About seven years ago I read a fascinating paper by Roy Spencer et al. that argued that clouds respond to Earth’s surface temperatures in ways that moderate them–cooling the surface in response to warming, warming it in response to cooling. It led me to an earlier paper by Richard Lindzen that argued that, at least in a major region above the Pacific, clouds respond to surface temperature changes in a way analogous to how the human eye’s iris responds to light. In response to dimmer light, the iris lets more light in; in response to brighter light, it lets less light in. Similarly, in response to higher surface temperatures, clouds let more infrared (heat) escape to space, and in response to lower surface temperatures, they let less escape.

Both papers fascinated me because they were consistent with my notion that a wise Designer would have made a climate system that was robust, resilient, and self-correcting (that is, dominated by negative rather than positive feedbacks), not fragile and prone to catastrophic change due to comparatively minute influences (such as CO2’s rising from 28 thousandths of a percent to 56 thousandths of a percent or the atmosphere). But of course, that didn’t prove their theory true. Actual empirical evidence needs to be brought to bear. Both Lindzen and Spencer offered a good deal of such.

Nonetheless, both papers were controversial because they implied that clouds were a negative feedback on surface temperature rather than a positive one–i.e., rather than increasing initial warming, they reduce it. That cut across the orthodoxy of the climate alarmist establishment in control of the UN IPCC. So, with little actual hard evidence, various AGW alarmists started referring to Lindzen’s paper as “discredited.”

Now a new paper, “Missing iris effect as a possible cause of muted hydrological change and high climate sensitivity in models,” by Thorsten Mauritsen and Bjorn Stevens, in the journal Nature Geoscience, is giving new life to Lindzen’s theory, providing substantial empirical evidence for it.

The implication for climate debates? “Climate sensitivity” (how much warming comes from doubled atmospheric CO2 concentration) would be significantly lower than the CMIP-5 computer climate models suggest, since they treat clouds as a positive feedback, but the iris effect would mean they’re negative.

Judith Curry reviews and comments on Mauritsen and Stevens’s article, as does Rud Istvan.

The case for high climate sensitivity, and hence for any expensive action to mitigate manmade climate change, grows weaker.

 

Featured image courtesy of Liane Metzler