Scientists at the University of Auckland in New Zealand analyzed the first 10 years of global cloud-top height measurements (from March 2000 to February 2010) from the Multi-angle Imaging SpectroRadiometer (MISR) instrument on NASA’s Terra spacecraft.
The analysis, published in the journal Geophysical Research Letters, 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.”
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.
MISR, built and managed by NASA’s Jet Propulsion Laboratory, Pasadena, Calif., is one of five instruments on NASA’s Terra spacecraft, launched in December 1999. The instrument uses nine cameras at different angles to produce a stereo image of clouds around the globe, allowing measurement of their altitude and movement. For more on MISR, visit: www-misr.jpl.nasa.gov. For more on Terra, visit: terra.nasa.gov.
Another NASA mission that studies clouds is NASA’s CloudSat, also built by JPL and launched in 2006. CloudSat is the first satellite that uses an advanced radar to “slice” through clouds to see their vertical structure, providing a completely new observational capability from space. CloudSat’s primary goal is to furnish data needed to evaluate and improve the way clouds are represented in global models, thereby contributing to better predictions of clouds and thus to their poorly understood role in climate change and the cloud-climate feedback. For information on NASA’s CloudSat mission, visit: cloudsat.atmos.colostate.edu and www.nasa.gov/cloudsat.
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