Lowering clouds may help fight against

The clouds might be fighting back against global climate change, said scientists in New Zealand Wednesday. The research from the University of Auckland on changes in cloud height in the decade to 2010 has provided the first hint of a cooling mechanism that could be in play in the Earth's climate. The analysis of data from the NASA Terra satellite showed an overall trend of decreasing cloud height of about 1 percent, or 30 to 40 meters, over the decade, said a statement from the university. Most of the reduction was due to fewer clouds occurring at very high altitudes. "This is the first time we have been able to accurately measure changes in global cloud height and, while the record is too short to be definitive, it provides just a hint that something quite important might be going on," lead researcher Professor Roger Davies said in the statement. Longer-term monitoring would be required to determine its significance for global temperatures. A consistent reduction in cloud height would allow the Earth to cool more efficiently, reducing the surface temperature of the planet and potentially slowing the effects of global warming, he said. This could represent a "negative feedback" mechanism: a change caused by global warming that worked to counteract it. "We don't know exactly what causes the cloud heights to lower, but it must be due to a change in the circulation patterns that give rise to cloud formation at high altitude," said Davies. Until recently, it was impossible to measure the changes in global cloud heights and understand their contribution to global climate change. "Clouds are one of the biggest uncertainties in our ability to predict future climate," said Davies. "Cloud height is extremely difficult to model and therefore hasn't been considered in models of future climate. For the first time we have been able to accurately measure the height of clouds on a global basis, and the challenge now will be to incorporate that information into climate models. "It will provide a check on how well the models are doing, and may ultimately lead to better ones." The research team analyzed measurements of the Multiangle Imaging SpectroRadiometer (MISR), one of the instruments on the Terra satellite launched by NASA in December 1999. The MISR used nine cameras at different angles to produce a stereo image of clouds around the globe, allowing measurement of their altitude and movement. The results showed a complex pattern of decreases in cloud altitude across some regions of the globe and increases in others, with the El Nino-La Nina phenomenon in the Pacific producing the greatest variation from year to year, but the overall trend was of decreasing cloud height from 2000 to 2010. "If cloud heights come back up in the next 10 years, we would conclude that they are not slowing climate change," said Davies. "But if they keep coming down it will be very significant. We look forward to the extension of this climate record with great interest." The research, published in the journal Geophysical Research Letters, was funded by NASA's Jet Propulsion Laboratory.