. Such relationships have also been used to demonstrate climatic fluctua- tions which took place during the life time of trees.
Dendrochronology also helps in recon- structing the advances and retreats of glaciers in the recent past on the basis of shapes of tree growth rings like concentric (symmetrical) growth rings or asymmetrical rings. It may be remem- bered that growth rings become concentric and symmetrical as long as the trees remain perfectly erect (perpendicular to the ground surface) but become asymmetrical when the trees are tilted. It is argued that trees are erect when the glacial ice is away from the position of trees but they are tilted when ice comes close to the trees. Thus, erect trees and resultant concentric and symmetrical growth rings are indicative of withdrawal or retreat of glaciers while asymmetrical rings reveal advanc- ing glaciers. It may be mentioned that tilting of trées and resultant asymmetrical growth rings may not be always due to glacial advancement, rather it may also be due to landslides, gusty winds, strong
storms etc.
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(2) Faunal Indicators
Identification and analysis of fossilized invertebrate animal remains embedded in the sedimentary formations provides significant clues about the climatic conditions that existed during the period of their survivals. In this respect
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the
'CLIMATIC CHANGE
physiological characteristics of fossils of inverte- brate animals (without backbones) and their chemistry are of vital significance wherein two techniques are used to determine their age, namely (1) radiocarbon dating (C-14 analysis), and (2) isotope analysis.
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The analysis of fossils of bones of inverte- brate animals helps in determining the lithological successions of sedimentary formations containing animal fossils and thereafter the sequences of palaeoclimates are reconstructed on the basis of dating of animal fossils and sedimentary layers. It be remembered that animal fossils are mostly preserved in the bottoms of oceans, lakes and rivers which provided them suitable habitats for their development and survival. Each species of tiny creature (e:g. foraminifera) survives in certain temperature and moisture condition. After death such creatures are embedded in the mud layers of water bodies. The mass deaths of certain species of micro-organisms take place when the temperature and moisture conditions change and become unfavourable to them. New species of organisms develop in accordance with new temperature and moisture regime. This process continues and fossils of animals are deposited in the successive layers of bottom sedimentary layers of seas, lakes and rivers. After determining the lithological successions, the animal fossils in each layer are identified and their dates are determined on the basis of carbon-14 analysis. On the basis of the principle of 'uniformitarianism' (the comparison of animals of a particular geologi- cal period as determined on the basis of C-14 analysis, with identical animals at present provides the climatic conditions in which they lived on the ground that same physical processes and laws which operate today operated throughout geologi- cal periods) the climatic conditions of that particu- lar period is inferred.
The radiocarbon dating method or carbon-14 analysis requires obtaining of cores of mud layers containing animal and plant fossils from the floors of oceans, lakes, and river valleys and the dating of the fossils is accomplished through the analysis of carbon-14 and carbon-12 contained in the skeletons of the fossilized animals. It may be mentioned that skeletons of dead animals 'contain
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both ordinary carbon and minute trace of isotope carbon-14. The proportion of carbon-14 to car- bon-12 remains fixed while the organism is alive. After it dies the carbon-14 begins to decay; by knowing the ratio of carbon-12 to carbon-14, one can determine the age of the shell' (Oliver and Hidore, 2003).
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dex The isotope analysis of the chemistry of skeletons of fossilized animals also helps in determining palaeoclimates on the basis of tem-
perature and moisture conditions which are inferred from the body chemistry of dead animals. Oxygen has three non-radioactive isotopes e.g. O17, 018 and O16 out of which the first two isotopes are not very common but the last one is common and normal isotope. After the evaporation of water, these isotopes crystallize at different rates in the shells i.e. O18 isotope settles down more rapidly than the O16 isotope because the latter is lighter than the former isotope (O18). It may be mentioned that the rate of crystallization of oxygen isotope is controlled by temperature. The number of settling O18 isotope decreases with increase in the temperature of ocean water. Thus, on the basis of number of oxygen isotopes contained in the shells of dead animals the prevailing temperature at the time of the existence of particular animal is determined. For this purpose again the cores of mud layers from the floors of oceans, lakes and river valleys are taken out, oxygen isotopes of shells of each mud layer are determined, tempera- ture condition for each mud layer is inferred and finally temperatures prevailing at the time of deposition of animal skeletons and mud layers are used to ascertain climatic changes.
Two important research projects, namely CLIMAP (Climate, Longrange Investigation, Mapping and Prediction) and COHMAP (Cooperative Holocene Mapping Project), were concerned with the study of climatic changes which might have occurred in the recent past on the basis of carbon-14 analysis and isotope analysis of skeletons of organisms embedded in the mud layers at the floors of the oceans. The CLIMAP studies concentrated on the reconstruction of palaeoclimates of about 1,000,000 years before present, while COHMAP project studied the palaeoclimates of the past 10,000 years only.
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The occurrence of large numbers of animal fossils in close proximity of a region denotes the fact that they might have been killed during a catastrophic disasters e.g. advancing ice sheets and freezing, severe droughts, meteoric collision with the earth (as is supposed to have happened during Cretaceous period killing dinasaurs enmass)
etc. On the basis of such evidences one can infer the climatic and environmental conditions at the time of concerned catastrophic disaster.
2. Geological Indicators
Geological indicators (evidences) of palaeoclimates include lithological characteris- tics of mainly sedimentary deposits such as lacustrine deposits (varves), evaporites, lime- stones and coal seams, marine deposits (sea floor deposits), soil profiles etc. Varves are the alternate sequences of layers of fine silts and clays deposited at the floors of lakes and large ponds in such an area which is characterized by alternate freezing and thawing during winter and summer seasons respec- tively. Thus, the lithological sequences of lacustrine deposits in the aforesaid condition denote periglacial climate. The analysis of annual layers of silt and clay provides an idea of seasonal changes in climate. It may be mentioned that when the surface freezes during winter season, very fine suspended clay particles are deposited in the lakes and ponds but during summer season, when frozen surface thaws (melts), silts with melt water are deposited in the lakes and ponds. Such type of annual banded alternate layers of clay and silt denote periglacial climate.
Evaporite deposits, represented by salt deposits, occur when climate is characterized by high temperature and aridity wherein evaporation exceeds precipitation. In such circumstances water is evaporated and salt contents are left behind. It may be mentioned that this happens only when water on land is saline and thus salt is in solution form. So, the massive salt rocks deposited on land surfaces, such as in South-West USA, Germany, Central Asia, North-West India (mainly Rajasthan and Gujarat), denote hot and arid climatic environment.
Limestones (CaCO3) containing calcium carbonates are deposited in tropical warm oceanic
CLIMATOLOGY
water and hence the occurrence of limestones in the regions having cold climates denotes the fact that the concerned region was under tropical warm climate at the time of limestone deposition.
Duricrusts are indurated hardened surfaces of different kinds such as laterites, silcretes, calcretes, alcretes, ferricretes etc. depending on the dominance of constituent minerals. Normally, lateritic crusts are supposed to have been formed in hot and humid climate of tropical and subtropi- cal areas and therefore these are indicative of hot and humid climate. Lateritic crusts are predomi- nantly found in Chotanagpur high lands (Patlands of Ranchi and Palamau plateaus of Jharkhand), and over many areas of Deccan plateau (e.g. Mahabaleshwar and Panchgani plateaus of Maharashtra). The presence of lateritic crusts in certain parts of Europe (e.g. U.K. and Germany) clearly demonstrates the fact that these are not the result of the present temperate climate. 'Such crusts are often interpreted as of Tertiary age, or as having been under continuous formation since the end of the Mesozoic. Exposures of silcretes and calcretes similarly are often related to past rather than present climatic conditions' (D. R. Stoddart, 1969).
Pedogenic criteria used to decipher past climates, include the analysis of palaeosols and fossils of plants and animals therein. The alluvial soils buried in older flood plains give indication of moist climate and the dominance of fluvial processes. The older loessic soils indicate dry climate and the dominance of aeolian process. Even the materials involved in the deposition of loess also give clues for climatic conditions. The extensive loess deposits of China having areal coverage of 774,000 km2 and thickness of 90 m to 300 m consist of materials blown from the deserts of central Asia while the European loess is example of glacial loess. The American loess, extensively found in Illinois, Iowa, Nebraska etc. is partly glacial and partly desert loess. The datermination of palaeoclimates on the basis of fossil remains in the soils is accomplished through radiocarbon dating and isotope analysis, which have already been explained in the previous subsection on floral
and faunal indicators.
CLIMATIC CHANGE
3. Cryogenic Indicators
Cryogenic indicators are related to the proxy data from the evidences of glaciation, glaciers and icesheets. The science dealing with glaciation and glaciers is known as glaciology. The processes of glaciation and deglaciation provide significant proxy data for climatic changes and fluctuation. The period of widespread glaciation of larger areas of the globe is called great ice age which comprises several glacial and interglacial periods. The glacial period denotes onset of cold climate and advance of ice sheets while interglacial periods indicate relatively warmer periods when ice sheets retreat. The geologists, glaciologists and climatologists have identified a few great ice ages (e.g. pre- Cambrian ice age, Carboniferous ice age, and Pleistocene ice age) when major portions of the globe were glaciated and covered with thick ice sheets which are definite indicators of major climatic changes.
Credit goes to European school of geomorphologists and glaciologists for identifica- tion and recognition of ice ages. Louis Agassiz (1807-1873 A.D.) is given credit for an early start in this precarious field and for the recognition and identification of the presence of ice age during Pleistocene period as he presented his ideas in the year 1840. A host of geoscientists, namely Jean de Charpentier, John Playfair (1815), Venetz of Switzerland (1821, 1829), Esmark of Norway (1824), Bernhardi of Germany (1832) etc., studied different aspects of Pleistocene ice age. The Scottish geologist James Geikie postulated the concept of 'great ice age' in 1894 which is comprised of several glacial periods which are separated by warm interglacial periods. A. Penck and Bruckner identified four glacial periods during Pleistocene ice age e.g. Gunz, Mindel, Riss, and Wurm which were separated by three warm interglacial periods. Similarly, four glacial periods (e.g. Nebraskan, Kansan, Illinoin, and Wisconsin) and three interglacial periods (e.g. Aftonian, Yarmouth, and Sangman) were identified during Pleistocene glaciation of North America. Most of Gondwanaland was glaciated during Carbonifer-
ous ice age.
The shapes of glaciated valleys, glacial drifts, erratics (glacial boulders), striations etc. provide important clues for climatic changes.
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The glaciers and ice layers provide evi- dences for temperature and precipitation condi- tions during different phases of ice ages. Advanc- ing glaciers and ice sheets indicate cold phase and lowering of temperature much below freezing point, while retreat of ice sheets heralds increase in temperature and subsequent ablation of glaciers demonstrates onset of warmer climate. The glaciologists have demonstrated the retreat of Alpine glaciers and resultant shrinking in their length and width about 3000 B.C. (i.e. about 5000 ybp, years before present), followed by re-advance of glaciers about 500 B.C. (i.e. 2500 ybp) and again retreat of glaciers. The Alpine and Scandinavian glaciers registered resurgence in the 17th and 19th centuries while they again started to retreat in the 20th century. All these indicate climatic fluctuations characterized by cooling (advance of glaciers) and warming (retreat of glaciers) periods during the past 5000 years in European continent.
