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Climate is an average weather condition for a long period. A minimum of 25 years have to be considered for reliable statistical determination of the characteristics of a climate. Climate is never static; it is subject to fluctuations. The term, ‘climate change’, is defined as the climatic variations in historical time over the span of a few thousand years. The study of climate changes assumes importance in the context of the fact that all living beings have to adapt themselves to changes in climate.
Reconstruction of Past Climates:
Morphological Indicators:
These include glacial features such as erratics; denudational features such as striations, cirques, arete, tarn, etc.; fluvial features such as river terraces; aeolian features such as dunes, inselbergs; periglacial features such as pluvial lakes.
The glacial features indicate the former snowline whereas inselbergs indicate the presence of dry conditions in the historical past. Animal footprints and marine fossils found in deserts are valuable in analysing the past climate of a particular place. The fall in sea level during the past ages can be detected by the study of river terraces.
Lithogenic Indicators:
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These include varves, evaporites, weathering processes, especially laterisation and resultant landforms, limestone deposits and coal deposits. Laterites are formed when wet and dry climates alternate. Redbeds prove the presence of tropical soil/climate in the past. Similarly, coal deposits indicate the past existence of forests in the area under study.
The varves reveal the most detailed chronology for the past thousands of years. Varves are the annual layers of silt and sediment accumulated on the beds of lakes and ponds. During winter, only the fine suspended clay is deposited on the lake bottom even when the lake is frozen. At the onset of summer, fresh water and coarse sediments are introduced in the lakes.
Therefore, no two successive layers of deposits have similar thickness. A comparison of varves in areas once under glaciers suggests the dates of ice removal. The varve records of Scandinavia reveal that the last glacial recession took place about 13,700 years ago.
Isotopic Evidences:
The oxygen isotope analysis technique was discovered by Harold G. Urey. It determines the ratio between Oxygen18 and Oxygen16 isotopes found in the water (H2O) molecules by using mass spectrometry. When temperatures of atmosphere remain high, more of heavier Oxygen18 isotope evaporates from the ocean and, subsequently, precipitates on ice caps. During cold periods, however, more of the lighter Oxygen16 isotope is easily evaporated and locked in ice caps, leaving the oceans rich in Oxygen18 atoms.
So, the ocean undergoes a process of compensating change in the ratio of Oxygen18 and Oxygen16 isotopes. The shells of micro-organisms in oceans contain calcium carbonate (CaCO3) which reflects the ratio of the two isotopes. It is thus possible to obtain a reliable record of past temperatures and periods of glaciation from analysing deep-sea organic sediments.
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The application of the isotope method of palaeotemperature analysis in the ice fields of Greenland and Antarctica has produced temperature fluctuation data for approximately the last 100,000 years. The oxygen isotope analysis technique, which can give a continuous temperature record for more than a million years, has played a very important role in research done by CLIMAP (Climate, Long-range Investigation, Mapping and Prediction).
CLIMAP endeavours to obtain fundamental knowledge of climatic variations over the past million years. This method, however, can merely help to estimate temperatures; it cannot provide a direct basis for dating climatic change. Earth-plating is accomplished on the basis of rate of decay of radioactive isotopes in geological and deep-sea deposits.
Fossil Evidence:
For estimates of temperature and precipitation from fossils, one has to assume that a given species required the same environmental conditions to survive in the past as they do today. Fossils of flora and fauna, and bones of arctic mammals have been discovered far south of their present range. Fossils of desert and steppe animals have been unearthed in the temperate countries of Western Europe. This probably indicates that climate has changed diametrically over time in various regions.
Floral Evidence:
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The spatial distribution of plants provides important clues regarding the presence of different types of .climates in the past. A great variety of fossil records proves the existence of a warm climate. The climate of the Carboniferous Period can be interpreted by the presence of large seams of coal in different parts of the world—the coal being related to the prolific vegetation that went into making it.
Pollen Analysis Pollens are fine substances discharged from the anther of flowers. Pollens are blown away by wind and water and sometimes deposited at the bottom of seas or lakes. A comparison of pollen deposits in different layers of sedimentary rock with those of modern pollens may help us to interpret past climates. Such analyses accomplished with the help of pollen studies are known as palynology.
