Exercise 13.2
Which of the following is primarily responsible for warming of the Earth? (i) Solar radiation is immediately absorbed by carbon dioxide, which then releases it as heat. (ii) The atmosphere’s tiny particles absorb incoming solar radiation, which directly heats the Earth. (iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases. (iv) The Earth’s environment is heated only by the solar radiation reflected by the clouds.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
NCERT’s answer
(iii)
(iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.
The chapter states that infrared radiation warms the Earth's surface, which then re-radiates this heat back into the atmosphere, and a portion of the outgoing heat is trapped by greenhouse gases such as \(\displaystyle CO_{2} \), \(\displaystyle CH_{4} \) and water vapour.
(i) is wrong — \(\displaystyle CO_{2} \) does not absorb the incoming sunlight; it absorbs the outgoing infrared radiated by the warmed surface.
(ii) is wrong — particles, clouds and gases do absorb and scatter some incoming radiation, but that reduces what reaches the ground; it is not what heats the Earth.
(iv) is wrong — radiation reflected by clouds is sent back towards space, so it is energy lost, not energy that warms the Earth.