Ice sheets and ice cores are most significant cryogenic indicators of palaeoclimates. It may be remembered that the ice sheets are formed by the deposition of several layers of ice. The accumulated snow from the annual snowfall is compressed and is changed to ice wherein the air bubbles and atmospheric dusts are trapped. Thus, every year a layer of ice is deposited upon underlying ice layer. It is evident that the great ice caps of Greenland and Antarctica are comprised of numerous ice layers wherein the then climatic conditions have been preserved. Thus, the study of each ice layer provides ample evidence of the weather and climatic conditions at the time of its formation. The drilling into ice caps is undertaken to obtain ice cores for analysis. The following properties of ice layers derived from the ice cores provide proxy data for temperature, precipitation, composition of atmosphere, storminess, volcunic events, atmos- pheric pollution etc.
(i) thickness of annual ice layers →
precipitation
(ii) chemical analysis of annual ice layers
→temperature-
(iii) air bubbles trapped in the annual ice
layers → atmospheric
composition
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(iv) dust contents trapped in the annual ice
layer atmospheric circulation
and storminess
(v) acid content in the annual ice layer →
volcanic events
A few studies of ice cores from Greenland ice sheets (namely, Greenland Ice Sheet Project 1 and 2, GISP-1 and 2) and Antarctica ice sheets have revealed sequences of climatic variations for the last 110,000 years and 160,000 years respectively. The analysis of ice cores to obtain climatic records is based on isotopes of oxygen (two very common isotopes are 160 and 180) wherein the ratio of 160 to 180 provides information about the existing environmental temperatures at the time of the formation of annual ice layers. The isotope analysis of ice cores also reveals relationship between global warming and concentration of greenhouse gases (e.g. carbon dioxide, methane etc.), and level of atmospheric pollution caused by anthropogenic sources since the period of industrial revolution. Besides, the concentration of radioactive elements in the atmosphere and their fallout emitted during nuclear plant disasters (for example, Chernobyl nuclear disaster) and testings of atomic devices, can be measured through the analysis of annual ice layers. The ice core analysis also reveals informa- tion about the levels of sulphate content in the air emitted from the volcanic eruption. With the improvements in the technologies of obtaining ice cores, studies are carried out to reconstruct the atmospheric conditions mainly temperature varia- tions and climatic fluctuations for the past 200 years or so on the basis of the analysis of ice cores derived from mountain ice sheets in the tropical and subtropical areas.
The evidences of periglacial features and deposits found in such areas which at present do not have periglacial environment, indicate earlier periglacial climate of those areas. The term periglacial literally means around the ice or peripheral to the margins of the glaciers but now this term is used for both 'periglacial landscape' and 'periglacial climate'. Periglacial areas are those which are in permanently frozen condition but without permanent ice cover on the ground surface. The periglacial climate is characterized by
TO CLIMATOLOGY
mean annual temperature ranging between -1°C and -15°C and mean annual precipitation of 120 mm to 1400 mm (mostly in solid form). Many parts of Europe and South-West USA presently do not have periglacial climate but have relict periglacial features which demonstrate that such areas were under periglacial climate in the past.
4. Tectonic Indicators
The tectonic movements involving plate movements, pole wandering, continental drifts, orogenesis, palaeomagnetism, topographic fea- tures etc. and seafloor spreading and sea level changes are significant indicators of palaeoclimatic changes. A. Wegener, a German meteorologist, was primarily concerned with the problem of past climatic changes. It may be pointed out that there are ample evidences, as discussed above, which indicate widespread climatic changes throughout the past history of the earth. Infact, the continental drift theory of Wegener 'grew out of the need of explaining the major variations of climate in the past'. The climatic changes which have taken place on the globe may be explained in two ways as follows:
(1) If the continents remained at their places throughout geological history of the earth, the climatic zones might have shifted from one region to another region and thus a particular region might have experienced varying climatic conditions from time to time.
(2) If the climatic zones remained station- ary, the landmasses might have been displaced and drifted.
Wagener opted for the second alternative and postulated the concept of 'continental drift' or 'displacement hypothesis' to explain the global climatic changes on the basis of a number of geographical, geological, palaeontological etc. evidences. The plate tectonic theory as an outcome of post-Second World War advancement (mainly in the 1960s) in geotectonics, the evidences of palaeomagnetism and seafloor spreading have validated the concept of polewandering (shifting of the positions of the poles) which help in the reconstruction of climochronology mainly global climatic changes during Carboniferous and Pleistocene Ice Ages and widespread glaciation.
CLIMATIC CHANGE
It has been demonstrated that plate move- ments during Cenozoic era displaced major continental blocks in the middle and high latitudes in the northern hemisphere and thus such north- ward displaced continental masses were subjected to the dominance of cold climate resulting into widespread glaciation (such as major parts of North America and Eurasia) during Pleistocene period (known as Pleistocene Ice Age and Pleistocene glaciation). On the other hand, moun- tain ranges of Tertiary period, formed due to collision of convergent plates and consequent subduction of heavier plate and lateral compres- sion, and high plateaus, became effective barriers in controlling the global atmospheric circulation and generating a few new climatic types such as monsoon climate of South Asia (due to relief barriers of the Himalayas and Tibetan plateau). The investigations regarding the impacts of tectonic movements on climatic conditions have revealed strong relationship between horizontal plate movements leading to seafloor spreading along divergent plate boundaries and orogenesis along the convergent plate boundaries, vertical movement (tectonism) leading to upliftment and subsidence and climate change. The changes in global pattern in plate motions cause changes in climates at global scale.
5. Geomorphological Indicators
The geomorphological processes and landforms resulting therefrom have been directly related to particular climatic type on the concept that each climatic type produces its own character- istic assemblage of landforms and set of geomorphological processes which shape them on the basis of the following themes:
(1) Landforms differ significantly in differ- ent climatic regions.
(2) Spatial variations of landforms in different climatic regions are because of spatial variations in climatic parameters (e.g. temperature, humidity, precipitation etc.) and their influences on weathering processes, erosion dynamics and surface runoff.
(3) Quaternary climatic changes could not obscure relationships between landforms and
climates.
349
In other words, there are certain diagnostic landforms which clearly demonstrate climate- landforms relationships.
Thus, on the basis of diagnostic landforms, the inference of climatic conditions at the time of the formation of such landform, is drawn which helps in the reconstruction of palaeoclimates.
The climatogenetic or climatically control- led landforms are identified and differentiated in two ways, e.g. (i) general observation and acquaintance of whole landscape of each climatic region, and (ii) identification of typical or distinc- tive landforms which represent the controls of particular climate. The typical landforms are, in fact, main tools of climatic geomorphologists which help them in determining climate-landforms relationships in different climatic regions. Such distinctive landforms are designated as diagnostic landforms which include duricrusts, inselbergs, pediments, tors, glaciated valleys, glacial boul- ders or erratics, sand dunes etc.
Duricrusts are indurated hardened surfaces of different kinds such as laterites, silcretes, calcretes, alcretes, ferricretes etc. and are normally supposed to have been formed in hot and humid climate of tropical and subtropical areas and hence they are indicative of warm and moist climate. The presence of lateritic crusts in certain parts of Europe (e.g. U.K., Germany etc.) clearly demon- strates the fact that these are not the result of present climate. 'Such crusts are often interpreted as of Tertiary age, or as having been under continuous formation since the end of the Mesozoic. Exposures of silcretes and calcretes are often related to past rather than present climatic conditions' (D. R. Stoddart, 1969).
18 20 Inselbergs representing steep sided residual hills are considered to be the representative landforms of hot and arid and semi-arid climates. It is argued that inselbergs are structurally controlled rather than climatically controlled and most of the present inselbergs were formed before Quaternary period, 'hence present climates are not necessarily those in which inselbergs were formed' (Stoddart, 1969). It may be possible that inselbergs might have been formed when the climate was warm and arid or semi-arid which might have changed after their formation.
350
Pediments, characterized by low-angle rock- cut surfaces surrounding mountains, are also considered to be the representative landforms of warm arid and semi-arid climates. Pediments are also found in a variety of climatic conditions e.g. tropical wet and dry climate, subtropical and temperate climates. A few geomorphologists argue that pediments are structurally and tectonically controlled rather than climatically controlled. L.C. King has opined that the process of pediplanation and pedimentation is universal and occurs in all environmental conditions. In fact, 'many arid zone pediments are clearly polycyclic, developed during the complex sequence of Pleistocene pluvials (period of prolonged rainfall) and interpluvials : Many appear to be being destroyed under present climatic conditions, rather than being formed' (D.R. Stoddart, 1969).
Tors, 'one of the most controvercial landforms, are piles of broken and exposed masses of hard rocks particularly granites having a crown of rock- blocks of different sizes on the tops and clitters (trains of blocks) on the sides' (Savindra Singh, 1977). Tors have been considered of periglacial origin by J. Palmer and R. A. Neilson (1962), of fluvial origin by D. L. Linton (1955), whereas L. C. King has opined that tors are the result of universal processes of pediplanation in different climatic conditions. In fact, the presence of tors right from the Dartmoor of England through Nicargua to India has complicated the problem of the origin of tors rather than solving it.
It may be concluded that the aforesaid diagnostic landforms are older than Pleistocene climatic changes, so they are definitely not related to present climates where they are found. It may be pointed out that climatic relation of landforms at least in glacial, periglacial and warm desert climates is undoubtedly confirmed. So, such landforms may provide definite clues to recon- struct past climates.
It is an established fact that different geomorphological processes work in different climatic regions because the geomorphic proc- esses originate from the atmosphere and their nature (type, e.g. whether fluvial, or glacial, or periglacial or aeolian etc.) depends on the combina- tions of temperature and precipitation. Thus, the
BOK CLIMATOLOGY
nature of geomorphological processes in the past geological history of the earth helps in deciphering the climatic conditions in which a particular process was predominant.
The presence of glacial boulders or erratics in a region presently having other than glacial climate, may it be warm tropical or temperate climate, reminds us the fact that the region was glaciated at the time of formation and deposition of these erratics. Similarly, the presence of frost- riven cliffs, tors, altiplanation terraces, patterned grounds (having stone rings, polygons, garlands, stripes etc.), thermokarst lakes, nivation hollows, involutions, pingos, congelifluctate landforms, blockfields (blockmeers), boulderfields etc. tells us the dominance of periglacial processes under periglacial climate at the time of the development of such landforms. The presence of glacial boulders and erratics in the coal seams of Talcher of Orissa (India) of Gondwana period clearly indicates an earlier cold and glacial climate in India having presently warm monsoon climate.
