Is water vapour more harmful to climate than carbon dioxide?
“I have read that water vapour is a greenhouse gas just like CO2 and that water vapour also causes heating. But which of the two gases is more potent, and which is more climate-friendly?”
Water vapour is certainly far more instrumental in governing what happens to energy in the atmosphere than CO2. Water vapour is responsible for about 60% of the greenhouse effect that raises Earth’s average temperature from well below freezing point to 15 degrees. And the effect on average global temperatures would be far larger if we removed all water vapour than if we managed to remove all CO2.
But water vapour, unlike CO2, does not cause climate change in itself. Instead, the quantity of vapour in the atmosphere follows changes that are caused by other factors. Indeed the temperature of the atmosphere determines the quantity of water vapour it can hold, rather than the other way around.
If we added more water vapour to the atmosphere, precipitation would just increase correspondingly. And if we somehow extracted water vapour from the atmosphere, it would eventually be replaced by increased evaporation from the ocean surface, though precipitation levels would fall for a period of time.
More CO2, on the other hand, increases the temperature by itself. Less CO2 lowers it. So carbon dioxide is clearly the most important greenhouse gas in relation to global warming. Other greenhouse gases such as methane and nitrous oxide contribute, whereas water vapour boosts the total effect of greenhouse gases. This boost is included in the calculation that ‘budget’ for how much each individual greenhouse gas affects the climate – also known as global warming potential (GWP).
So when scientists calculate CO2’s greenhouse effect and future global warming, they automatically allow for water vapour in the atmosphere. So water vapour forms part of all climate calculations.