Man-made warming is increasing the amount of evaporation from the oceans. Photograph: David Pu’u/Corbis

It’s a positive feedback loop: humans release CO2, which warms the air, which boosts evaporation, which increases water vapour – and thus significantly amplifies the warming

This question and answer is part of the Guardian’s ultimate climate change FAQ

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It’s true that water vapour is the biggest overall contributor to the greenhouse effect and that humans are not responsible for directly emitting this gas in quantities sufficient to change its concentration in the atmosphere. However, the scientific evidence suggests that the warming caused by man-made emissions of CO2 and other greenhouse gases is increasing the amount of water vapour in the air by boosting the rate of evaporation.

Unlike CO2, which can persist in the air for centuries, water vapour cycles through the atmosphere quickly, evaporating from the oceans and elsewhere before back coming down as rain or snow. Since the rate of evaporation rises with temperature, the amount of water vapour in the air at any one time (and the amount of warming it causes) is strongly related to the amount of other greenhouse gases in the air.

This is an example of a positive feedback loop: humans release CO2, which causes warming, which boosts evaporation, which in turn amplifies the warming. Indeed, scientists believe that evaporation is the most important positive feedback loop in the global climate system, doubling or tripling the sensitivity of the climate to CO2 emissions.

This explains why, despite water vapour constituting the largest part of the greenhouse effect, the world’s science academies have agreed, there is strong evidence that human activities are driving the observed change in temperatures.