6. Sea Level Fluctuations
Fluctuations in sea levels are considered significant indicators of past climatic changes. Sea level changes are of two types, namely positive change marked by rise in sea level above datum line, and negative change denoting fall in sea level below datum line. Changes in sea level are effected by (i) tectonic movements, and (ii) climatic changes. The tectonic movements cause rise in sea level when there is upward movement of sea floor (ocean bottoms) or down warping of coastal areas whereas fall in sea level is caused by downwarping leading to subsidence of sea floor or upwarping of coastal lands. It may be mentioned that tectonically induced changes in sea levels are very slow. The climatically induced changes in sea levels are rapid and are effected by glacial age and advancement of glaciers and ice sheets (fall in sea levels) and deglaciation leading to retreat of glaciers and ice sheets (rise in sea levels). The sequences of rise and fall in sea levels during Pleistocene Ice Age have revealed several phases of glacial periods (Gunz, Mindel, Riss and Wurm in Europe, and Nebraskan, Kansan, Illinoin and Wisconsin glacial periods in
CLIMATIC CHANGE
Fall
R
+20
Present Sea Level
0
-20
B-40
-60
-80
'(w)T
-100
-120
(-)
(-)
(--) (--)
(-)
25000
75000
125000 W
0
50,000
100.000
150,000
Age Before Present (B.P.)
Fig. 15.2: Fluctuations in sea level during last 150,000 years based on raised coral terraces and core oxygen 18 isotope data from deep sea deposits. Source: after: K.K. Turekian, 1996, in Oliver and Hidore, 2003.
Mean Sea Level in Meters
+ 20
Present 0
level
- 20
- 40
-60
80
- 100
20 18 16
14
12 10
8 Thousands of years ago
6
CA
0021
ilo
Kar
Fig. 15.3: Fluctuations in sea level with reference to present sea level (present datum line at o meter). Source: after
Fairbridge, in H.J. Critchfield, 2002.
351
352
North America and interglacial periods (relatively warm periods) separating two glacial (cold) peri- ods. Figures 15.2 and 15.3 denote fluctuations of sea level (rise and fall) with reference to present sea level (i.e. present datum line at o meter) about 150,000 ybp and 20,000 ybp (years before present).
7. Indicators of Historical Records
The recorded events during the existence of human species provide valuable data for recon- structing the palaeoclimatic history (palaeochimo- chronology) for the past 6000 years. It may be mentioned that the recorded past events are related mostly to extreme events of weather conditions rather than regular weather conditions. Such extreme weather events include freezing of rivers and lakes, unprecedented floods and droughts leading to famines, mass exodus of human migra- tion deserting their settlements etc. A few exam- ples will be sufficient to demonstrate the signifi- cance of such indicators of climatic changes. The records of temperatures of the Thames (U.K.) from 9th century A.D. to 18th century A.D. denote increasing trend of frequencies of freezing of the Thames river per 100 years e.g. the Thames was frozen only once or twice per 100 years from 800 to 1500 years ago, 4 times during 16th century, 8 times during 17th century, and 6 times during 18th century. This denotes cooling of U.K. and environs from 800 to 1800 years before present (YBP). The recorded data from Iceland from 900 to 1900 YBP also demonstrate cooling of the Northof North Atlantic Ocean between 900 and 1900 YBP. The records of high floods in the Nile Valley since 640 A.D. reveal increase in rainfall in the source regions of the Nile river. The records of droughts in the south-western arid and semiarid United States of America and Sahel region or Sub-Sahara region denote acute deficiency in normal rainfall and resultant extreme aridity.
15.4 RECONSTRUCTION OF
CLIMOCHRONOLOGY
Climochronology may be defined as system- atic description of climatic conditions and climatic changes in terms of geological history of the earth i.e. past climates of each period of the earth's history. The reconstruction of palaeoclimates i.e.
CLIMATOLOGY
climochronology means rearranging of climatic history of the globe or part thereof on the basis of indicators/evidences of plaaeoclimates. The de- scription of climochronology of the world may be attempted in the following heads.
(1) Climatic changes during geological
periods,
(2) Climatic changes during Quaternary period, and
(3) Climatic changes in the recent past (about past 1000 years or so).
1. Climatic Changes Through
Geological Periods
The geological history of the earth or the 'geological clock' refers to the reconstruction of evolutionary sequences of the geological events involving the information of various zones (crust, mantle and core) of the earth, formation and evolution of geomaterials (rocks), formation and development of mountains and faults, evolution of different lives etc. The whole geological history right from the origin of the earth to its present form has been divided into major and minor periods on the basis of forms of life (organic remains), characteristic rock deposits, places of rock forma- tion, major tectonic events etc. The whole geologi- cal history of the earth has been divided into five eras (the largest time division of the earth's history has been termed Era) based on five major groups of deposits as follows: Major Groups of
Deposits
Cenozoic group Mesozoic group
Proterozoic
Archeozoic
Eras
(from youngest to oldest)
Cenozoic
(era of recent life)
Mesozoic
(era of medieval life)
Proterozoic
(era of earlier life) Archaeozoic
(era of primeval life) Each era is numbered in sequence as first (primary), second (secondary), third (tertiary) and fourth (quaternary) epoch. Further, each epoch is
CLIMATIC CHANGE
190
200
225
136
Jurassic
End of ice ages- Human evolution
100-
70
Cretaceous
Cenozoic
0 millions of years
1 billion
years
2 billion
years
3 billion
Oldest fossil
years
280
300
Missis-
sippian
sylva-
nian
Permian
345
006-
Devonian
395
400
cells
Youngest Moon rocks
Silurian
440
blo
4 billion
years
Oldest rock dated
4.7 b.y.
Oldest Moon rocks
on
Earth
Precambrian
000.01
Ordovician
Cambrian
Shelled organisms evolve
570
600
- 800
700
500
Fig. 15.4: The geological clock (modified after F. Press and R. Siever, 1974). Numbers denote years in millions
before present.
Cambrian
Ordovician
Cambria or Wales (place) in U.K. Ordovices (a British tribe in N. Wales)
Silurian
Devonian
Silurs (a British tribe in S. Wales)
divided into several periods. The names of periods have been assigned on various grounds e.g. names of the places of characteristic systems of deposits, the names of tribes, the characteristics of deposits, dominance of certain elements and minerals etc. as follows:
palaeo (ancient), zoe (German)- life
Palaeozoic
Mesozoic
mesos (German) means middle
Permian
Cenozoic
Kainos (German) means new
Devonshire (place and region in U.K.)
Carboniferous dominance of carbon (coal)
perm (a province in erstwhile.
USSR
353
354
Triassic
Jurassic
Cretaceous
three-fold division of deposits in Germany, 'trias' means triple after Jura mountains in Switzer- land
creta (Latin) means chalk, domin- ance of abundant deposits of white writing chalk
Eos means day break
Oligos (German) means little Meion (German) means smaller Pleion (German) means greater Pleistos means most
Eocene
Oligocene
Miocene
Pliocene
Pleistocene
Holocene
Holo means complete
CLIMATOLOGY
Some scientists have put together all the geological events of the past history of the earth in the form of a clock. Thus, the spiral system representing the whole geological and geomorphic history together is called as 'geological clock' wherein one billlion years represent each revolu- tion of the clock's arm. Each revolution is further subdivided into 'hours' where each divi- sion (hour) corresponds to 100 million years and 'minutes' represent the time period of 10 million years. Fig. 15.4 represents the geological clock suggested by Frank Press and Raymond Siever
(1974).
Table 15.1: Geological Time Table (from youngest to oldest)
CI
Pi
to
P
ba
th
ea
af
CO
pl at
bi
ar
be
re
is
ea
Eras
Epochs
Periods
пром
Duration (million years) Age
Ice
Starting time before present
SU
41
SU
(million years)
M
Or
Neozoic
Quaternary
2. Holocene or Post-glacial 1. Pleistocene
di
Pleistoecene
ea
0.990
ice age 4
1.000
P
Cenozoic
Tertiary
4. Pliocene
10.000
11.000
gi
3. Miocene
14.000
25.000
m
2. Oligocene
15.000
40.000
de
1. Eocene
30.000
70.000
se
Mesozoic
Secondary
3. Cretaceous
65.000
135.000
R
O
2. Jurassic
45.000
180.000.
re
1. Triassic
45.000
3
3ools loigulong si 225.000
kr
Palaeozoic
Primary
6. Permian
45.000
Carboniferous
270.000
La
5. Carboniferous 80.000
ice age
350.000
su
m
4. Devonian
50.000
age
400.000
3. Silurian
40.000
2
440.000
ag CO
2. Ordovician
60.000
amojaIce age
500.000
1. Cambrian
ha
100.000
Pre-Palaeozoic
Azoic or Archaean
Pre-Cambrian
or Algonican Archaean
(450-430 m ybp) 600.000
Pre-Cambrian
thi
700.000
m
ice age (850-
Pr
600 m ybp)
800.000
ΕΣ
pre
CLIMATIC CHANGE
Pre-Palaeozoic Era
ninamoasige
Very few geological evidences are available to reconstruct the climatic history of early pre- Palaeozoic era. Inferences have been drawn on the basis of lithological evolution and evidences thereof, numerical models and deductions. The earth changed from gaseous state to liquid state after its origin. The solid crust was formed due to cooling and solidification of liquid materials. This phase was followed by the formation of dense atmosphere surrounding the earth. Due to gradual but continued cooling and contraction of the earth and resultant condensation of water vapour there began the precipitation process which ultimately resulted into the development of rivers and seas. It is assumed that the earliest climatic phase of the earth's atmosphere was warm with average (as- sumed) atmospheric temperature of 37°C about 4250 MYBP (million years before present) which is supposed to have dropped to 25°C about 2500 MYBP. Such assumptions and calculations are based on the estimate that the concentration of carbon dioxide (CO2) might have been much higher in the earliest atmosphere than at present and thus the pre- Palaeozoic climate might have been very warm due to green house effect of the early atmosphere.
The sequence of warming was broken by many glacial periods as indicated by glacial deposits, a few glacial features, striated surfaces, sedimentary data etc. found from Gowganda, Ramsey lakes, Bruce formations etc. in the Ontario province of Canada. These indicators reveal the existence of probably the first ice age, known as Huronian Glaciation (on the basis of Lake Huron of Canada and USA) which is supposed to have occurred about 2700 MYBP and might have continued upto 1800 MYBP. The earth again experienced warm climatic phase which continued upto 950 MYBP.
The late pre-Cambrian period is supposed to have been again under cold climatic phase wherein three glacial periods might have occurred when most of the earth's surface was glaciated.
Precambrian Period one visit onT
The geological characteristics of rocks of pre-Cambrian period which started about 700 MYBP (million years before present) denote that
355
dense atmosphere was formed around the earth. Due to gradual but continued cooling and contrac- tion of the earth and resultant condensation of water vapour there began the process of precipita- tion which ultimately resulted into the develop- ment of rivers and seas. The sequence of warm climate was broken by many glacial periods resulting into the beginning of perhapse the first ice age known as pre-Cambrian ice age having a time span from 850 MYBP to 600 MYBP. Among the plant kingdom only marine grasses were evolved. The three pre-Cambrian glacial periods known as Gnejso, Sturtian, and Varangian glacial periods, continued from 950 to 650 MYBP.
