The earth as a whole does not accumulate or lose heat. It maintains its temperature. This can happen only if the amount of heat received in the form of insolation equals the amount of lost by the earth through terrestrial radiation.
Suppose, the insolation received at the top of the atmosphere is 100%.
Out of this 100%, 35% is reversed back the uppermost part of the atmosphere. In other words, 35% of the sun’s rays do not even get enter the atmosphere of the earth. This is called the ‘Albedo effect’. Albedo means the amount of sun’s energy radiated back by the earth.
The remaining 65% of the energy enters the atmosphere. Out of which 14% has received the elements of the atmosphere. These elements include water vapour, carbon dioxide and other greenhouse gases.
The energy that the surface of the earth receives is 51% (i.e. 65% – 14%).
Now, it is the time for the earth to radiate back the 51% of energy, which it accumulated.
The earth radiates back all of its energy i.e. 51%. Out of this 51%, 34% is held by the atmosphere for a short period of time and the remaining 17% is directly sent back to space.
Now, the 14% energy received by the earth’s atmosphere at the time of insolation plus the 34% energy received by the earth’s atmosphere while radiating it back, amounts to a total of 48% (i.e. 14% + 34%). In other words, 48% of energy is received by the earth’s atmosphere at the last.
This 48% of energy is also radiated back to the space.
So, in total, 48% + 17% = 65% is radiated back to the space. Hence, the amount of energy received equals the amount of energy radiated back. This is how earth maintains its heat.