76.

What is green house effect and global warming?

i) The earth receives a large amount of energy from the sun. Out of the total energy received, about 66% is absorbed by the earth while the rest 34% is reflected (or scattered) back into space.

ii) The reason for this energy reaching the earth is that visible and ultraviolet radiations (short wavelength) can pass through CO₂ and water vapours present in the atmosphere. Ultraviolet radiations are absorbed by ozone layer but visible light reaches the earth. As a result, the temperature of the earth increases. However, when the earth cools, the energy is re-emitted from the earth’s surface in the form of infra-red radiations (longer wavelength and have a heating effect). I.R. radiations are heat radiations. These I.R. radiations can be absorbed by CO₂ and H₂O vapours but can not pass through them.

iii) The heat thus absorbed by CO₂ and H₂O vapours cannot go to the upper atmosphere and is radiated back to the surface of the earth. In this way, additional heat is kept within the lower atmosphere and warming of the earth occurs. This warming of the earth leads to the warming of air (due to the presence of green -house gases) is called global warming. This warming of the earth or global warming due to re-emission of the sun’s energy absorbed by the earth followed by its absorption by CO₂ molecules and H₂O vapours present near the earth’s surface and then heat radiated back to the earth by CO₂ and H₂O vapours is called ‘green- house effect.’