Palaeozoic Era (650-600 to 250 MYBP)
The early Cambrian period is supposed to have been in cold climatic phase leading to ice age during which the glaciation was more widespread than pre-Cambrian ice age. This connotation is validated on the basis of the presence of ancient early Cambrian morainic deposits in Greenland, Scottland, Scandinavia, China, South America southern parts of Africa, Australia etc. representing both the hemispheres. This cold phase was followed by warm climatic phase which became responsible for the evolution of life mainly plants in water bodies i.e. seas. Most of the vertebrate animals including 1000 species were evolved in the seas but these are not found at present. These animals depended on marine grasses for their food. Though evidences from ancient organisms and sedimentary deposits suggest warm climatic phase during Cambrian and early Ordovician periods but some evidences from Sahara indicate little ice phase, if not ice age, during late Ordovician period. The Silurian period again became warm and this phase continued into Devonian period. The evidences denote warm tropical climate of most of the northern hemisphere including North America, Europe and China, characterized by warm and semi-arid climate. The Devonian period was also characterized by the evolution of green land plants and a large number of species of fish. Amphibians were evolved towards the end of Devonian period. There was dispersal of vertebrate animals from seas to land
areas due to such flora on land areas which could provide them food.
356
The Carboniferous period experienced con- trasting climatic conditions in the northern and the southern hemispheres. Most of the coals of North America and Europe were deposited during this period. The northern hemisphere was characterized by warm tropical climate having both warm dry and warm wet climates which became responsible for dense vegetation cover in the northern hemisphere. On the other hand, most of the southern hemi- sphere, say Gondwanaland, was under cool cli- matic phase leading to widespread glaciation known as Carboniferous Ice Age which is sup- posed to have continued from middle and late Carboniferous period to early Permian period. The climatic hazard of such widespread glaciation became responsible for extinction of numerous plant and animal species. With increasing seasonal variations in the climatic conditions the ratio of evergreen trees continued to decrease. Conse- quently, the deciduous trees which could resist dry weather and frost, were evolved. The number and species of land animals further increased and numerous species of mammal-like reptiles were evolved but these soon perished. By the end of Permian period the southern hemisphere recovered from Carboniferous glaciation due to retreat and ablation of ice sheets.
Mesozoic Era (225-70 MYBP)
The climatic condition during Triassic period was warm and dry but it became wet by the end of this period. Consequently, coniferous trees and ferns were developed in the northern hemi- sphere. For the first time, mammals evolved from reptiles on land areas. The Jurassic climate became subtropical. The rainfall was such that dense vegetation could be evolved and developed in many areas. For the first time, flowering plants (angiosperms) were evolved during this period. Land areas were dominated by forests and swampy plains having lakes and meandering rivers. Creta- ceous period was marked by warming of high latitudes which made the growth of vegetation possible upto Greenland. Deciduous trees flour- ished because of seasonal regime of climate. The oxygen isotope analysis of deep sea cores having benthic and plankton fossils (300 planktons) provide ample data for inferring temperatures of oceans in both low and high latitudes. The later half
CLIMATOLOGY
of Cretaceous period witnessed cooling of high latitudes due to northward displacement of land areas of Angaraland. This time of earth history Isaw the world in its greenhouse mode, when climate was predominantely warm, polar ice caps nonexistent, and sea level high. The change from this to an eventual ice house mode may not have been smooth, but rather episodic' (Oliver and Hidore, 2003).
Cenozoic Era
The early Tertiary witnessed drop in tomperatures but still the climate remained warm. In the Eocene period (70 to 40 MYBP) or early Tertiary epoch England was characterized by tropical vegetation similar to present Malaysia. Warm climate extended upto Greenland and hence tropical palm trees grew upto Greenland. During Oligocene period (40 to 25 MYBP) most of the areas were dominated by warm and temperate climate but the cycle of cold climate also started in this period. The onset of cold climate caused disappearance of forests in some areas mainly in high latitudes but there was expansion in grasslands which became responsible for the evolution of many species of grass eater mammals. During Miocene period (25-11 MYBP) the earth's surface was characterized by varying climatic conditions as these varied from dry and desert climatic conditions to wet and cold climate. Humid climate became responsible for the growth and develop- ment of deciduous forests having species of maple, oak and poplar in North America and Europe while cedar grew in highlands. The plains of North America were covered by prairie grasses. The Pliocene period (11-1 MYBP) witnessed wide range of fluctuations in temperatures i.e. repetition of warm and cold phases. The glaciers began to form over Antarctica. The continued lowering of tempera- ture culminated into the formation of continental glaciation and onset of ice age in Pleistocene period of Quaternary epoch. This is being discussed in the following heading.
2. Quaternary Climate Changes
The Quaternary epoch of Cenozoic era started about one million years before present and continues at present. This epoch comprises Pleistocene and Holocene (post-glacial period) periods. The
CLIMATIC CHANGE
Pleistocene witnessed most pronounced climatic changes for which much authentic data derived through different techniques such as pollen analy- sis, isotope analysis, carbon dating, potassium- argan dating etc. are available and the sequences of events are well documented. Major parts of North America and Europe were extensively glaciated during Pleistocene ice age which comprised of four glacial and four interglacial periods.
The Pleistocene glaciation of North America (Fig. 15.5) witnessed four periods of cooling and resultant formation of glaciers and ice sheets and their equatorward advancement. The glacial peri- ods have been identified and named as Nebraskan (300,000-260,000 YBP), Kansan (205,000-167,000
357
YBP), Illinoin (135,000 -100,000 YBP) and Wiscconsin (70,000-10,000 YBP). These four glacial periods were separated by warmer periods, called as interglacial periods, namely Aftonian interglacial period between Nebraskan and Kansan glacial periods (260,000 to 205,000 YBP), Yar- mouth interglacial period between Kansan and Illinoin glacial periods (167,000-135,000 YBP), Sangamon interglacial period between Illinoin and Wisconcin glacial periods (100,000-70,000 YBP) and recent Holocene interglacial period. The ice sheets advanced from 3 major sources (ice caps) e.g. (1) Labrador ice sheet; (2) Hudson Bay or Keewatin ice sheets, and (3) Cordillarean or Rocky ice sheets. Two ice sheets of Labrador ice caps and
PACIFIC
C
C
K
N. AMERICA
OCEAN
ATLANTIC
OCEAN
S, AMERICA
Fig. 15.5 Glaciation of North America during Pleistocene Ice Age; C-Cordillarean ice sheet, K-Keewatin ice sheet,
and L-Labraa" ice sheet.
358
Keewatin ice caps after their initial southward advance combined together and became much wider and extensive ice sheets which moved as far south as upto the province of Nebraska. The combined ice sheets were called as Laurentide ice sheet. The St. Lawrence Valley and central lowland were completely covered by Laurentide ice sheet. Its eastward movement covered the Appalachians. Besides, Alaska, western Canada, Washington, Idaho, Montana etc. were also covered by thick ice sheets measuring 1000 meters to 1500 meters in thickness. The withdrawal or retreat of ice sheets during interglacial periods left behind numerous terminal moraines. Several lakes mainly the Great Lakes (comprising Superior, Michigan, Huron, Erie and Ontario) are the legacy of Pleistocence Ice Age and consequent glaciation.
Table 15.2: Pleistocene Glaciation of North America
Glacial and Interglacial
Periods
1. Nebraskan
1. Aftonian
(inter glacial)
2. Kansan
2. Yarmouth
(inter glacial)
3. Illinoin
3. Sangamon
(inter glacial)
4. Wisconcin
4. Holocene
(inter glacial)
Duration
(years before present)
300,000-260,000
260,000-205,000
205,000 167,000 167,000-135,000
135,000-100,000 100,000-70,000
70,000-10,000 10,000
Europe was glaciated during Pleistocene ice age by the advancing ice sheets from three major sources of snow fields (i.e. caps), namely Scotland, Scandinavia, and the Alps. The advancing ice sheets from Scotland and Scandinavia covered Great Britain, Scandinavian countries, Denmark, Belgium, Luxemberg, Netherlands, Germany while Switzerland, Austria, Italy, France, southern Ger- many etc. were glaciated by ice sheets advancing northward from the ice caps of the Alps. Like glacial and interglacial periods in North America, four glacial periods have also been identified and
CLIMATOLOGY
named in Europe, e.g. Gunz, Mindel, Riss and Wurm glacial periods wherein two glacial periods were separated by warmer interglacial period. The tropical and subtropical regions of the globe remained warm.
The last but most recent early Wisconcin glacial period started about 70,000 YBP but main Wisconcin glacial period in North America is supposed to have started about 30,000 YBP (years before present) and retreated about 12,000 YBP. The then temperature was about 4°C less than the present temperature.
3. Climate in the Post-Glacial Period
The final retreat and withdrawal of recent ice sheets started about 18,000 YBP and continued upto 10-12,000 YBP when most of the ice covers were withdrawn from the USA. On the other hand, the margins of retreating ice sheets in Scandinavia and Scotland (Europe) started to expand and advance about 10,200 YBP and thus there was reappearance of short period glacial phase involv- ing limited areal extent. This little short-period glacial phase is known as Younger Dryas but this temporary glacial phase soon came to an end as the ice sheets melted and finally withdrew. The period ranging from 18,000 to 5,500 YBP is considered as a period of deglaciation denoting rapid changes in climate and rise in temperature.
It may be mentioned that the Pleistocene Ice Age caused lowering of sea level at least by 100 meters about 18,000 YBP and the deglaciation during Holocene period resulted in the recovery of sea level to its present level about 5000-6000 YBP (fig. 15.3). As mentioned above after Younger Dryas the warming of previously glaciated areas resulted in complete withdrawal of ice sheets from North America and Europe by 5,500 YBP. Green- land and Antarctica still remained under thick ice sheets. 'All evidence points to this being a time when the mean atmospheric temperature of the mid-latitudes was 2.5°C (4.5°F) above that of the present. This time has been described as the Climatic Optimum a term originally applied to Scandinavia when temperatures were warm enough to favour more varied flora and fauna' (Oliver and Hidore, 2003).
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3
2
1
0
1-
-2--Deglaciation
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Little
Climatic optimum
Ice
Age
goog
3
-4-
Younger Dryas
5
18
16
14
12
10
8
6
4
2
0
Thousands of year
Fig. 15.6: Trend of changes in surface temperature for the past 14,000 years. The temperature change is from
present average global temperature. After J.E. Oliver and J.J. Hidore, 2003.
4. Climatic Changes in the Recent Past
Christian Era)
The proxy data from geological and biologi- cal sources and indicators, instrumental records, historical records, documentation etc. provide ample authentic information to reconstruct yearly climatic fluctuations since 1st century A.D. The records show that the temperature and precipitation conditions of European continent and Mediterra- nean regions were similar to presentday climatic conditions in the 1st century of Christian era. These areas experienced further more humid conditions characterized by increased precipitation and ameleorating temperature upto mid 4th century A.D. (i.e. by 350 A.D.). Europe and North America experienced dry phase resulting into semi-drought condition in the 5th century A.D. The increased aridity caused drying of several lakes in the western USA. The climate in the northern hemisphere
0 became further harsh during 600-700 A.D. when climate became warmer and drier. The increased aridity resulted in melting of valley glaciers in the Alps of Europe and opening of frozen passes allowing free movement of people across the Alpine mountains. It may be mentioned that these passes are again in frozen condition at present time due to reestablishment of valley glaciers. The moist condition returned again in the 9th century A.D.
