Water vapour is the most abundant greenhouse gas in the atmosphere, yet other greenhouse gases (such as carbon dioxide and methane) are often portrayed as the main drivers of climate change. Why is that?

Answer:

Water vapour is the most abundant greenhouse gas in the atmosphere, both by weight and by volume (1), (2). Water vapour is also an effective greenhouse gas, as it does absorb longwave radiation and radiates it back to the surface, thus contributing to warming.

When compared to other greenhouse gases, water vapour stays in the atmosphere for a much shorter period of time. Water vapour will generally stay in the atmosphere for days (before precipitating out) while other greenhouse gases, such as carbon dioxide or methane, will stay in the atmosphere for a much longer period of time (ranging from years to centuries) thus contributing to warming for an extended period of time.

The addition of water vapour to the atmosphere, for the most part, cannot be directly attributed to human generated activities. Increased water vapor content in the atmosphere is referred to as a feedback process. Warmer air is able to hold more moisture. As the climate warms, air temperatures rise, more evaporation from water sources and land occurs, thus increasing the atmospheric moisture content. The increase in water vapour in the atmosphere, because water vapour is an effective greenhouse gas, thus contributes to even more warming: it enhances the greenhouse effect.

Water vapour is often discussed and recognized as being an important part of the global warming process. The water vapour feedback process is most likely responsible for a doubling of the greenhouse effect when compared to the addition of carbon dioxide on its own (3).