On the Edge

One of the most important things to know about earth’s climate is a quantity called climate sensitivity. Actually there are two uses of this term. One is the temperature change caused by a change in climate forcing: if we increase the heat budget at earth’s surface by, say, one watt per square meter (1 W/m^2), how much will temperature rise? The other is the temperature change caused by a doubling of CO₂ in the atmosphere. This can be called dT_2x, and that’s the one I’d like to discuss.

If there weren’t any feedbacks in the climate system, it would be straightforward to compute; dT_2x = 1.2^oC. But there are feedbacks. For example, there’s ice-albedo feedback. Ice and snow are highly reflective, bouncing much of the incoming solar energy right back into space. Global warming means less snow and ice, means less solar energy reflected back to space, means more solar energy absorbed by earth, means even more warming. This is an example of a positive feedback; “positive” doesn’t mean “good,” it simply means that the feedback tends to amplify a change. Negative feedbacks tend to suppress a change. Another example is water vapor feedback. Water vapor is also a greenhouse gas; warming means more water vapor in the air, means more greenhouse effect from water vapor, means even more warming — another positive feedback.

It turns out that most of the feedbacks in the climate system are positive, so man-made global warming from increased CO₂ (and other greenhouse gases) tends to be amplified. We expect that the climate sensitivity to doubling CO₂ (dT_2x) is greater than 1.2^oC. How much greater is a hotly debated topic.