The period from 950 to 1250 A.D. i.e. 300 year period is called as Phase of Little Climatic
Optimum in the climatic history of the earth when climate became warm and relatively dry as average temperature increased by 10 to 2oC from the present-day global average temperature. The cli- mate of Greenland and Iceland became mild and attracted the Vikings from Iceland to settle in Greenland. It may be mentioned that Vikings migrated from Europe to settle in Iceland in the 9th century A.D. because of favourable conditions characterized by warmer climate. The climatic conditions in southern Greenland allowed the growth of stunted vegetation, pasture and agricul- ture to support newly settled human population.
The period from 1250 A.D. to 1450 A.D. was characterized by the reversal of mild climate of 10th to 13th centuries A.D. as referred to above as temperature began to drop causing accumulation of more ice over Greenland, drifting of ice sheets and numerous icebergs in the North Atlantic Ocean. The drifting icebergs disrupted human physical connection of Greenland with Iceland and Europe. The 13th century saw very stormy weather in the Atlantic Ocean and the north Sea while extreme arid condition leading to severe drought prevailed in the western USA.
The trend of worsening climatic conditions in the middle and high latitudes of the northern hemisphere continued and the climate became further harsh for another period of around 400 years (i.e. from 1450 A.D. to 1880 A.D.) as the
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temperature fell below freezing point giving birth to another glacial period known in the climatic history of the earth as Little Ice Age. The climate of Greenland became so cold that the earlier settled people perished and their settlements and other infrastructures were buried under thick cover of ice sheets. The Alpine glaciers became more active, all the glacial valleys were covered with thick ice sheets, the advancing glaciers engulfed several villages at the foot-hills of the Alps mountain chains, several lakes and rivers were frozen, which are presently unfrozen etc. for example, the Thames river of England was frozen 4 times in the 16th century, 8 times in the 17th century, and 6 times in the 18th century. The historical records and Icelandic sagas very much indicate the severity of climate in Iceland. Extreme cold condition led to human deaths due to severe famines. It may be mentioned that glacial period (i.e. Little Ice Age) from 1450 to 1880 A.D. was not always characterized by continuous extreme cold condition and advancing glaciers rather it was punctuated by several cold and warm intervals. It is believed that each cold period continued at least for 30 years in each century (between 15th and 19th centuries) and two cold periods were separated by relatively warm period. The coldest period culmi- nated in the year 1816 when major parts of northern Europe and the USA did not experience summer season. The year 1816 is known as 'the year without a summer' in the climatic history, when the glaciers became most active after Pleistocene Ice Age, thereafter climate began to improve due to rise in temperature and by the end of 19th century the ice age was terminated.
The records of global temperatures since Industrial Revolution (1860) denote slow but irregaular trend of rise of temperature i.e. ranging between 0.3°C and 0.60. Following R.G. Barry and R.J. Chorley (2002) the phases of rise in tempera- tures after 1860 have been identified as follows:
(i) Highly irregular trend of rise in temperature ranging between 0.2°C to 0.4°C between 1860-1920 A.D.
(ii) Consistent trend of rise of temperature of mean value of 0.4°C between 1921 and 1945.
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(iii) Oscillating trend of rise of temperature between 1946 and 1975 A.D. around 0.4°C. The northern hemisphere was warmed while the temperature in the southern hemisphere remained more or less constant
i.e. neither increase nor decrease.
(iv) Overall warming trend continued between 1976 and 1989 A.D. when temperature registered an upward rise by 0.2°C. (v) The global rise of temperature reached its peak in the last decade of 20th century (i.e. 1990s). Six out of seven warmest years on record occurred since 1980.
It may be summarised that overall increase in the surface air temperature in the 20th century has been about 0.5°C to 0.7°C against an average rate of increase of 0.3°C per 1000 years at global level. According to another view the average air temperature in the northern hemisphere increased by 0.4°C between 1880 and 1940 because of rapid rate of combustion of fossil fuels during this period but the temperature dropped after 1940 inspite of continued rapid rate of combustion of fossil fuels due to fast industrial growth but soon after 1940 air temperature in the southern hemisphere showed rising trend which registered an overall increase of 0.6°C between 1940 and 1960.
Another source has indicated an increase in air temperature by 1.5°C upto 1995 while other sources have shown general air temperature rise ranging between 0.3°C and 0.6°C. It may be mentioned that a rise of 2°C temperature from the normal temperature was recorded in the Indian Ocean during 1997-98 which caused catastrophic coral bleaching leading to 70 per cent death of corals in the Andman Nicobar and Lakshwadeep inlands.
Various models have been developed to predict global rise in air temperature. S.H. Schneider (1950) pointed out that the temperature could rise upto 1.5°C to 3°C if the concentration of atmos- pheric carbon dioxide could be doubled from the 300 ppm (by volume) level to 600 ppm. General Circulation Model (GCM) developed by S. Manabe and R.T. Wetherald (1975) predicts that if the present amount of carbon dioxide (1975 level)
The
CLIMATIC CHANGE
of the atmosphere is doubled, the temperature of the earth's surface wil increase by 2.9°C.
The major sources and processes of global warming, for example, ozone depletion and greenhouse gases, will be discussed in the
succeeding chapter.
15.5 CAUSES AND THEORIES OF
CLIMATIC CHANGES
As mentioned earlier climatic changes are effected by changes in atmospheric circulation and interactions among five components of the earth- atmosphere system, namely atmosphere, hydro- sphere, lithosphere, biosphere, and cryosphere (frozen surface of the earth) wherein the amount of received solar energy, and the process of distribu- tion, redistribution, and absorption of solar radiant energy at the earth surface are important considera- tions of the state of climate of an area in specific time period. The causes for such interactions leading to climatic changes are related to (1) outside sources, say extraterrestrial sources, and (2) inside sources, say eath-atmosphere system or terrestrial sources
The causes and theories of climatic changes are viewed in terms of periodicity of climatic changes which are generally of two types, namely (1) short-term climatic changes, and (2) long-term climatic changes. Since the nature and patterns of climatic changes vary temporally and hence the causes of such changes are also of varied nature. This is why no single theory can explain all types and patterns of climatic changes and thus we have a host of causes and theories of climatic changes.
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The significant causes and related theories of climatic changes may be stated as follows:
(1) Solar irradiance (variation in solar radiation),
(2) Sunspot cycles,
(3) Astronomical theories (eccentricity of earth's orbit, obliquity of the ecliptic, precession of the equinoxes, earth-sun relationship) etc.,
(4) Atmospheric dust hypothesis (mainly volcanic eruptions and dusts thereof),
and
(5) Carbon dioxide hypothesis,
(6) Continental drift and pole wandering, (7) Tectonic and topographic control theory, (8) Oceanic variation hypothesis,
(9) Extra terrestrial bodies collision theory,
(10) Anthropogenic sources (changes in the earth's surface and atmospheric composition).
Solar Irradiance Theory
Solar radiative forcing is considered to be a significant factor of climatic changes. It is important to note that there are fluctuations in the energy radiated from the sun's outer surface (photosphere). It may also be mentioned that the amount of solar energy received at the earth's surface determines the nature and pattern of energy exchanges and atmospheric circulations which in turn determine temperatures and precipi- tation. The amount of solar radiant energy received at the earth's surface is also subject to changes due to (i) changes in the composition of the atmosphere in terms of its transparancy to incoming shortwave solar electromagnetic radiation waves, (2) changes in the relative distances between the sun and the earth, (3) the amount of the energy radiated from the earth's surface, (4) changes in the surface covers of the earth's surface etc. The variations in solar irradiance are viewed as (i) long-term change,
Since the Industrial Revolution (1860 A.D.) the man's increased economic activities and the application of advance technologies are introduc- ing significant modifications and changes in climatic conditions. This has led to the emergence of a new dimension in climatic changes and an additional source thereof. Thus, the sources of and (ii) as short-term or periodic change (i.e. climatic changes may be grouped in the following
3 broad categories:
(1) Outside of extra-terrestrial sources (2) Inside or terrestrial sources (3) Anthropogenic sources
sunspots cycles).
It is a common belief that increase in solar radiation for longer duration will cause warming of the atmosphere leading to onset of warm climate and melting of ice sheets and glaciers. It may be mentioned that regular measurement of tempera-
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tures of the sun's surface started at the Kitt Peak
National Observatory in Arizona, USA, from 1975. It is believed that even 0.1 per cent decrease in the
average annual solar radiation for a decade in continuation may introduce measurable climatic changes in terms of changes in temperature and precipitation. The recorded data of sun's surface temperature revealed a drop of temperature by 11°C in January, 1977. If the solar radiation drops by even one to two per cent, the temperature at the earth's surface in the middle and high latitudes may come down and may cause climatic changes similar to Little Ice Age which occurred from 1450 to 1880 A.D. The year 1816 was known as the year without summer in the USA. The computer model has further revealed that the drop in solar radiation by 2-3 per cent for 50 years in continuation may cause regeneration of glaciation and may reactivate older glaciers, and if the solar radiation decreases by 5 per cent or more, the earth may experience new ice age and widespread glaciation.
The expansion and contraction of the core of the sun has been accepted by a few scientists as basic cause of changes in the amount of energy radiated from the outer surface of the sun. According to E.J. Opik the core of the sun expands after long intervals. The sun consumes a portion of its energy to expand its outer surface to cope with the expansion of its core. In such circumstance the radiant solar energy decreases because of con- sumption of substantial portion of solar energy by the sun itself. The decrease in solar radiation results in lowering of atmospheric temperature of the earth and consequently cool phase of climate is introduced, which causes ice age. Conversely, when the core of the sun contracts, the internal consumption of solar energy is remarkably re- duced. Consequently, the solar radiation increases, which results in the increase of temperature of the earth's surface, termination of glacial period and beginning of interglacial period.
Simpson Theory
Contrary to the general view of the warming of the earth's surface and its atmosphere during the period of increased solar radiation, Sir George Simpson presented an entirely different concept related to variations in solar radiation and climatic
CLIMATOLOGY
changes in 1938 A.D. According to Simpson during the period of moderate increase in solar irradiance, the middle and high latitudes will experiencre extension in glaciers and their ad- vances while decrease in solar radiation and resultant decrease in air temperatue would cause melting of ice sheets and glaciers and their retreat, resulting into onset of interglacial period. Simpson propounded the concept of cyclic pattern of increase and decrease in solar radiation. According to him the atmospheric temperature increases with increase in solar radiation. The increase in atmos- pheric temperature causes increase in evaporation and cloudiness, strong meridional air circulation and increased precipitation in higher latitudes. The precipitation in higher latitudes is in the form of snowfall which allows more accumulation of ice and extension of ice sheets and glaciers. The greater cloudiness during summer season prohibits melt- ing of accumulated snow and ice rather protects the ice cover. On the other hand, during the phase of decrease in solar radiation, the atmospheric tem- perature decreases, meridional air circulation is weakened, evaporation and precipitation remark- ably decreases in high latitudes, melting of ice sheets causes their retreat. 'Thus, paradoxically a lowering of mean atmospheric temperature might cause a recession of ice sheets, whereas tempera- ture increase would lead to their advance. Although the Simpson theory appears not to fit recent instrumental evidence, it is a warning against oversimplified explanations of complex processes' (H.J. Critchfield, 2002).
