About 75% of the solar energy reaching the earth is absorbed by the earth's surface, which increases its temperature. The rest of the heat radiates back to the atmosphere.
Some of the heat is trapped by gases such as carbon dioxide, methane, oxone, chloroflorocarban compounds (CFCs) and water vapour in the atmosphere. Thus, they add to the heating of the atmosphere. This causes global warming.
We all know that in cold places flowers, vegetables and fruits are grown in glass covered areas called greenhouse.
Do you know that we humans also live in a greenhouse? Of course, we are not surrounded by glass but a blanket of air called the atmosphere, which has kept the temperature on earth constant for centuries.
But it is now undergoing change, through slowly. Just as the glass in a greenhouse holds the sun's warmth inside, atmosphere traps the sun's heat nere the earth's surface and keeps it warm.
This is called natural greenhouse effect because it maintains the temperature and makes the earth perfect for life. In a greenhouse, visible light passes through the transparent glass and heats up the soil and the plants.
The warm soil and plants emit infrared radiations. Since glass is opaque to infrared (heat) radiations, it partly reflects and partly absorbs these radiations
This mechanism keeps the energy of the sun trapped in the greenhouse. Similarly, carbon dioxide molecules also traps heat as they dioxide molecules also traps heat as they are transparent. If the amount of carbon dioxide crosses the delicate proportion of 0.03 per cent, the natural greenhouse balance may get disturbed.
Carbon dioxide is the major contributor to global warming. Besides carbon dioxide, other greenhouse gases are methane, water vapour, nitrous oxide, CFCs and ozone. Methane is produced naturally when vegetation is burnt, digested or rotted in the absence of oxygen. Large amounts of methane are released in paddy fields, coal mines, from rotting garbage dumps and by fossil fuels.
Chloroflorocarbans (CFCs) are man- made industrial chemicals used in air conditioning etc. CFCs are also damaging the ozone layer. Nitrous oxide occurs naturally in the environment. In recent years, there quantities have increased significantly due to the use of chemical fertilizers and burning of fossil fuels.
If these teands continue, the avarage global temperature will increase to a level which may lead to melting of polar ice caps and flooding of low lying areas all over the earth. Increase in the global temperature increases the incident of infectious diseases like dengue, malaria yellow fever, sleeping sikness etc.
