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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Evidence is building that net cloud feedback is likely positive and unlikely to be strongly negative.

The effect of clouds in a warming world is difficult to predict. One challenge is that clouds have both warming and cooling effects. Low-level clouds in particular tend to cause a cooling effect by reflecting sunlight. High-level clouds tend to cause warming by trapping heat.

As the planet warms, clouds can cause cooling if there are more low-level clouds or more high-level clouds. Clouds can cause warming if the opposite is true. So to work out the total effect from clouds, it's not enough to measure just the total amount of clouds. Scientists also need to know which types of clouds are increasing or decreasing.

Some climate scientists are skeptical that greenhouse gas emissions will cause dangerous warming, such as Richard Lindzen and Roy Spencer. Their skepticism hinges mainly on uncertainty related to clouds. They tend to believe that as the planet warms, low-level cloud cover will increase. This would mean the Earth's overall reflectiveness would increase, causing cooling. This cooling would cancel out some of the warming from an increased greenhouse effect.

However, recent research has provided evidence that this is not the case. Most of the cloud feedback uncertainty is due to cloud changes near the equator, in the tropics and subtropics. Two separate studies both looked at cloud changes in these regions using a combination of ship-based cloud observations, satellite observations and climate models. They both concluded that the cloud feedback in this region appears to be positive, meaning more warming.

Another study used satellite measurements of cloud cover over the entire planet to attempt to determine the cloud feedback from 2000 to 2010. The scientists concluded that although a very small negative feedback (cooling) could not be ruled out, the overall short-term global cloud feedback is probably positive (warming), and may be strongly positive. Measurements showed that it is very unlikely that the cloud feedback will cause enough cooling to offset a significant amount of human-caused global warming.

So while clouds remain a significant uncertainty, the evidence is building that clouds will probably cause the planet to warm even further, and are very unlikely to cancel out much of human-caused global warming. It's also important to remember that there many other feedbacks besides clouds. There is a large amount of evidence that the net feedback is positive and will amplify global warming.

Updated on 2013-03-12 by John Cook.

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