Sunspot Theory
The sunspot activity has been related to variations in solar irradiance. The increased sun- spot activity (increase in the number of sunspots) causes warming of the earth's surface and its atmosphere whereas decreased sunspot activity (decrease in the number of sunspots) causes lowering of atmospheric temperature. Sunspots are darker and cooler areas in the photosphere of the sun. The increased sunspot activity means increase Fin the number of sunspots while decreased sunspot activity is related to decrease in the number of sunspots. The study of sunspot activity for the last 200 years has revealed cyclic pattern of increase and decrease in sunspot activity. On an average, 11
CLIMATIC CHANGE
years cycle has been accepted where as the period of one cycle may be as short as 8-9 years and as long
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powerful volcanic eruptions reduces the tempera- ture of the earth's surface at its lower atmosphere
and climatic conditions atleast at shorter temporal scale. It may be remembered that the stratospheric
temperature increases at the time of greater volcanic activity because most of the scattered, reflected and absorbed energy remain there, but the temperature of the lower troposphere and the earth's surface drops significantly, and it is the temperature of the lower atmosphere that controls weather and climate at the earth's surface. This corollary may be substantiated with a few examples of volcanic eruptions and resultant cooling of the earth's surface and its atmosphere.
as 16 years. Certain subcycles of sunspot activity at remarkably and introduces fluctuations in weather longer period have also been postulated e.g. 35 years cycle, 80 years cycle etc. It has been estimated that the output of ultraviolet radiation from the sun's surface at the time of maximum sunspot activity (maximum number of sunspots) is 20 times more than during the period of minimum sunspot activity. The prolonged period of mini- mum sunspot activity, called as Maunder Mini- mum, is supposed to introduce cooling of the earth's surface and its atmosphere, whereas the prolonged period of maximum sunspot activity may cause warming. It may be mentioned that perfect correlation between sunspot activity and atmospheric temperature has not been substanti- ated. It may be that periodic fluctuations in sunspot cycles may introduce some sort of weather changes at shorter temporal scale. 'Repeated studies trying to correlate rainfall with the fluctuation in sunspot cycles have not yet produced statistically signifi- cant results' (Oliver and Hidore, 2003).
Atmospheric Dust Hypothesis
The atmospheric solid particulate matters include dust particles, salt particles, pollen, smoke and soot, volcanic dusts and ashes etc. Most of the solid particles are kept in suspension in the atmosphere. It is an established fact that these solid particulate matters (SPM) present in the atmos-0 phere reduce the amount of solar radiant energy reaching the earth's surface by scattering, reflec- tion and absorption of incoming shortwave electro- magnetic solar radiation. About 23 per cent of incoming solar radiation is scattered by dust particles and haze, of which 6 per cent energy is sent back to the space while 17 per cent energy reaches the earth's surface as diffuse day light, of course much later. The scattering of incoming solar radiation waves by dust particles when the diameter of such particles is longer than the wavelengths of incoming solar radiation waves, is called diffuse reflection which sends some portion of incoming solar energy back to space while some portion remains in the lower atmosphere.
It is also an established fact that sudden increase in dust particles caused by violent
(1) There was annihilating violent explo- sive eruption of a volcano on 27 August, 1883 in Krakatoa island located in the Sunda Strait between Java and Sumatra in the East Indies. The powerful volcanic blast was 9TNT equivalent of about 100 million tons (2 x 10 pounds). Nearly 20 cubic kilometers (some sources put it 53 cubic kilometers) of fragmental materials, ashes and dusts were thrown in the air upto 32 kilometers in the atmosphere (upto middle of stratosphere), which were later on distributed, due to their fall, on an area of 700,000 square kilometers. The fine dusts were ejected upto 32 kilometers in the atmosphere which produced global decrease in solar radiation re- ceived at the earth's surface by 10-20 per cent, there was total darkness in the sky because the dusts and ashes blotted out the sun for several days, the effect of total darkness was observed upto the distance of 150 kilometers from the center of eruption. The ejection of fine dusts and ashes in the stratosphere and their circulation and drifting around the earth by upper air atmospheric circulation produced brilliant red sunsets for several years. The reduced solar radiation received on the earth's surface and resultant drop in temperature matches with cold years from 1884 to 1886.
(2) The violent eruption of Mt. Asoma in 1783 in Japan is correlated with severe cold years of 1784, 1785 and 1786. The exceptionally cold year of 1816 A.D., known as a year without summer in the climatic history of the world, followed the famous explosive eruption of Mt. Tamboro in Dutch East Indes in the year 1815. The
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volume of dusts ejected from the violent eruption of this volcano was so enormous that thick dust veil covered the sun resulting into complete darkness for 3 days in continuation, which extended upto a distance of 500 kilometers from the center of
eruption.
(3) The explosive eruption of Mt. Katmai in 1912 in Alaska (USA) ejected about 21 cubic kilometers of volcanic materials and dusts in the atmosphere resulting into 20 per cent reduction in the amount of solar radiation received at the earth's surface. About 2 percent drop in solar radiation was noted at the Mauna Loa Observatory in Hawaii at the time of the eruption of Mt Agung in Bali in the year 1963.
Contrary to the general belief of correlation between major volcanic eruptions and lowering of temperatures, the empirical studies of a few recent volcanic eruptions such as El Chichon eruption of 1982 in Mexico, Mt. St. Helens eruption of 1980 (USA) a few earlier explosive eruptions such as Mount Cosequina eruption of 1835 in Nicaragua show no such correlation. It may be mentioned that the impact of volcanic eruptions in lowering the temperature depends on a variety of factors, numely the penetration of the stratosphere by ejected volcanic dusts and gases, the volume of sulphur dioxide, the amount of dusts etc. If the eruption is very explosive and powerful, the volume of ejected sulphur dioxide is very large, and the amounts of dusts are very high, definitely these materials will reach the stratosphere and will reduce the temperature at the earth's surface and its atmosphere. The ejected sulphur dioxide gas after combining with atmospheric water vapour forms tiny droplets of sulpluric acids. These tiny sulphu- ric acid droplets remain in the atmosphere for longer period and reflect sizeable portion of incoming solar radiation and thus the amount of solar radiation received at the earth's surface is remarkably reduced resulting into the lowering of earth's surface temperature.
Recently, two indices related to volcanic eruption and its impact on climate change, namely (1) Volcanic Explosive Index (VEI) and (2) Dust Veil Index (DVI), have been prepared. It is argued that high VEI would indicate powerful and
CLIMATOLOGY
effective penetration of the stratosphere by vol- canic dusts and gases and thereby would cause lowering of temperature. Similarly, high DVI would be indicative of reduction in solar radiation received at the earth's surface and consequent drop in temperature. It may be mentioned that resident period of volcanic dusts and gases is very important factor for glacial climate. If the resident period of volcanic materials is for longer duration i.e. if the volcanic materials remain in the atmosphere for longer period, the resultant lower- ing of temperature may initiate glacial period. On the other hand, short-term resident period would cause only local effects on weather and climate.
Carbon Dioxide Theory
It is important to note that it is the receipt of solar energy at the earth's surface and absorption of incoming solar radiation and outgoing terrestrial radiation by the atmosphere which has significant control on weather and climate, and the amount of energy received at the earth's surface depends on (1) changes in the composition of the atmosphere, (2) changes in the transparency of the atmosphere, (3) modification of energy in the transit (i.e. flowing through the atmosphere) etc. The changes in the gaseous composition of the atmosphere are effected by both natural and anthropogenic sources. The increase of relative proportion of greenhouse gases (e.g. carbon dioxide, methane, nitrogen oxides etc.) in general and carbon dioxide in particular causes global warming and initiates warm climate while decrease in their relative percentage causes global cooling and helps in initiating cold climate if other factors remain constant. Thus, the carbon dioxide theory states that increase and decrease in temperatures of the earth's surface and its atmosphere is effected by increase and decrease of its (CO2) relative percent- age in the gaseous composition of the atmosphere respectively.
It may be remembered that the increase of trial infrared radiation by certain gases (mainly earth's temperature by absorbing outgoing terres- carbon dioxide) is called greenhouse phase of the atmosphere whereas icehouse phase refers to lowering of earth's temperature leading to begin- ning of glacial period. The greenhouse effect
CLIMATIC CHANGE
(Oxford
means 'progressive warming-up of the earth's surface due to the blanketing effect of man-made carbon dioxide in the atmosphere' Dictionary). 'In a green house, visible sunlight passes through the glass and heats up the soil warming the plants. The warm soils emit radiation in longer wavelengths particularly in the infrared band. Because the glass is opaque to these wavelengths (long wavelengths of infrared radia- tion waves), it absorbs and reflects (reradiates back to the soils) the infrared (radiation)' (D.B. Botkin and E.A. Keller, 1982). This mechanism keeps the greenhouse warmer than the outside environment. In nut shell it may be summarized that a greenhouse is a body which allows the shortwave incoming solar radiation to come in but does not allow the longwave outgoing terrestrial infrared radiation to escape. Carbon dioxide and water vapour act as greenhouse in that these allow visible sunlight to reach the surface of the earth but absorb and reflect back (reradiate) the longwave outgoing terrestrial radiation mainly infrared radiation (back to the earth's surface) and thus help in keeping the earth's surface warmer.
The most significant greenhouse gas is carbon dioxide which is released to the atmosphere by burning of fossil fuels (coal, mineral oil and natural gas) for different purposes in various ways, burning of firewoods etc. Deforestation also helps in increasing the concentration of carbon dioxide in the atmosphere. The pre-industrial level of atmospheric content of carbon dioxide was fixed at 280-290 ppmv (part per million by volume) or 0.028 per cent to 0.029 per cent (the base year of the beginning of industrial revoltuion in 1860A.D.) It is also important to note that there are certain natural processes and sources which regulate the atmospheric concentration of carbon dioxide, namely vegetation covers and oceans are major sinks(absorbers and users) of atmospheric carbon dioxide. Deforestation and burning of forests for shilfting cultivation remarkably reduces the use of carbon dioxide by vegetation covers and hence helps in increasing the concentration of carbon
dioxide in the atmosphere.
The atmospheric content of carbon dioxide increased from the pre-industrial level of 280-290 ppmv to 350-360 ppmv during 1998, registering an
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overall increase by 25 per cent from the pre- industrial level. It is believed that the rate of increase of atmospheric carbon dioxide through anthropogenic sources will be accelerated due to uncheked industrial development and increasing urbanization worldover.
