THE heated debate over the causes of global warming took a new turn last week with the publication of a controversial study suggesting that changes in cloud cover could be to blame for rising temperatures. The authors of the study say they have discovered a warming effect stronger than that attributed to greenhouse gases, but critics accuse them of jumping to unjustified conclusions, and some question whether the work should have been published at all.
The storm is raging round a study of earthshine, the sunlight reflected off the Earth onto the moon, giving a dim glow to the shadowed areas. This is relevant to climate studies because it reveals how much sunlight Earth absorbs. Enric Palle and colleagues at the Big Bear Solar Observatory in Big Bear City, California, and the California Institute of Technology in Pasadena have been measuring earthshine since 1998, and wanted to work out how it had changed before then. They reasoned that changes in earthshine were the result of changes in global cloud cover, and sure enough, when they compared their earthshine measurements with satellite measurements of cloud cover, they found a straightforward correlation. They then used earlier satellite data to infer changes in earthshine back to 1984. Palle says this reveals that earthshine dropped steadily between 1984 and 2000, although the trend has since reversed.
This method is controversial, however. Measurements of cloud cover are notoriously unreliable, says Kevin Trenberth, a climate modeller at the National Center for Atmospheric Research in Boulder, Colorado. Many of these measurements come from meteorological satellites, and these are difficult to calibrate for a variety of reasons 鈥� such as changes in the satellites鈥� altitudes over time. Trenberth says that most of the apparent change in cloud cover is the result of measurement problems. 鈥淚 don鈥檛 believe the downward trend is real,鈥� he says.
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But Palle is confident that the change is real and is drawing further inferences from it. Given that the sun鈥檚 radiance is roughly constant, a drop in the amount of light reflected by the Earth means it must be absorbing more. He and his colleagues suggest that this extra absorbed energy could account for the increase in global temperatures over the period of their observations. The result is 鈥渃onsistent with the large tropospheric warming that has occurred over the most recent decades,鈥� their paper says.
Other climate scientists insist that there is no validity in the link Palle has made between earthshine and climate change. Bruce Wielicki from NASA鈥檚 Langley Research Center in Hampton, Virginia, says that warming in the troposphere (lower atmosphere) cannot be inferred from these results. He leads the science team of a mission called CERES 鈥� the Clouds and the Earth鈥檚 Radiant Energy System 鈥� which has instruments on three satellites measuring both the amount of visible light reflected by the Earth and the amount of infrared radiation it is emitting.
Without measuring both, he says, it is impossible to infer anything about warming. CERES measurements, for instance, show that clouds not only reflect visible light, they also trap infrared radiation in the lower atmosphere. The two effects almost cancel out, so cloud cover makes little difference to the energy the atmosphere retains. Earthshine tells only one side of the story.
Wielicki even goes so far as to say that the paper, published in Science this week, should not have made it to print. 鈥淚f it had been mine to review, I would have said no,鈥� he says. John Harries, an expert in Earth observation at Imperial College London recommends taking the authors鈥� conclusion 鈥渨ith a pinch of salt鈥�.
Though Trenberth does not accept their conclusions, he does see value in the work as a new tool in the armoury for investigating climate change. Future earthshine measurements using the telescope at the Big Bear Solar Observatory could be an innovative way of probing the effect of clouds on climate, he says.