Clouds Mitigate Global Warming, New Evidence Shows

Clouds Mitigate Global Warming, New Evidence Shows
November 1, 2007

In a study published in the American Geophysical Union's Geophysical Research Letters on August 9, researchers at the University of Alabama-Huntsville (UAH) provide more real-world evidence of the self-regulating nature of the Earth's atmosphere.

If the self-regulatory mechanism is confirmed by additional research, it will represent yet another deal-breaker for the hypothesis that has propped up climate alarmism thus far.

Positive Feedback Theory

Key to predictions of runaway global warming are alleged "positive feedback" cycles that supposedly will build upon each other to cause runaway global warming. Existing climate models, for example, assume a warmer atmosphere will cause an increase in high-altitude cirrus clouds--a positive feedback into the climate system since cirrus clouds trap outgoing radiation emitted by the Earth.

When you feed a warming scenario envisioning the doubling of atmospheric carbon dioxide levels--which would on its own create no more than 1.2º C of warming--into a climate model that has been turbocharged with positive feedback factors such as cirrus clouds, the resulting estimated warming increases by 250 percent to 3º C.

Self-Regulating Clouds

However, many scientists have questioned the validity of the hypothetical positive feedback mechanism. Massachusetts Institute of Technology atmospheric physicist Richard Lindzen, for example, proposed the "iris effect" in 2001 as an explanation for why the amplified warming has never materialized.

Analyzing a limited set of data, Lindzen hypothesized cirrus clouds and associated moisture work in opposition to surface temperature changes. The data seemed to indicate that when the Earth's surface warms, clouds open up to allow heat to escape. A cooling surface, in turn, causes clouds to close and trap heat.

This elegant, self-regulatory, atmospheric mechanism was soon attacked for being based on limited data and the inability of other researchers to identify the effect in other cloud and temperature data sets.

New Data Support Theory

But the new research from the University of Alabama-Huntsville supports the validity of the iris effect.

Analyzing six years of data from four instruments aboard three NASA and NOAA satellites, the UAH researchers tracked precipitation amounts, air and sea surface temperatures, high- and low-altitude cloud cover, reflected sunlight, and infrared energy escaping out to space.

As opposed to the hypothesized positive feedback of the climate models, the UAH data show a strong negative feedback. As the tropical atmosphere warms, cirrus clouds decrease, allowing infrared heat to escape from the atmosphere to outer space.

"To give an idea of how strong this enhanced cooling mechanism is, if it was operating on global warming, it would reduce [climate model-based] estimates of future warming by 75 percent," said UAH researcher Roy Spencer in a media release.

"The role of clouds in global warming is widely agreed to be pretty uncertain," Spencer said. "Right now, all climate models predict that clouds will amplify warming. I'm betting that if the climate models' 'clouds' were made to behave the way we see these clouds behave in nature, it would substantially reduce the amount of climate change the models predict for the coming decades."

Steven Milloy ( publishes and He is a junk science expert and an adjunct scholar at the Competitive Enterprise Institute. This article first appeared on and is reprinted with permission.