The sky is falling - but it may be good news for the planet. New Zealand scientists say that the Earth's clouds are getting lower, in a mechanism that could help to counter global warming.
An analysis of the first ten years of data from the NASA Terra satellite shows that global average cloud height has fallen by 30 to 40 metres, or about one percent. Most of the reduction was due to fewer clouds occurring at very high altitudes.
There were 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 a noticeable fall between 2000 and 2010.
"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," says Professor Roger Davies.
A consistent reduction in cloud height would allow the Earth to cool to space more efficiently, and could represent a 'negative feedback' mechanism.
"We don’t know exactly what causes the cloud heights to lower, but it must be due to a change in the circulation patterns that give rise to cloud formation at high altitude," says Davies.
Until recently, however, it was impossible to measure changes in global cloud heights.
“Clouds are one of the biggest uncertainties in our ability to predict future climate. Cloud height is extremely difficult to model and therefore hasn’t been considered in models of future climate," says Davies.
"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 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."