The gaseous mantle around our globe allows a considerable portion of solar radiations to enter right upto the surface of earth which absorbs it and radiates back infra-red and heat waves. This heat is transferred to layers above, as warm layer rises and is in turn passed on to higher and higher layers. Finally much of the solar radiations are radiated back to space as infra-red and heat waves.

The system consisting of our globe and its atmosphere is in a state of dynamic equilibrium with the rate of absorption of solar radiations and its emission back to space as infra-red and heat waves, nearly balancing each other. Those gases and vapours which allow free passage to radiations of relatively shorter wavelengths (2900 A – 7000 A) while absorbing effectively infra-red and heat waves (700 A onwards) play a very important role in maintaining surface temperatures within a range in which life can exist.

They form a blanket around the globe which checks the passage of infra-red and heat waves from earth’s crust back to space and keep it warm and hospitable. The phenomenon is similar to that of green-house in which the glass enclosed atmosphere gets heated up due to its insulation from the rest of the environment. Hence, global warming is also known as Green House effect and the gases responsible for it are called green house gases.

(1) Causes of Global Warming:

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Very slow, almost imperceptible rise of about 4-5°C in the global temperatures has occurred in the past 20,000 years. However, a rise of about 0.3°-0.7°C was recorded during the last century alone, which is remarkably faster as compared to change that occurred in the past.

This acceleration in the pace of global warming coincides with a rise in the concentration of green house gases in the atmosphere. The insulation of earth’s surface from the outer space caused by green house gases tends to become more and more effective as the concentration of these gases rises. More heat and infra-red radiations are trapped by the gaseous mantle around the globe which accelerates the pace of global warming.

(2) Gases Responsible for Global Warming:

There are a number of gases present in the atmosphere which are capable of absorbing effectively heat waves and infra-red rays while being transparent to radiations of lower wavelengths. Carbon dioxide, methane, oxides of nitrogen, sulphur dioxide, ozone, chlorofluorocarbons and water vapours are some of the gaseous constituents of troposphere which come in this category.

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From the point of global warming, however, only those gases are important which maintain an effective concentration in the troposphere, i.e., the region of atmosphere immediately covering earth’s surface. There are five such gases rising concentration of which has been implicated in causing noticeable rise in the mean global temperatures. These gases are: carbon dioxide, methane, chlorofluorocarbons, nitrogen oxide and water vapours. Other gases like sulphur dioxide, ozone are not able to contribute much; they are quickly cleared from the atmosphere.

(a) Carbon Dioxide:

It is one of the most important green house gases of which about 18 billion tons are being introduced into the atmosphere annually. A rise of about 26% has already been recorded in a period of 200 years only. Between the years 1980-90, carbon dioxide has been estimated to be responsible: for at least about 55% of global rise in temperatures. The concentration of this gas is still raising at a rate of about 0.5% per year. (Fig. 15.3.B)

(b) Methane:

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Methane is another greenhouse gas which is produced when organic matter decays under anaerobic conditions. In 1950 its concentration was about 1.1 ppm while in the year 1985 it was estimated to be 1.7 ppm. The concentration of this gas is rising at a rate of about 1% per year, between the years 1980-90 about 15% of the total warning has been attributed to this gas alone. In the atmosphere methane undergoes oxidation to carbon dioxide and water both of which tend to accentuate the greenhouse effect.

(c) Nitrous Oxide:

Nitrous oxide is another troublesome gas. Other oxides of nitrogen are reacted upon or cleared rapidly while nitrous oxide undergoes decomposition very slowly and hence it tends to accumulate in the atmosphere. In 1950 its concentration was about 280 ppb while in 1985 it had reached 380 ppb. Between the years 1980-90, nitrous oxide accounted for at least 6% of the total global warming. The concentration of this gas is still rising at a rate of about 0.3% per year.

(d) Water Vapours:

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About 70% of earth’s surface is covered with water wherefrom an enormous quantity of water evaporates. Likewise the process of transpiration introduces a substantially large amount of water vapours into the atmosphere. Precipitation brings down about 496.1 x 103 cubic kms of water to earth’s surface whereas vapours equivalent to 14,000 cubic kms of water stay back permanently in the atmosphere (Serruya and Pollinger, 1983).

