There’s a deceptively simple number at the heart of the science of climate change: the sensitivity of Earth’s climate system to an addition of greenhouse gas like CO2. This variable gets defined in technical terms on several different timescales, but it all boils down to how much global warming we’ll get if we increase CO2 by a given amount.
All kinds of complex interactions are contained within this number, including all the feedback loops that amplify or dampen the warming response. One of the harder feedbacks to pin down has been changes in clouds. As the world warms, more water vapor ends up in the atmosphere—and water vapor is an important greenhouse gas. But the bright, low clouds that water vapor can form reflect sunlight, shading and cooling the Earth.
It turns out the net result of increased water vapor enhances warming rather than limiting it. A new study by Lawrence Livermore National Laboratory’s Chen Zhou, Mark Zelinka, and Stephen Klein reveals an interesting interaction that makes this even more complicated. It’s not enough to figure out what clouds will do in general—there's not some single number that you can get to stand in for clouds. Instead, there are spatial patterns to clouds' effects, and they vary over time. This has some pretty interesting implications for understanding the last few decades and what’s coming in the future.
The researchers used a number of climate model simulations of the last century-and-a-half to examine how this works. Some simulations were run with all the historical changes to “forcings” like greenhouse gas concentrations and solar activity, as well as the observed patterns of sea surface temperatures. Other simulations were run with a twiddled knob—either holding the forcings constant at their pre-Industrial-Revolution levels or their current levels or removing the long-term warming trend from sea surface temperatures.
By comparing the balance of energy entering and leaving these virtual atmospheres to the global temperature change, researchers could work out the contribution of the cloud feedback over time. That contribution varied significantly from decade to decade. Even though clouds were an amplifying feedback overall, there were periods of time where they counteracted warming....