The sky may be falling -- a little closer to the Earth, according to NASA satellite image data. Over the last decade, the height of cloud cover has been lowered by 1 percent or 30 to 40 meters.

Scientists at the University of Auckland have analyzed the first 10 years of global cloud heights from 2000 to 2010 based on data gathered by the Multi-Angle Imaging SpectroRadiometer instrument on NASA's Terra satellite. The results show that the global average cloud height declined by around 1 percent over the decade and most of the reduction was due to fewer clouds forming at high altitudes.

"We don't know exactly what causes the cloud heights to lower," study researcher Roger Davies of the University of Auckland in New Zealand said in a statement. "But it must be due to a change in the circulation patterns that give rise to cloud formation at high altitude."

If the cloud cover continues to descend, it could have an important effect on global climate change. The lower cloud cover could allow the Earth to cool and offset the global warming trend.

"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," said Davies.

The researchers haven't pinpointed the exact cause of why the clouds are suddenly dropping in altitude, but it could be the result of a change in circulation patterns in high-altitude clouds. Although clouds may seem insubstantial to affect the Earth's climate they could slow down the effects of global warming or exacerbate them.

"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 team's study was published in the journal Geophysical Research Letters.