Dendrochronology and Lichenometry:
Dendrology studies past climates by the analysis of tree rings. In regions experiencing regular seasonal changes of weather, trees exhibit growth of one ring per year. The ring may be very thick if, for instance, rainfall conditions are optimum for the growth of trees. On the other hand, in dry conditions such rings become narrow.
The light coloured part of each ring suggests growth during the spring and early summer whereas the darker coloured part indicates slow growth at the end of summer. Records contained in tree rings date back to more than 3000 years in living trees and another 5000 years or more in the case of fossil woods. Studies done by the laboratory of Tree- Ring Research, University of Arizona, reveal that tree-ring widths indicate temperature, pressure and atmospheric circulation patterns which may help scientists to analyse past climatic fluctuations.
A similar technique called lichenometry reveals the date of recession of glaciers with reference to the size of lichens, certain species of which grow constantly in a year.
Flood Records:
Records of floods that occurred in the past provide evidence useful for research in climatic changes, the study of the Nile floods from 640 AD to 1921 AD showed that floods of low intensity occurred in 930-1070 AD and 1100-1350 AD.
Drought Records:
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Past records related to drought give evidence of climatic fluctuations. The abandoned human habitats in Chp.co Canyon and Mesa Verde, for example, indicate that people had to evacuate the area due to the occurrence of drought.
Migrations:
People tend to migrate from drier to wetter regions. The flow of migrations can be studied for evidence of climatic changes.
Contemporary Literature:
Literature mentions climatic events like unusual freezing of the Tiber river in the ninth century and ice formation on the Nile. Such literary evidences are extremely valuable 7or studying climatic changes.
Agricultural Records:
Some crops like wheat produced in temperate countries are extremely sensitive to temperature fluctuations, so their crop histories provide information on past climates.
Instruments:
The climatic variables are accurately measured with the help of instruments. With the invention of the thermometer by Galileo in 1593 and the barometer by Torricelli in 1643, climatic observations became more accurate. Some valuable weather data were recorded in Europe in 1649 after which the number of weather stations increased.
The invention of telegraph revolutionised the process of collection, observation and analysis of weather data. Nowadays, the use of satellites and supercomputers for data interpretation has made the entire process of weather observation more efficient and precise. It has made the task of future scientists relatively easier.
Climate during Geological Past:
The climate record is fragmentary for 90 per cent of the earth’s lifetime. In order to interpret climatic change which took place 2600 million years ago, we need to take the help of computer-simulated models, because direct evidence from rocks does not go back beyond-2800 million years.
The early Proterozoic was characterised by Huronian glaciation which occurred around 2700 to 1800 million years ago. This was followed by a warm period until about 950 million years ago. During the late Precambrian period, the glacial epochs;—the Gnesjo (940 million years ago), the Strutian (770 million years ago), and the Varangian (615 million years ago)—occurred, each lasting for about 100 million years.
The Phanerozoic Eon:
The Phanerozoic Eon includes the Palaeozoic, the Mesozoic and the Cenozoic Eras. Palaeozoic climates (570 to 225 million years ago) were warmer than the Cambrian and Ordovician periods, interspersed as the latter were by brief glacial events. The Silurian period was warm and with the advent of the Devonian period, North America, Europe, China and Australia experienced tropical climate. Much cooler climate prevailed during the Carboniferous period and by the Permian period the great deserts started expanding.
During the Mesozoic Era, climate was warm, and temperature fluctuated from 10°C to 20°C at the poles and from 25°C to 30°C at the equator. In the middle of the Triassic period, the climate was hotter than today. The Jurassic Period witnessed a slight drop in temperature.
For analysing the Cretaceous Period, oxygen isotope analysis of deep sea cores using marine fauna is important. The end of the Cretaceous Period is characterised by global cooling.
The Cenozbic Era was a time of continuous cooling. At the start of the Eocene Epoch (55 million years ago), there was a long-term cooling. The early Tertiary Period was much warmer than today.
There was a great advance of massive glaciers during the Quaternary period of the Cenozoic Era. The Quaternary period is divided into the Pleistocene Epoch and the Holocene Epoch. The Pleistocene is characterised by ice ages. (The present ice sheets in different parts of the world are remnants of the Pleistocene glaciation.) There were ice advances and retreats.
