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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 complicated. One challenge is that clouds cause both warming and cooling. Low-level clouds tend to cool by reflecting sunlight. High-level clouds tend to warm by trapping heat.

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As the planet warms, clouds have a cooling effect if there are more low-level clouds or less high-level clouds. Clouds would cause more warming if the opposite is true. To work out the overall effect, scientists need to know which types of clouds are increasing or decreasing.

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

However, recent evidence indicates this is not the case. Two separate studies have looked at cloud changes in the tropics and subtropics using a combination of ship-based cloud observations, satellite observations and climate models. Both found that 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 measure cloud feedback. Although a very small negative feedback (cooling) could not be ruled out, the overall short-term global cloud feedback was probably positive (warming). It is very unlikely that the cloud feedback will cause enough cooling to offset much of human-caused global warming.

Other studies have found that the climate models that best simulate cloud changes are the ones that find it to be a positive feedback, and thus have higher climate sensitivities. Steven Sherwood explains one such study:

While clouds remain an 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 2014-01-02 by John Cook.

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