You’ll often hear a climate change denier claim that water vapour is the most important greenhouse gas, and that carbon dioxide is unimportant by comparison. While there is a grain of truth to it, this argument is ultimately a misleading distraction.
Water vapour does have more than twice the total warming effect of carbon dioxide, but the amount of water vapour in the atmosphere is at capacity (equilibrium) while CO2 is not. Yes, we’re emitting lots of both, but increasing water vapour emissions just means that more overflows (as rain or snow). Increasing CO2 emissions means more CO2 in the atmosphere — for decades. So it’s CO2, not water vapour that drives climate change -- here we focus on CO2, but other human-emitted greenhouse gases are also climate forcers: methane, NO2, ozone-destroying CFCs.
The trick is that despite the equilibrium of water vapour (the overflowing glass), water vapour concentrations are increasing. Huh? It sounds contradictory, but it’s not: the amount of water vapour the atmosphere can hold is limited by temperature, and temperature is increasing because of the extra CO2.
So there is more water vapour in the atmosphere, and it does contribute to warming, but it all starts with CO2. CO2 warms the atmosphere, which grows the water-vapour glass, which further warms the planet. Therefore, water vapour is known as a climate feedback, but not a climate forcer.
There are several other climate feedbacks, but carbon dioxide remains the single most important contributor to climate change. The next time someone tells you it’s water vapour, tell them they’re just spewing hot (wet) air.
Kai Chan is an assistant professor and Canada Research Chair at the Institute for Resources, Environment, and Sustainability (IRES) at UBC. Stephen Ban is a PhD candidate with the Australian Research Council’s Centre of Excellence for Coral Reef Studies at James Cook University in Townsville, Australia.