Earth's Clouds are Getting Lower
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 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.
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 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 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 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 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 current research was funded by NASA’s Jet Propulsion Laboratory, which is managed for NASA by the California Institute of Technology.