Water vapour like any other greenhouse gases contributes significantly to the global warming. With an overall rise in temperatures the rate of global transpiration and evaporation shall also go up which shall introduce more water vapours into the atmosphere and could in turn influence the pattern of global warming.

(e) Chlorofluorocarbons:

Chlorofluorocarbons represent a group of man-made, colourless, odourless, easily liquifiable chemicals which have more potential for global warming than any other greenhouse molecules. They are very stable compounds which may persist in the atmosphere for periods as long as 80-10) years.

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Though first introduced only in the fiftes, chlorofluorocarbons have rapidly attained such levels that between the years 1980-90, they were responsible for 24% of global warming. Till 1985, about 15 million tons of these compounds had been released in the atmosphere. In spite of much international efforts to check the use of these chemicals CFCs are still rising at a rate of about 5% per year.

Apart from contributing substantially to global warming the persistent nature of chlorofluorocarbons enables them to accumulate and rise at random to reach the stratosphere. Strong ultra-violet radiations present in the stratosphere decompose these compounds to yield chlorine atoms which catalytically destroy the vital ozone shield.

It is human activity which is mainly responsible for the accumulation of green house gases in the atmosphere. Energy production and its use, intensive agriculture, maintenance of huge livestock population, use of chlorofluorocarbons, land use modification and industrial production are some of the aspects of human activity which are responsible for this accumulation.

These practices, apart from introducing gaseous pollutants in the atmosphere have also modified natural ecosystems and vegetation which act as an efficient sink for many of these pollutants. So while the input of green house gases into the atmosphere continues at a greater pace their removal or output from the atmosphere has definitely been reduced. The acceleration in the pace of these activities is tremendous and is intimately linked with growth, development and prosperity.

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Power generation, refrigeration facilities, transportation activities, extensive deforestation for human habitations, his industries, agriculture etc. are necessities of today. Putting restriction on these activities involves drastic lowering of living standards and amenities and an equally serious reduction in growth, development and technological advancement.

To cap up all these come international rivalries and conflicting interests which blame the developed world for all the environmental deterioration caused so far demanding the advanced nations to take up responsibilities for the cleanup. Though much environmental awareness has been created in the recent past the key issues are lost in the conflict and confusion and little effective efforts have so far been done to redress the situation.

(3) Green House Effect and Stratospheric Ozone Depletion:

In the troposphere greenhouse gases provide an effective thermal insulation while in the stratosphere many of these gases are responsible for causing ozone depletion. In spite of the discontinuity caused by mutually opposite thermal gradient which prevents a free exchange of materials between the troposphere and the stratosphere, some transfer of gaseous material does take place between the two layers. Because of a greater build up of greenhouse gases and longer life span in the troposphere, these molecules rise at random and reach the stratosphere where they undergo dissociation yielding chlorine atoms, hydroxyl ions and nitric oxide which react with ozone to degrade it to oxygen.

A significant amount of nitric oxide is produced in the stratosphere by nitrous oxide derived from the troposphere below. More chlorine atoms are derived from the disintegration of chlorofluorocarbons than from any other source. Hydroxyl ions are produced in plenty when water molecules dissociate under influence of ultra-violet radiations in the stratosphere. Though there may be other sources as well it is mainly the pollution of the troposphere which contributes significantly to the ozone-depleting constituents of the stratosphere.

During the last two or three decades there has been a slow thinning out of the vital shield and the overall reduction in ozone content is now estimated to be about 8%. An inevitable consequence of this will be an increase in the intensity of solar radiations containing harmful ultraviolet rays. Apart from other damages this will further enhance the heating effect caused by sun’s rays.

However, adimunition in the ozone content of the stratosphere shall also reduce the heat retaining capacity of the gaseous mantle around the globe and it has also been suggested that this could neutralize some of the warming effect caused by higher concentration of greenhouse gases.

(4) Consequences of Global Warming:

Global records of earth’s surface temperatures indicate that a warming of about 0.5°(0.3°- 0.7°C) has occurred during the last century alone. Results from recent climatic models suggest that mean global temperatures shall rise by 2°-6°C during the next century if we assume that carbon dioxide concentration in the troposphere increases to 600 ppm.

