Water Vapour is the biggest contributor to the 'natural greenhouse effect' and varies the most in the atmosphere. Cold air can hold little water and so the atmosphere over the polar regions contains very little water vapour. In contrast, air over the tropics is very humid and the atmosphere can contain up to 4% water vapour.

It is this 'positive feedback' that makes water vapour important in climate change as a small increase in global temperature would lead to a rise in global water vapour levels thus further enhancing the greenhouse effect.

Human activities have little impact on the level of water vapour in the atmosphere.