Understanding the behavior of clouds is key to any accurate projection of future climate (as we’ve covered in the past) because of the complex, competing effects that clouds have on Earth’s energy budget.
Radiation from the sun in the form of visible light (which climate scientists refer to as "shortwave radiation") is reflected back into space by clouds. An increase in clouds (caused by increasing evaporation that comes with higher temperatures) will thus decrease temperatures by reducing incoming shortwave radiation. We call that a negative feedback—it acts to keep temperatures stable.
But water vapor is also a strong greenhouse gas. In fact, compared molecule for molecule, water vapor absorbs 100 times more outgoing infrared (or "longwave") radiation than does carbon dioxide. That means increasing evaporation will cause temperatures to increase even more. That’s a positive feedback—it acts to exacerbate rising temperatures.
(Despite its greenhouse potency, water vapor provides a feedback, not a forcing—it can’t initiate temperature increases because its presence in the atmosphere depends on temperature, unlike carbon dioxide or methane.)
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