An outcome of the atmospheric pollution is warming of the earth’s environment. This is a serious problem and in twenty-first century this may be a major global problem. The temperature of the earth has increased to some extent during the past fifty years has been exhibited in the following (fig. 15.6).
This increase in temperature of atmosphere is dangerous for living organisms. The human activities are mainly responsible for global warming. Though there are several causes for global warming, however, some important ones are mentioned here: They include deforestation, industrialisation, energy production from fossil fuels, such as diesel, petroleum, coal, etc., and urbanisation.
The conversion of a forest to a grazing land or a cropland through deforestation causes loss of carbon stored in soil and vegetation to the atmosphere, and affects the global carbon cycle.
Biomass burning associated with agricultural practices releases a large amount of CO2 into the atmosphere. Besides, due to domestic and industrial coal burning, huge amount of carbon dioxide is also released in the atmosphere.
Besides carbon-dioxide, some other gases like methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs) are also responsible for increasing the temperature of the earth and its environment.
These gases CO2, CH4, N2O and CFCs are radiatively active gases, also called greenhouse gases as they can absorb long wave infrared radiation.
The increased amounts of greenhouse gases in the atmosphere affect the global climate, and this phenomenon is called global change.
Greenhouse Gases (Greenhouse Effect):
The gaseous mantle around the globe allows a considerable portion of solar radiations to enter right up-to 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 in turn passes on to higher and higher layers.
The outgoing long-wave infrared radiation is absorbed by the greenhouse gases normally present in the atmosphere. The atmosphere radiates part of this energy back to the earth. This downward flux of radiation, known as greenhouse flux, keeps the earth warm.
Thus, the greenhouse gases of the atmosphere form a blanket over the earth that controls the 13.61 escape of heat from the surface of earth to outer space so as to keep it warm and cosy. This phenomenon is called greenhouse effect.
The name, greenhouse effect is derived from the fact that inside a glass enclosed green house, temperature is warmer than outside. The mean annual temperature of the earth is about I5°C. If there would have no greenhouse gases in the atmosphere, the earth’s mean temperature would go sharply down to about—20°C.
This capacity of the atmosphere to keep the earth warm depends upon the concentration of the greenhouse gases. The excessive increase in concentrations of greenhouse gases in the atmosphere would retain more and more of the infrared radiation, resulting in enhanced green-house effect. The consequent increase in the global mean temperature is referred to as global warming.
The rise in temperature of earth’s atmosphere caused due to greenhouse effect depends on the amount of CO2 present in the atmosphere. At normal CO2 concentration (0.03 per cent) in the atmosphere the surface temperature of the earth remains constant due to energy balance of the sun rays which strike on the earth, heat it and then radiate back into the space. This is called energy budget.
But when there is increase in CO2 concentration in the atmosphere, the thick layer of CO2 prevents the heat from being radiated out into space. This layer of CO2 thus functions as a glass pane of green-house which allows the sunlight to filter through it but prevents heat from being radiated back into the outer space.
However, an international panel on climate change periodically makes an assessment of the atmospheric abundance of greenhouse gases and its possible impact on climate and other such issues.
The trends in the increase in concentrations of greenhouse gases since pre-industrial times are as follows:
Carbon Dioxide:
CO2 is the most abundant greenhouse gas in the atmosphere. This is about sixty per cent of the greenhouse gases. The level of the concentration of carbon dioxide in the atmosphere has increased from the pre- industrial level of 280 ppm (parts per million) to about 368 ppm in 2000 AD.
Here, pre-industrial time is considered 1750 AD. This increase in concentration is mainly due to fossil fuel burning, deforestation and change in land use.
Methane:
CH4 is another greenhouse gas which is produced when organic matter decays under anaerobic conditions. Methane concentration in atmosphere was about 750 ppb (parts per billion) in pre-industrial level, i.e., 1750 AD., which had become 1750 ppb in 2000 AD. Methane is generally produced as a result of incomplete decomposition by a group of bacteria called methanogens, under anaerobic conditions.
The concentration of this gas is rising at a rate of about one per cent per year. Between the years 1980-90 about 15 per cent of the total warming has been attributed to this gas alone.
