Roger Davies, University of Auckland: Cloud height changes may lower global temperature

February 24, 2012

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 may be in play in the Earth’s climate.

Published in the journal Geophysical Research Letters, the analysis of the first ten years of data from the NASA Terra satellite revealed an overall trend of decreasing cloud height. Global average cloud height declined by around 1 per cent over the decade, or around 30 to 40 metres. 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,” explains lead researcher Professor Roger Davies. Longer-term monitoring will be required to determine the significance of the observation for global temperatures.

A consistent reduction in cloud height would allow the Earth to cool to space more efficiently, reducing the surface temperature of the planet and potentially slowing the effects of global warming. This may represent a “negative feedback” mechanism – a change caused by global warming that works to counteract it. “We don’t know exactly what causes the cloud heights to lower,” says Professor Davies, “but it must be due to a change in the circulation patterns that give rise to cloud formation at high altitude.”

Until recently however, 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,” says Professor 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.”

University of Auckland physicists Professor Davies and Matthew Molloy, a BSc Honours student, analysed measurements of the Multiangle Imaging SpectroRadiometer (MISR), one of the instruments on the Terra satellite launched by NASA in December 1999. The instrument uses 9 cameras at different angles to produce a stereo image of clouds around the globe, allowing measurement of their altitude and movement.

The results to date reveal a complex pattern of decreases in cloud altitude across some regions of the globe and increases in others, with the El Niño / La Niña phenomenon in the Pacific producing the strongest effect and greatest variation from year to year. After taking into account all these differences, however, the overall trend was of decreasing cloud height from 2000 to 2010.

The Terra satellite is scheduled to continue gathering data through the remainder of this decade. “If cloud heights come back up in the next ten years we would conclude that they are not slowing climate change,” says Professor 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.”

Professor Davies holds the Buckley Glavish Chair in Climate Physics at The University of Auckland. The current research was funded by NASA’s Jet Propulsion Laboratory, which is managed for NASA by the California Institute of Technology.