The Cenozoic Era had four well-marked short- term cooling episodes:
(i) The middle Palaeocene Epoch about 60 million years ago;
(ii) The middle Eocene Epoch about 45 million years ago;
(iii) The Eocene-Oligocene Epoch transition, 38 million years ago;
(iv) The mid-Oligo-cene Epoch around 28 million years ago.
The transition bf the Eocene and Oligocene Epochs’ witnessed the most severe short-term cooling episode. Within a span of 100,000 to 1,000,000 years, the entire ocean water column had cooled by 3°C to 5°C in high latitudes while in low latitudinal regions the cooling took place at depth alone.
The Antarctica ice sheet probably formed between about 25 million to 15 million years ago, and by the late Miocene the Antarctica ice sheet assumed the present status. During the past two million years, three distinct phases in the world climate occurred—ice age, warm (as today) and interstadial (intermediate).
The final (Eemian Epoch) interglacial took place about 120,000 years ago, and it lasted about 10,000 years. The last major glaciation in the northern hemisphere took place around 18,000 years ago. The world mean summer ocean temperature became 2.3°C lower than present.
World Climate during Historical Past:
The Cordilleran ice sheet of western North America disappeared by 10,000 years ago, having reached its peak around 14,000 years ago. The Scandinavian ice sheet melted and was reduced to its present size by 8,500 years ago. The climatic optimum (warm time) was reached between 5,000 and 8,000 years ago.
Between AD 900 and 1200 winters became colder, followed by rising temperature which is known as the little climatic optimum. Between AD 1400 and 1850, what is called the Little Ice Age occurred with European glaciers advancing farther than at any time since the continental ice receded. The rise in temperature was observed between 1850 and 1940 and by 1970 the cooling trend which started after 1940 seems to have stopped.
Climatic Cycles:
Although climatic fluctuations are recorded, the regularity of climatic cycles is not so certain. The lengths of well-recognised daily and seasonal cycles vary by large percentages. At high latitudes, for example, the character of daily cycle differs much from summer to winter. The onset of the monsoon in south and east Asia occurs at widely differing dates. Studies suggest the existence of a great number of cycles but few show dependable regularity.
Most of them, therefore, are termed rhythms or quasi-periodicities ranging from a year to millions of years. The climatic cycles seem to occur randomly and due to random causes. A vigorous research is going on to identify the true nature of solar activity, earth-sun relations, atmospheric composition and other related phenomena which may throw more light on climatic cycles.
Theories of Climatic Change:
Hypotheses Related to Solar Activity:
The variations in solar output are caused by the variation of nuclear activity inside the core of the sun. The changing number of sunspots appearing on the face of the sun after intervals of 11 years is also believed to cause solar output fluctuations. The solar flares released by the sun during maximum sunspot periods are known to affect the magnetic field of the earth.
A strong geomagnetic field causes deflection of the charged particles towards the north and south poles whereas a weak field allows them to enter through the atmosphere at low latitudes. It results in the creation of an ozone hole in the earth’s atmosphere allowing increased penetration of ultraviolet radiation of the sun.
Sunspot activity in combination with other solar phenomena like solar rotation and solar wind has a tremendous influence on earth’s heat budget, the general circulation and precipitation. The geomagnetic field gets completely reversed at intervals of relative frequency on the geological time-scale and the magnetic field disappears during such intervals. Palaeoclimatologists claim that a strong correlation exists between earth’s magnetism and climatic changes (especially those climatic changes associated with ice ages and the extinction of species).
According to the hypothesis of expanded sun, hydrogen diffuses from the sun’s mantle to its core, and the slowly diffusing metals are left behind. The metals ultimately form a barrier to the sun’s radiation, and the sun contracts. The metal barrier turns hot, causing convection currents, and the core becomes extremely large. As an effect of the enlarged core, the content of hydrogen in the sun is increased, rotation decreases, and the planets revolving round the sun cool.
The Orbital Eccentricity of the Earth:
The Croll-Milankovitch hypothesis postulates that the orbital variations of the earth produce climatic changes. As Howard Critchfied explains, astronomical theories consider five factors. The first three are generally grouped together as the Milankovitch theory, after Milutin Milankovitch, a Yugoslav geophysicist who combined them in a mathematical model to explain expansions and contractions of Pleistocene ice sheets.
