Life on earth as we see it today has been made possible by the sun energy most of which arrives in the visible spectrum. About one third of sunlight is scattered back into space from the outer layers of the atmosphere while the rest heats up the earth's surface. The heated earth looses a part of this energy through the infrared radiations that are carried by air currents. A small part of this energy is captured and stored by several trace gases in the atmosphere such as water vapour, carbon dioxide and methane. These trace gases that make up only about 1 per cent of the atmosphere, called the greenhouse gases, act like a blanket around the earth, or like a greenhouse, and that is why the name. They trap heat and keep the planet some 33 degrees C warmer than it would be in the absence of the greenhouse gases. The life on earth as we see it today owes its existence to this blanket of greenhouse gases around the earth' surface.
But now this blanket is becoming thicker with enormous additions of these trace gases into the atmosphere from the burning of fossil fuel; by agricultural activities and deforestation and other changes in land use; and by several industrial processes that release gases into the atmosphere that not only trap infrared rays but are also very stable and remain intact in the atmosphere for centuries.
Carbon dioxide is responsible for almost two third of the enhanced greenhouse effect. We are burning coal, oil, and natural gas at a rate that is incomparably faster than the rate at which these fossil fuels were created. This is upsetting the carbon cycle, by which carbon is exchanged between the air, the oceans, and land vegetation. If the present trends of greenhouse gas emissions is not arrested the concentration of carbon dioxide in the atmosphere will double from pre-industrial levels during the 21st century.
A higher concentration of greenhouse gases traps more infrared radiation and raises temperatures. The average temperature of the earth's surface has already risen by 0.6 degrees C since the beginning of industrial age. Climate models estimate that the average global temperature will rise by 1.4 to 5.8 degrees C by the end of this century.
The earth's climate is both the cause and the outcome of the flow of energy on the surface of the earth and even a small rise in temperature will be accompanied by profound changes in the seasons, in rain and snow, cloud cover, and wind speed and dimension. Life on earth revolves around the weather patterns and changes will be difficult and disruptive for most, particularly the poorest. Computer models indicate a trend toward greater severity of storms and hotter, longer dry periods and wider swings in what is considered normal whether. Dry regions would lose more moisture under hotter conditions thus making droughts severe in regions already affected by droughts in the continental interiors in equatorial and tropical countries of Africa and Asia.
The effect of the warming earth is being felt in the coldest parts of the world and, contrary to expectations, it is not a welcome change to the people residing in these areas. Non-polar glaciers are under retreat everywhere. Mount Kilimanjaro in Africa has lost the snow cover on its peak in all but name. Gomukh and Pindari glaciers in India have retreated upwards significantly in the past few decades. Arctic air temperatures have increased by about 5 degrees C during the last hundred years and buildings are collapsing with the melting of the permafrost underneath.
Higher temperatures cause waters in the sea to expand in volume and the glaciers to melt and add more water thus causing the sea level to rise. Already the average sea level has risen by 10 to 20 cm during the 20th century, and an additional increase of 9 to 88 cm is expected by the year 2100. The results would be nothing short of catastrophic if the worst case scenario were to occur with complete disappearance of some of the island countries and submergence of a large part of the heavily populated coastal Bangladesh. The sea waters would also make a large part of the fresh water sources in the coastal belt unusable and agriculture impossible thus causing mass uncontrolled migration to higher localities in the inlands. Even with consequences milder than those projected in the worst case scenario the cost of addressing the human distress would be beyond the reach of all but the richest countries in the world.
The global warming trend is also likely to result in the extinction of a large number of floral and faunal species and the affect would be worse on more specialized species higher on the evolutionary scale. In the Alps, some plant species have been migrating upward by one to four meters per decade, and some plants previously found only on mountaintops have disappeared. In Europe, spring events in the life cycle of many bird species have started occurring earlier with egg-laying by 20 of 65 species of birds in United Kingdom having advanced by an average of eight days between 1971 and 1995. Butterflies and other insects are now living at higher latitudes and altitudes, where previously it was too cold to survive.
Agricultural productivity would slide down in most non-temperate regions while in temperate regions there might be significant increases in the productivity if the temperature increase is modest. Climate models suggest drying of continental interiors and reduced availability of moisture combined with higher dessication would result in sharp reduction in food productivity in central and western India, central Asia, the African Sahel, and the Prairies of the United States.
Human and veterinary health would also come under high stress with rise in temperature as the environs become more favourable to a number of viruses and bacterias. The range of vector borne diseases such as dengue, malaria, filaria may expand.