Saturday, February 25, 2012

NASA Satellite Finds Earth's Clouds are Getting Lower

 
Clouds Over Southern Indian Ocean
This image of clouds over the southern Indian Ocean was acquired on July 23, 2007 by one of the backward (northward)-viewing cameras of the Multi-angle Imaging SpectroRadiometer (MISR) instrument on NASA’s polar-orbiting Terra spacecraft. The area covered by the image is 247.5 kilometers wide and 660 kilometers long, and is shown in an approximate perspective view at an angle of 60 degrees off of vertical. The solar zenith angle ranges from about 83 degrees at the top of the image to 88 degrees at the bottom, hence the lengthening of shadows cast by the clouds on the underlying ocean surface and reddening of the hues in the foreground. Stereoscopic analysis of the data from multiple MISR cameras indicates that the cloud tops visible here range in altitude from about 0.6 to 2.5 miles (1 to 4 kilometers). A new university study using MISR data revealed an overall trend of decreasing global cloud height during the last decade.

Earth's clouds got a little lower -- about one percent on average -- during the first decade of this century, finds a new NASA-funded university study based on NASA satellite data. The results have potential implications for future global climate.

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 study, published recently in the journal Geophysical Research Letters, revealed an overall trend of decreasing cloud height. Global average cloud height declined by around one percent over the decade, or by around 100 to 130 feet (30 to 40 meters). Most of the reduction was due to fewer clouds occurring at very high altitudes.

Lead researcher Roger Davies said that while the record is too short to be definitive, it provides a hint that something quite important might be going on. Longer-term monitoring will be required to determine the significance of the observation for global temperatures.

A consistent reduction in cloud height would allow 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 Davies. "But it must be due to a change in the circulation patterns that give rise to cloud formation at high altitude."

NASA's Terra spacecraft is scheduled to continue gathering data through the remainder of this decade. Scientists will continue to monitor the MISR data closely to see if this trend continues. 

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