1. Changes in the angle which the earth makes with the plane of the ecliptic. The tilt angle varies slowly between 22.1° and 24.5° during a cycle of about 41,000 years, presumably affecting the seasons, temperature distribution and the general circulation.
2. Changes in the eccentricity of the earth’s orbit— period 96,000 years. Resulting variations in the mean distance from earth to sun could affect temperatures on earth.
3. Precession of the equinoxes, the regular change in the time when the earth is at a given distance from the sun. At present, the earth is closest to the sun in the Northern Hemisphere winter (about January 3). About 10, 5000 years ago the Northern Hemisphere winter came at a time of year when the earth was farthest from the sun. Other things being equal (which they never are), winters should have been colder and summers warmer than they are now. In the Southern Hemisphere the reverse applies.
4. Shifting of the earth on its polar axis. This hypothesis, suggested by Robert Hooke in 1686 to explain tropical fossils in England, has been abandoned by most climatologists in favour of theories based on plate tectonics and continental drift, which also could account for apparent ‘Polar wandering.
5. Changes in the rate of the earth’s rotation on its axis, affecting the diurnal heat budget and ultimately world climates.
Volcanic Dust Hypothesis:
Volcanic dust deflects light in short wavelength whereas long wave terrestrial radiation passes through the dust easily. Most probably, the presence of the large amount of volcanic dust in the atmosphere caused the Little Ice Age on earth. The eruption of Krakatoa in 1883 may have contributed to severely cold winter that followed in the Northern Hemisphere. Theoretically, the hypothesis can be put to test by comparison of records of ice age climates with – sedimentary record evidences of volcanic activity, though accurate measurements over a large area to test the validity of the hypothesis seem difficult to achieve.
Albedo-based Hypothesis:
The degree of the reflection and absorption of insolation influences the earth-atmospheric system. With increasing afforestation, for example, albedo decreases and deforestation increases albedo. The higher albedo surface leads to global cooling in winter seasons.
Another mechanism is related to the change in the form of the major waves in the general atmospheric circulation. The Milankovitch mechanism suggests that an increased height of both the Tibetan Himalayas and Southwestern USA would alter the planetary wave structure such that, in the long run, the North American and European landmasses would be cooled and made susceptible to orbitally driven insolation changes.
The past 180 million years have been witness to changes in the land-sea ratio by about 20 per cent and this has affected climate: there have been changes in the net radiation balance, the geographical distribution of albedo, seasonality and ocean circulation.
Changes in the Atmosphere of the Earth The changes in climate are rooted in variations in the transmissivity and absorptivity of the earth’s atmosphere. Carbon dioxide (CO2), ammonia and water vapour are opaque to outgoing long-wave solar radiation while they remain transparent to incoming short-waves. This leads to the greenhouse effect and global warming. The proportion of CO2 has increased by 15 to 17 per cent from 1890 to 1980 leading to a rise in temperature by 2° to 4°C.
It has been suggested that the decreased levels of carbon dioxide in glaciers is due to phytoplanktons which consume C02 and iron. Martin noted that the last glacial age was much dustier than today and the dust was rich in iron. As the dust entered oceans it fuelled plankton growth. The atmospheric levels of C02 dropped as the phytoplanktons consumed the gas.
Variations in the amount and height of maximum ozone concentration in the upper atmosphere might also affect air temperatures. Sulphur dioxide and chlorine emitted during volcanic eruptions are among the gases that can react chemically, or photochemically, to reduce ozone, which absorbs ultraviolet radiation from the sun as well as portion of infrared terrestrial radiation. Its increase would lead to a small rise in surface temperatures; a decrease would tend to produce surface cooling.
Crustal Movements and Climatic Change:
The influence of the earth’s surface on the heat and moisture budgets suggests another category of theories. Continental drift during past geologic eras would account for climatic changes of major proportions as land masses shifted in relation to one another and assumed different latitudinal positions. Refinement of plate tectonics theory by geophysicists since the middle of the twentieth century has given support to explanations of climatic change based on crustal movements.
If vertical temperature lapse rates similar to those of the present prevailed in the past, changes in elevation owing to major crustal upheavals might have initiated glacial stages and redistribution of vegetation, both of which are important clues to past climates. Structural warping of ocean basins alters sea level and oceanic circulation, consequently affecting the transport of heat and moisture.