Different models have been developed to reveal the relationships between the increase in the concentration of atmospheric carbon dioxide through anthropogenic sources and climate changes but the predictions of these models are very much confus- ing and contradictory. 'If there is uncertainty in the prediction of carbon dioxide trends, then the predictions of the resulting climatic effects are even more uncertain' (J. E. Hobbs, 1980).
(1) Schneider Model (1950); S.H. Schneider while reviewing the results of various climatic models dealing with the predictions in the change of thermal conditions of the atmosphere and the earth's surface resulting from the increased content of atmospheric carbon dioxide concluded that the temperature could rise upto 1.5C-3.0°C if the concentration of atmospheric carbon dioxide could be doubled from 300 ppmv level to 600 ppmv. He further predicts that increased tempera- ture would cause increase in evaporation and cloudiness which would reduce incoming solar radiation (because of increased albedo i.e. more clouds would reflect more solar radiation back to space). Thus, the reduced solar radiation reaching the earth's surface would counteract the warming of the earth's surface. It is obvious that such feedback mechanisms negate the impact of green- house effect of increased atmospheric carbon dioxide and the whole process of the heating or cooling of the lower atmosphere and the earth's surface becomes highly complicated. Another model envisages cooling of the earth's surface due to enormous increase in the atmospheric carbon dioxide.
(2), General Circulation Model (GCM): The general circulation model by S. Manabe and R.T. Wetherald (1975) predicts that if present amount (1975 level) of carbon dioxide of the atmosphere is doubled, the temperature of the earth's surface will increase by 2.9°C and there will be 7 per cent increase in the activity of hydrological cycle but there will not be any feedback and thus there will
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not be either increase or decrease in the amount of cloudiness as predicted by S. H. Shneider. In other words, the increase in surface temperature caused by increase in the atmospheric carbon dioxide will not be negated by feedback mechanism and hence increased greenhouse effect would certainly intro- duce climate change (warming of the earth's surface and its lower atmosphere).
(3) Atmospheric-Oceanic General Circula- tion Model (AOGCM): As per the report of the Intergovernmental Panel on Climate Change (IPCC, 2001) if the concentration of atmospheric CO2 increases to 540-970 ppmv by the end of the 21st century, the average surface air temperature at global level would register an increase between 1°C to 5.8°C. This increase in average air temperature has been estimated in relation to average air temperature during 1990-2000 A.D.
The trends of probable future climatic change due to increase in average surface air temperature at global level have been estimated on the basis of increase in the concentration of greenhouse gases in the atmosphere in future.
It has been estimated that the overall increase in surface temperature over the past one hundred years (upto 2000 A.D.) has been about 0.5°C to 0.7°C. According to another view the average air temperature in the northern hemisphere increased by 0.4°C between 1880 and 1940 because of rapid rate of combustion of fossil fuels during that long period but the temperature dropped after 1950 inspite of continued rapid rate of combustion of fossil fuels due to fast industrial development after 1960. The air temperature in the southern hemisphere showed rising trend which registered an overall increase of 0.6°C between 1940 and 1960. It is evident that though there is maximum consumption of fossil fuels in the northern hemisphere and consequent increase in the concen- tration of atmospheric carbon dioxide but the air temperature dropped whereas there is minimum consumption of fossil fuels in the southern hemisphere but air temperature increased. It does not mean that there is no direct impact of increasing carbon dioxide on air temperature rather some other factors might have dominated over the factor of greenhouse effect. It is opined that large amount
HOMA CLIMATOLOGY
of volcanic dusts thrown into the atmosphere through the eruptions of Heckla (Iceland) in 1947, Mt. Spurr (Alaska, USA) in 1953, Agung (Bali) in 1963, Mount Taal (Philippinse) in 1965, Mayon and Fernandina in 1968 etc. might have reduced air temperature in the northern hemisphere between 1940 and 1970.
Inspite of contrasting opinions about the impact of increasing concentration of atmospheric carbon dioxide due to greenhouse effect on air temperature it may be conclusively opined that there is definite positive effect of increased greenhouse effect due to increased concentration of carbon dioxide in the atmosphere. The increase in temperature introduces several changes in climatic conditions at local, regional, and global levels as fallows:
abixoib nodis ogasa
OM (1) According to A.B. Pittock (1972) the change in global mean temperature by a few degrees celcious caused by greenhouse effect would greatly change climate.
(2) Increased temperature due to increased greenhouse effect would cause decrease in precipi- tation and soil moisture content.
(3) In case of global warming oceans would be required to absorb more and more carbondioxide, this will raise normal level of acidity of the oceans, which would decrease biological activity in the oceans and oceanic plant cover, which ultimately would alter the albedo of ocean surface.
(4) It may be possible that the carbon dioxide concentration in the atmosphere may increase to such an extent that the total atmospheric pressure would increase. Such increase in atmospheric pressure would broaden the absorption bands and increase the opacity of the atmosphere to the outgoing terrestrial radiation which would in turn increase the surface temperature to such an extent that all the atmospheric processes may come to grinding halt.
(5) Recently, atmospheric black clouds (ABC), as a result of concentration of pollutants in the atmosphere due to burning of fossil fuels, have been related to unprecedented snowfall in Dubai, heavy snowfall in the lower altitudinal areas of Jammu and Kashmir, Himachal Pradesh and
CLIMATIC CHANGE
Uttaranchal Himalayas (India) etc. during Decem- ber (2004) and January (2005). Continental Drift and Polewandering
The plate tectonic theory, based on the evidences of palaeomagnetism and seafloor spreading, the result of post-1950 advancement in geophysi- cal researches worldover, has validated the concept of continental drift. It has now been proved that different plates are in constant motion and hence the continents and ocean basins change their relative locations. The plate tectonics have two major implications, namely changes in relative positions of the continents (and ocean basins) due to continental displacement (drift), and origin of mountains of varying heights due to convergence of destructive plate boundaries (the effects of the later on climate will be discussed under the next heading). It is believed that the relative locations of continents in relation to the poles have paramount control on climatic changes. The clustering of continental masses around the pole causes glacia- tion of major land areas while scattering of continents away from the poles causes deglaciation of the land areas which are places at greater distances from the poles, as 'the primary require- ment for the formation of great ice caps is the polar location of continents' (J.E. Oliver and J.J. Hidore, 2003).
A lot of convincing evidences are available for two great ice ages e.g. Permo-Carboniferous ice age, and Pleistocene ice age. It is believed that about 350-250 million years B.P. (before present) i.e. during Carboniferous-Permian periods all the landmasses were united in one landmass which has been named as Pangaea II. Most of the land areas of Gondwanaland (comprising present South America, Africa, India, Australia and Antarctica) clustered around south pole which was located near the present position of Durban in Natal (South Africa). Thus, the south pole was located almost in the middle of Pangaea. Consequently, ice sheets might
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have parted away due to disruption of Pangaea and consequent movement to present locations because of plate movements, and finally might have experienced deglaciation and termination of Permo- Carboniferous ice age.
The Pleistocene glaciation of northern land areas of the northern hemisphere is supposed to have occurrred around one million years B.P. due to closer location of North American and Eurasian landmasses to north pole. The Pangaea began to break during early Jurassic about 180 million years B.P. and the disruption was completed by 70-100 million years B.P. Consequently, the northern part of North America upto present Nebraska in the south (in USA), Greenland, Iceland, Europe and Siberia were covered with ice sheets during Pleistocene period (about one million years B.P.). As mentioned in the preceding section the Pleistocene Ice Age comprised of four glacial periods separated by four interglacial periods of warmer climate. It may be mentioned that interglacial periods within a comprehensive ice age cannot be explained on the basis of continental drift and plate tectonics. It may also be remembered that glaciers of the Rockies, Alps, Himalayas etc. were more extensive than their present position. The glaciation of high mountains, which were far away from the location of pole also cannot be explained on the basis of plate tectonics and continental drift.
Tectonism and Topographic Controls
It is a commonly agreed fact that topo- graphic factor (relief) plays an important role in shaping weather and climate at regional and global levels. High mountains control temperature and upper air circulation patterns. This is why efforts have been made to correlate ice ages with active tectonism and mountain building, 'as with increas- ing height of landmasses, the potential for ice formation is greatly increased' (Oliver and Hidore, 2003). The advocates of tectonic theory of ice ages and glaciation have tried to demonstrate close
and Pleistocene ice age and glaciation with large- scale vertical tectonic movements and orogenesis (mountain building) as these two great periods of ice age and widespread glaciation preceded large- scale mountain building activities. In other words,
have spread out from south pole at the time of relationship between Permo-Carboniferous ice age glaciation and Brazil, southern South America upto Falkland, southern Africa, peninsular India, Australia, Antarctica etc., which were closer to south pole, might have been covered with thick ice sheets. At much later date, these land masses might
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the Caledonian mountain building was followed by Permo-Carboniferous ice age and late Cenozoic mountain building, leading to the formation of Alpine-Himalayan mountain chains, North Ameri- can cordillera, upliftment of Tebatan plateau, western North America including Great Plains etc, was followed by Pliocene-Pleistocene ice age when major parts of North America and Eurasia were glaciated. Rudiman and Kutzbach (1989) have also demonstrated such relationships through computer models wherein they used the models with 'no mountains', 'half mountains', and 'full mountains' to predict the patterns of general atmospheric circulation and their impact on producing cool climate to introduce glaciation. The computer models based on 'half mountains' and 'full mountains' during late Cenozoic period predicted colder climate over North America, North-Western Europe, and northern Asia during Pliocene-Pleistocene periods and likely ice age and glaciation. The actual occurrence of Pleistocene ice age after late Cenozoic mountain building validates the prediction of computer models, as 'in general 2 to 4 km of late Cenozoic surface uplift in Tibet and the Himalayan mountains, and at least one kilometer uplift across a broad area of the western North America, including the Sierra Nevada, Basin and Range province, Colorado Pleateau, Rocky Mountains, and the western Great Plains, successfully pro- duced changes in the model climates that are comparable to, although less extreme than, the inferred actual climate changes of the late Cenozoic Era' (A.L. Bloom, 2002).
Vertical tectonic movements leading to origin of mountains and their further uplift results in high reliefs which augment erosional and weathering processes which in turn affect atmos- pheric carbon dioxide. It has already been stated earlier that decreased content of atmospheric 1 carbon dioxide cools the earth's surface and its lower atmosphere to such an extent that glacial climate is induced. The weathering of rocks has been correlated with climatic cooling. In other words, chemical weathering requires dissolution of atmospheric carbon dioxide. The increased rate of chemical weathering results in decrease in the concentration of atmospheric carbon dioxide and resultant reduction in greenhouse effect, with the
CLIMATOLOGY
result cooling of the earth's surface and its lower atmosphere induces glaciation.
Astronomical Theories
The astromical theories are related to three variables, namely (1) earth's orbital eccentricity, (2) obliquity (tilt) of the earth's rotational axis relative to the plane of earth's orbit, and (3) precession of equinoxes, which determine the amount of solar radiation received at the earth's surface and its temporal variations. The earth's orbital eccentricity refers to a ratio between the elliptical orbit of the earth around the sun and a true circle. It may be remembered that the earth revolves around the sun in an elliptical orbit, thus, the eccentricity of the earth's orbit is derived by comparing the orbital elliptical path of the earth to theoretical circular path.
