Lower clouds may lower global temperature - study

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The sky may not be falling, but the clouds certainly are - and it could offer a glimmer of hope in the fight to slow global warming.

A Kiwi study has revealed clouds around the world dropped an average one per cent, or 30m-40m in the decade ending 2010.

The change was largely due to fewer clouds forming at very high altitudes, say Auckland University scientists who analysed data from a Nasa satellite.

Lead researcher Professor Roger Davies said a consistent reduction in cloud height would allow the Earth to cool more efficiently, reducing the planet's surface temperature and potentially slowing the effects of global warming.

The scientists analysed cloud heights from the years 2000 to 2010 using a Multiangle Imaging SpectroRadiometer instrument on Nasa's Terra spacecraft.

The instrument uses nine cameras from different angles, allowing researchers to measure the altitude and movement of clouds around the world.

The overall trend was for decreasing cloud height.

The biggest variations were seen in the central Pacific and over Indonesia, believed to be connected to weather patterns El Nino and La Nina.

In New Zealand, the changes were more subtle from year to year. But wherever you are you're unlikely to have noticed the change.

"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," said Professor Davies.

Until recently it was impossible to measure the changes in global cloud heights and to understand their effect on climate change.

Longer-term monitoring was needed to determine the significance of the finding for global temperatures, said Professor Davies.

The satellite is scheduled to continue gathering data for the remainder of this decade.

"If cloud heights come back up in the next 10 years we would conclude that they are not slowing climate change. But if they keep coming down it will be very significant. We look forward to the extension of this climate record with great interest."