Changes in Oceans:
Mounting evidence points to a link between climatic fluctuations and circulation systems in the oceans. But the relation is reciprocal. Changing climate may be reflected in water temperatures and circulation, which in turn affect climate. These and other effects that result from interactions within the earth’s climate system are termed autovariations.
Future Implications of Climate Change:
A thorough understanding of the past climate systems helps us to predict climates of the future.
But, unfortunately, no climatic cycle longer than one year has been demarcated so far which may enable us to forecast with reasonable precision. Nevertheless, attempts to build statistical probabilities have been made for broad range applications in planning. The basic limitation of climate forecasting is that the data, unlike that for weather forecasting, cannot be evaluated after the end of a climatic period.
However, the indirect methods applied nowadays such as meteorological and solar observations beyond the atmospheric limits may reinforce our information reserves in future. Climate forecasting is, on the whole, unpredictable since nobody can guarantee that volcanic eruptions or some unknown factor will not produce more (or less) severe climatic fluctuations in future.
The most popular technique for building future climate models based on complicated simulations is called General Circulation Models (GCM). All GCM experiments suggest an average increase in global temperature. Models predict that global warming in the future will be greater in the higher latitudes, probably 50 to 100 per cent more than the global mean temperature, whereas warming will be less in the tropics.
Most models suggest an above-average rise in global temperature in mid- latitude continental interiors in the northern hemisphere dining summer. S.H. Schneider (1990) predicted that some of the regional climatic changes due to global warming include higher temperature and longer growing seasons in high latitude regions, wetter winter and subtropical monsoon over high and mid-latitude regions, lesser rainfall in some mid-latitude regions during summer and more severe tropical cyclones.
The most visible impact of an increased availability of CO2 in the atmosphere will be an accelerated growth of plants. Global warming may result in permafrost wetting leading to the formation of thermokarst, as the surface of previous ice-laden ground breaks down. As a result, mass movement and slope instability would increase which would have a direct impact on surface forms, drainage networks, river sedimentation and man-made structures.
The consequence of climate change will be negative in the case of animals like polar bears; some species of birds whose geographical distribution is linked to specific ecosystems could face extinction. So, individual species would respond differently to changing climate, destroying the structure of the existing ecosystem and creating new combinations of flora and fauna.
This could expose many existing species to new and exotic species. In recent years, the destruction of rainforests—the richest, most diverse and complex ecosystem—has raised an uproar. Incidences like forest fire in rainforests have contributed between 15 and 30 per cent CO2 to the atmosphere.
Climatic change may cause serious disruptions in demand and supply of hydrological resources. A wide range of hydrological variables, such as humidity, soil moisture, evapotranspiration, surface runoff, groundwater recharge, snowfall, stream flow etc., will face the negative impacts of global warming.
A huge climatic variability could increase the occurrence of flash floods and severe droughts. In order to ensure security, it is of urgent necessity to identify catchment areas facing the greatest risk. The link between climate change and agriculture is often non-linear. To establish a direct relationship between the two is rather difficult as different crops in an agro-climatic region will respond variously to any particular climatic change.
The impact of climate on agriculture may be through the effect on the distribution of soil moisture, the growth or decrease of pests, or the creation of arid conditions causing soil erosion and reducing agricultural productivity. The broad relationship between climate change and agriculture is also influenced by a complex of decision-making at the individual level and at the government level as reflected in policies.
Agriculture can adapt itself to climatic change at three levels: (i) development of new crops, new patterns of crop rotation, and different managerial practices in response to changing conditions; (ii) the response of cultivators to changing climate; and (iii) the response of markets at regional, national and international levels to changing supplies of agricultural goods.
Global warming may induce the following land use changes: (a) changes in the cultivated area, (b) changes in the types of crops, and (c) changes in the cropping pattern. Global warming has far- reaching implications for future food security. At present, a few countries enjoy the status of food exporters, such as, the USA, Canada, Australia, Argentina and France.
But recently, concern has been raised over the possibility of a decline in productivity, particularly in the USA and Canada. As the low income developing countries of the world, located in the parts of Africa, south and southeast Asia, Central and South America, are vulnerable to dire social, economic and political consequences in the case of slump in food production, it is to be seen how shortfalls in agricultural productivity may be met.