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(1) The eccentricity of the earth's elliptical orbit simply means deviation of its elliptical orbital path from true circular path. The present position of earth's orbital path has been shown to be closer to circular path. When the earth's orbit is nearest to a circle, the earth's eccentricity becomes almost zero. It has been demonstrated through measurement for the past million years on the basis of inferred data that variations in the earth's eccentricity take place in cyclic manner. The variations in eccentricity during the past 1,000,000 years have changed between a minimum value of 0.001 to maximum value of 0.054. It takes about 95,000 years for the earth's eccentricity to attain its maximum value of 0.054. This is called 95,000 years cycle or Milutin Milankovitch cycle (after the name of Serbian scientist M. Milankovitch). The temporal variations in the earth's eccentricity influence the amount of solar radiation to be received by the earth's surface and also determine the temporal variations in solstices and equinoxes. It has been estimated that the maximum variation in the earth's eccentricity (0.054) 'results in a maximum variation of the incoming annual radia- tion of only 0.2 per cent of the total over a cycle of about 95,000 years' (Berger, 1988, quoted in A.L. Bloom, 2002).
(2) The obliquity of the earth's rotational axis or tilt of the earth's rotational axis 'refers to the angle of the axis (of the earth's rotation) in relation
CLIMATIC CHANGE
to the plane in which the earth revolves around the sun' (Oliver and Hidore, 2003). The obliquity or between 220 and 24° 27'. The net angular variation tilt of the earth's rotational axis varies temporally is 1.50 which oscillates around mean obliquity value of 23.10. The present angle of the earth's rotational axis is 66.50 giving an obliquity of 23.50 i.e. the tilt angle of the earth's rotational axis with respect to the plane of its orbital path is 23.5 degree. The significance of the obliquity factor lies in the fact that it controls the latitudinal distribution of solar radiant energy and the intensity and duration of different seasons. It may be mentioned that if the obliquity angle is zero (i.e. if the rotational axis of the earth is perpendicular to its plane of orbital circle) the length of day, and night would be of equal duration throughout the year, there would be no seasonality i.e. the same season would prevail throughout the year, and the horizontal distribution of climatic zones would be static but such situation is not possible because no such evidences could be found in the climatic history of the earth. It has been demonstrated through computer models, and manual calculation by Milutin Milankovitch that there have been angular variations in the earth's obliq- uity in the past and such variations, though small (maximum value of 1.50), are sufficient enough to induce changes in the climatic conditions and its world distribution. The change of the earth's obliquity from the minimum angle value of 22° to the maximum value of 24o 27' takes about 41,000 years. Thus, the complete change in the earth's obliquity occurs in cyclic manner and one cycle is completed in a period of 41,000 years. There is direct relationship between the amount of angular variation of the earth's obliquity and seasonal contrasts i.e. difference of temperature in summer and winter seasons. The smaller the changes in the inclination of the earth's rotational axis (obliq- uity), the smaller is the change of temperature between summer and winter seasons and vice versa. It may be mentioned that warmer winters induce more snowfall and accumulation of ice due to increased evaporation and resultant condensa- tion, while cool summers taboo melting of ice sheets. Thus, it is apparent that if the temperature difference between summer and winter seasons is minimised and if such condition persists for longer duration, the climate would be cold enough to
induce glacial period.
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(3) Precession of equinoxes: The dictionary (Webster) meaning of precession denotes 'slow, conical motion of the earth's axis of rotation, caused by the gravitational attraction of the sun and the moon, and to a smaller extent, of the planets, on the equatorial bulge of the earth,' while the precession of equinoxes 'refers to earlier occur- rences of the equinoxes in each successive sidereal year because of the slow retrograde motion of the equinoctial points along the ecliptic, caused by the precession of the earth's axis of rotation' (Webster Dictionary). In more simple words, the precession of equinoxes may be defined as the time of a year or say the days of the year on which the earth's position is nearest to the sun (perihelion) or farthest to the sun (aphelion) due to varying motions of the earth. Thus, the precession of equinoxes denotes 'the regular change in time when the earth is at given distance from the sun.' 'Climatic precession is a complex variable with principal periods of 23,000 and 19,000 years. The climatic precessional parameter refers to the time of year when the earth is at perihelion, or closet to the sun, which is now in early January (more precisely 3 January). Climatic precession controls the difference in the length of the seasons and has an opposite effect on each hemisphere' (A.L. Bloom, 2002). In other words, at the time of perihelion position of the earth during northern hemispheric winter, the winters in the northern hemisphere become much longer and 7 per cent more warmer while in the southern hemisphere the summers become much longer and 7 per cent less
warm.
Presently, the earth's perihelion position with respect to its distance from the sun is in the northern hemispheric winter. The astronomical calculation denotes that this position will be reversed after about 10,000 years from present i.e. the perihelion position of the earth will change to northern hemisphere summer season, with the result winter season will be more extreme and cold due to receipt of less amount of solar radiant energy. The scientists have made successful attempts in computing the dates of occurrences of perihelion and aphelion positions in the past and
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have tried to reconstruct palaeoclimates on this
basis.
Milutin Milankovitch, a Serbian scientist, manually calculated the dates of occurrences of perihelion and aphelion positions of the earth for the past thousands of years (600,000 years) and presented a mathematical model based on the
aforesaid three astronomical variables (e.g. eccen- tricity of the earth's elliptical orbit, obliquity including of the earth's rotational axis, and precession of equinoxes) to explain the advance- ment (expansion) and retreat (contraction) of ice sheets during Pleistocene Ice Age. His model or theory is popularly known as 'Milankovitch Theory' or 'orbital variation theory.' The earth's orbital variations are known as 'Milankovitch cycles'. The salient features of Milankovitch theory are stated as follows:
(i) The change of obliquity angle of the earth's rotational axis from 22.10 to 240 27' (say 24.50) is completed in a cycle of 41,000 years. Such variations in the inclination of the earth's rotational axis affect the amount of radiant solar energy to be received at the earth surface, temperature distribu- tion, general atmospheric circulation and seasons.
(ii) The change in the eccentricity of the earth's orbit from minimum value of 0.001 to maximum value of 0.054 takes place in a cycle of 95,000 years. Such variation causes variations in the distance of the earth from the sun which in turn affects the amount of insolation and temperature distribution on the earth's surface.
(iii) The precession of equinoxes in terms of the perihelion position of the earth with respect to the sun is completed in about one-half cycle of 11,000 years duration. It is believed that 'in about one-half cycle or 11,000 years from now, the northern hemisphere winter season will be cooler, and more than 23 days longer than the summer season, because the northern hemisphere winter will occur while the earth moves more slowly from the sun' (Berger, 1978, quoted by A.L. Bloom, 2002). The above view is based on the calculation of past events that about 10,500 years before present the northern hemisphere winter was at the time when the earth was farthest (aphelion position of the earth) from the sun.
CLIMATOLOGY
According to Milankovitch orbital theory
cool summers and relatively warmer winters in the latitudinal zone of 500 to 700 in the northern hemisphere are prerequisite conditions to induce cold phase of the climate leading to continental glaciation because cool summers prevent melting of annual accumulation of snow while relatively warmer winters induce more evaporation and condensation leading to more snowfall and accu- mulation of ice sheets.
It may be concluded that the eccentricity of the earth's orbit affects the receipt of insolation at the earth's surface while the obliquity of the earth's rotational axis and precession of equinoxes deter- mine the horizontal (latitudinal) distribution of temperature over the earth's surface and length and intensity of summer and winter seasons in terms of temperature. All such changes and variations induce climatic changes.
It has been argued by the critics of Milankovitch orbital theory of climatic changes that 'the ampli- tude of the fluctuations of solar energy inferred by Milankovitch theory are inadequate to produce the documented climatic changes of ice ages' (A.L. Bloom, 2002). The advocates of this theory are of the opinions that the aforesaid three astronomical variables, known as Milankovitch orbital vari- ables, have operated throughout past geological periods and may be effective in explaining climatic fluctuations at least during ice ages with minor corrections and improvements. They argue that 'other tectonic, atmospheric, oceanographic, and biological changes of the late Cenozoic Era, also created an environment in which the relatively weak orbital parameters could be expressed as major climate changes' (A.L. Bloom) and may act as climatic pacemaker.
15.6 IMPORTANT DEFINITIONS
Atmospheric black clouds (ABC): simply means formation of layers of pollutants in the atmosphere due to burning of fossil fuels (coal, petroleum, natural gas etc.). Climochronology: Climochronology may be de- fined as a systematic description of climatic conditions and climatic changes in geological history of the earth.
CLIMATIC CHANGE
Climatic precession: 'Climatic precession is a complex variable with principal periods of 23,000 and 19,000 years. Climatic precessional parameter refers to the time of year when the earth is at perihelion or closet to the sun, which is now in early January (more precisely January, 3)' (A.L. Bloom, 2002). Duricrusts: Duricrusts are indurated hardened surfaces of different kinds such as laterites, silcretes, calcretes, alcretes, ferricretes
etc.
Eccentricity: of the earth's elliptical orbit simply means deviation of its elliptical orbital path
from true circular path.
Evaporites: Evaporites are the deposits repre- sented by salt deposits in warm and arid climatic conditions when evaporation exceeds precipitation.
Glaciology: The science dealing with glaciers and
glaciation is called glaciology. Greenhouse effect: Greenhouse effect means 'progressive warming-up of the earth's surface due to blanketing effect of carbon dioxide in the atmosphere.'
Greenhouse phase: Greenhouse phase means increase in earth's temperature by the absorp- tion of outgoing terrestrial infrared radiation by certain gases i.e. greenhouse gases, mainly carbon dioxide.
Icehouse phase: The icehouse phase refers to lowering of earth's temperature leading to beginning of glacial period.
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Maunder minimum: The prolonged period of minimum sunspot activity is called Maunder
minimum.
Milankovitch cycle: The earth's orbital variations
are known as Milankovitch cycles. Palynology: The science of the study of pollen
grains and spores of plants is called palynology. Obliquity of the earth's rotational axis, or inclination of the earth's rotational axis 'refers to the angle of the axis in relation to the plane in which the earth revolves around the sun.'
Precession of equinoxes: may be defined as the time of the year or say the days of the year on
which the earth's position is nearest to the sun (perihelion position of the earth) or farthest to the sun (aphelion position) due to varying motions of the earth.
Proxy data: Proxy data refer to inferred data as
substitutes for real data.
Varves: Varves are the alternate sequences of
layers of fine silts and clays deposited at the floors of lakes and large ponds in such an area which is characterized by alternate freezing and thawing during winter and summer sea- sons respectively.
Year without summer: The year 1816 is known as the year without summer in the climatic history of the earth when the glaciers became most active after Pleistocene ice age due to extreme cold winter and cool summer mainly in North America.
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