Global warming refers to gradual rise in atmospheric and ground surface air temperature and consequent change in global radiation balance caused mainly by anthropogenic processes leading to climatic change at different levels (e.g. local, regional and global). It may be pointed out that the pattern of global rise in air temperature has been studied and reported by different scientists and agencies but their results are not uniform. Radiative forcing, as defined and used by Intergovernmental Panel on Climate Change (IPCC, 1990), refers to the effects which green house gases have in altering the energy balance of the ‘earth-atmosphere system’ (GWP) is used as a tool to compare the relative warming effect of different gases.
It has been estimated that the overall increase in the surface air temperature over the past one hundred years has been about .05°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 this period but the temperature dropped after 1950 in spite of continued rapid rate of combustion of fossil fuels due to fast industrial growth but soon after 1940 the 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 up to 1995 while other sources have shown general air temperature rise ranging between 0.3°C and 0.6°C. It may be mentioned that rise of 2°C temperature was recorded in the Indian Ocean during 1997-1998 which caused catastrophic coral bleaching leading to 70 percent death of corals in the Andaman Nicobar and the Lakshwadeep Islands.
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 atmospheric carbon dioxide could be doubled from the 300 ppmv level to 600 ppmv. The general Circulation Model developed by S.Manabe and R.T. Wetherald (1975) predicts that if the present amount of carbon dioxide of the atmosphere is doubled, the temperature of the earth’s surface will increase by 2.9°C.