The projected change in mean surface temperatures may appear insignificant because variations of this magnitude are experienced in course of seasonal or even daily weather. In fact this is not so. During the last great Ice Age, about 12,000 years ago, when much of Northern America and Europe was covered with a sheet of ice, the mean surface temperatures were only about 5°C lower than today.

The world climate was very much different from what it is now. The transition from the great Ice Age to present-day climate during which average surface temperature rose by 5°C took almost twelve thousand years. But the variations of almost similar magnitude shall be experienced within course of single century if greenhouse gases continue to rise at the present rate.

Though there are considerable uncertainties regarding the precise consequences of global warming one of the obvious results of the general heating up of earth’s surface shall be a rather rapid rise in mean sea level. During the last fifteen thousand years mean sea level has slowly been rising. Evidences suggest that about 12,000 years back it was nearly 100 m lower than the present-day level.

It is expected that global rise in temperatures shall further enhance the rate of already rising: levels in two ways. Firstly, large deposits of ice present on earth’s surface shall melt which will add more water to the oceans. Secondly, rise in temperatures shall also cause thermal expansion of upper layers of water.

An increase of 4°-5°C could cause enough expansion of this enormous mass of water so as to raise the mean sea level 5-6 cms. If all ice present on earth’s crust was to melt, a level could rise by about 60 m. Large stretches of low lying areas shall be submerged. As much of the world population lives near shore, this could be a total catastrophe. About sixty odd island countries shall face deep encroachments by sea water and some like Maldives may disappear altogether.

The rise in global temperatures shall not be uniform all over the surface area of the world. Most of the workers agree that Polar Regions of the world would undergo larger increase in temperature about ten to twelve times as much as the tropics. This shall bring unprecedented changes in wind and precipitation patterns within a span of a single century.

During the last Ice Age, with a colder Arctic region, North Africa, the deserts of Arabia and the Thar Desert of India were fertile regions. But with its warming the precipitation belt has shifted northward today. More warming shall move it further north. So North Africa, Europe, parts of Russia and the fertile corn belt of the United States could become drier, while much of peninsular India, parts of Australia and some of the Central Africa shall become more humid.

In India the deserts of Rajasthan could expand right upto Punjab, Delhi and western part of Uttar Pradesh while eastern part of India shall experience little change. Higher temperatures shall cause a rise in evapo-transpiration and ground water table may be affected as the rate of recharge of sub-surface water table shall change.

Salt waters encroaching upon the low lying areas may result in wastage of many of our fresh water reservoirs and could spoil much of our underground water resources also. These are all speculations only – based on scientific observations and logic. What is, however, certain, is that changes of enormous magnitude shall occur within a short span of time during the next century.

As the climatic belts shift away from equator towards poles, vegetation shall have to shift in the same direction to stay in favourable climatic conditions. Those species which are unable to do so shall die. There will be losses of genetic resources on large scale. Hardy and resistant forms shall come up and survive.

An altogether changed biotic spectrum shall replace the earlier ones and almost all important biomes shall be affected. As temperature changes will affect wind and precipitation patterns also water could play an important part in altering the biotic communities. It has been suggested that some rise in precipitation, however, shall be balanced by an enhanced evapo- transpiration and this could lead to water deficit and moisture stress in many regions of the world (Mather and Feddema, 1986).

Insects and pests may increase as warmer conditions could be more favourable to their growth and coupled with higher humidity pathogenic diseases shall multiply Cycling of mineral nutrients may be affected and with it leaching and desertification may follow many areas. The effect of global warming on agriculture will be of a varied type in different parts of the world. Wheat and maize crops may suffer from moisture stress.

More fertilizers shall have to be used to sustain productivity. In places where temperature conditions are already near the tolerance levels, even a rise of 1-2°C may be quite harmful. Alterations in cropping pattern stall occur and pest resistant varieties more suitable to warmer conditions shall have to be developed, short green house warming shall bring with it an entirely new environment in which life though not impossible yet its existence shall be tougher to maintain.