The Major Sources of Methane are:
Freshwater wetlands, enteric fermentation in cattle, and flooded rice fields. Biomass burning is also a source of methane. In the atmosphere, methane undergoes oxidation to carbon dioxide and water, both of which tend to emphasize the greenhouse effect.
Nitrous Oxide:
N2O is another troublesome greenhouse gas. Other oxides of nitrogen are reacted upon or cleared rapidly, white nitrous oxide undergoes decomposition very slowly, and therefore, it tends to accumulate in the atmosphere. The concentration of nitrous oxide in the atmosphere has increased from about 270 ppb in pre-industrial time (i.e., 1750 AD.) to about 316 ppb (parts per billion) in year 2000 AD.
The main sources of nitrous oxide are agriculture, biomass burning and other industrial processes. N2O is produced during nylon production, burning of nitrogen-rich fuels, livestock waste, breakdown of nitrogen-rich fertilisers in soil and nitrate contaminated ground water.
Chlorofluorocarbons (CFCs):
They 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 to 260 years.
CFCs are synthetic gaseous compounds of carbon and halogens. Though first introduced only in the fifties of twentieth century, chlorofluorocarbons have rapidly attained the concentration of CFC-11 (chloro-fluorocarbons) and HFC-23 (hydro-fluorocarbons) in the air about 282 ppt (parts per trillion) in 2000 AD.
Till 1985, about 15 million tons of these compounds had been released in the atmosphere. Inspite of international efforts to check the use of these chemicals, CFCs are still rising at a rate of about 5 per cent per year.
Major Sources of CFCs are:
Leaking air conditioners, refrigeration units and evaporation of industrial solvents; and production of plastic foams and propellants in aerosol spray cans.
The human activities are mainly responsible for the accumulation of greenhouse gases in the atmosphere. Energy 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 accumulation of greenhouse gases.
Percentage of Greenhouse Gases:
It is estimated that carbon dioxide contributes about 60 per cent of the total global warming. The share of methane is 20 per cent, while that of chlorofluorocarbons is 14 per cent. However, a smaller contribution to global warming is made by nitrous oxide, i.e., 6 per cent.
International Concern for Mitigating Global Change:
Global warming is the global concern. The long term challenge of stabilizing the atmospheric concentrations of greenhouse gases requires that global emissions be significantly lowered than what they are today. In this context, the representatives of many countries assembled from time to time to pass some resolutions, which can be implemented.
Montreal Protocol:
In 1987, seventy seven industrially developed countries signed the Montreal Protocol. This conference was held at Montreal. This was a landmark international agreement to protect the stratospheric ozone by agreeing to limit the production and use of ozone- depleting substances.
It was decided in the conference to limit the use of CFCs and cut level of CFCs production to 50 per cent by 1999. To-date, more than 175 countries have signed the Montreal Protocol.
However, India and China did not sign the Protocol. Developed countries have also agreed to establish a multilateral find under Montreal Protocol to provide financial support to developing countries for that purpose.
London Protocol:
In 1990, London Protocol was signed. In London Protocol, India, China and several other countries demanded more time to reduce CFCs consumption. However, the developed countries agreed to stop CFCs consumption completely by 2000 AD, and for developing nations the time limit was extended to 2010 AD.
Toronto Conference:
In 1988, the world conference on the changing atmosphere was held in Toronto (Canada). There it was decided to reduce 20 per cent carbon dioxide emissions by the year 2005.
Earth Summit:
In 1992, The First Earth Summit was held in Rio de Janeiro, Brazil. The Earth Summit was held under the banner of UNCED (i.e., The United Nations Conference on Environment and Development). This summit, established the principles for reducing greenhouse gas emission.
Kyoto Protocol:
The Kyoto Protocol, approved by a follow-up conference held in Kyoto, Japan, during December 1997, has specified the commitments of different countries to mitigate climate change. This protocol has established the rules to reduce overall greenhouse gas emissions to a level at least 5 per cent below the 1990 level by 2008-2012.
At present, on the basis of Montreal Protocol, India has established ozone cell under the Ministry of Forests and Environment with the help of UNDP (United Nations Development Programme). By now, India has developed R-22 and R-134 as CFCs substitutes which are presently at trial stage.