How hot will Earth be by 2100? The answer's in the clouds

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.)

If you want to project future climate change, you need a solid handle on how these competing feedbacks balance. What are the conditions where increasing evaporation will lead to more cloud formation? How much will cloud formation increase? Will it balance out the greenhouse effect of the water vapor?

These aren’t easy questions to answer, and this uncertainty accounts for much of the uncertainty seen in climate projections. Models that project a planet that is 1.7 to 4.4°C warmer by 2100 would give us a tighter range if we could model clouds more confidently. (...)

Knowing what we don't know will help direct future research that aims to reduce uncertainty in climate projections. Considering that a recent study found that climate models may be underestimating the positive feedback from clouds and that temperatures could therefore end up above the projected range, it’s critical that we nail down the role clouds will play in climate change.

Geophysical Research Letters, 2011. DOI: 10.1029/2011GL047632 (About DOIs).

Photo by Nicholas_T

 

By Scott Johnson | 05.07.2011

 

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