Credit: istockphoto/@tbradford

As Earth's average temperature rises, clouds will, on the whole, accelerate global warming by trapping more heat. The first planet-wide, observation-based study of the climatic effect of clouds confirms estimates previously produced by models.

Atmospheric scientist Andrew Dessler of Texas A&M University in College Station analysed satellite data gathered between March 2000 and February 2010 to assess short-term variations in the amount of visible and infrared radiation emitted to space1. Subtracting influences such as Earth's surface reflectivity and the heat-trapping effect of atmospheric water vapour, he estimated how clouds affected the planet's radiation balance as a function of temperature over the decade. Dessler's analysis suggests that clouds enhance warming by trapping, on average, an extra 0.54 watts per square metre for every 1-degree-Celsius rise in global average temperature.

But uncertainty in the estimate indicates that clouds could actually exert a small cooling effect as temperature rises, says Dessler. Even so, he notes, that slight negative feedback wouldn't be nearly enough to cancel out larger, well-constrained positive climatic feedbacks, such as that provided by water vapour.