Solar radiation warms the Earth’s surface and the atmosphere. About onethird of the radiation is reflected back into space, about 20% is absorbed by the atmospheric gases and the remaining amount reaches the earth surface and is absorbed by it. The energy so absorbed is remitted in the form of infrared radiation. The atmospheric gases absorb some of this radiation and hence do not allow all the emitted energy to escape into the space. Thus, some of the heat is trapped by these gases and the atmosphere becomes warmer. It is this phenomenon which raises the average temperature of earth from -18°C to +15°C and is very vital for life on the earth. The situation is analogous to a greenhouse which traps heat and its glass walls do not allow the heat to go out thereby increasing the inside temperature. Therefore, this effect is called greenhouse effect.
The gases such as carbon dioxide,
methane, ozone, chlorofluorocarbons and water vapours are responsible for
greenhouse effect and are called greenhouse gases. The contribution of water to
greenhouse effect being about two-thirds and that of carbon dioxide being about
one-quarter. The other gases nitrogen (N2 ), oxygen (O2 ), argon (Ar) present
in the atmosphere are incapable of absorbing infrared radiation. The
concentration of water vapours in the atmosphere has not changed significantly
but that of greenhouse gases has shown a marked increase since the industrial
revolution. The anthropogenic (human) activities such as generation of energy
from fossil fuels and deforestation have increased CO2 concentration.
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