JIM Allan points out that the biggest contributor to the greenhouse effect is water vapour and questions whether an increase in carbon dioxide can have much of an effect on the global climate (HAS, Jan 14).

Water vapour makes up about one per cent of the atmosphere and is responsible for between 36 and 70 per cent of the greenhouse effect.

Carbon dioxide makes up 0.038 per cent of the atmosphere and is responsible for between nine and 26 per cent of the greenhouse effect.

So water vapour is responsible for a maximum of 7.8 times the amount of warming for which carbon dioxide is responsible, but there is 26 times more of it in the atmosphere. Proportionately, carbon dioxide is a “worse”

greenhouse gas.

Chris White, Spennymoor, Co Durham.