Clouds play contradictory roles in climate change. Photo: Shannon Hall

The world's storm tracks are shifting polewards and clouds are rising higher in the atmosphere, two trends that are likely to exacerbate global warming, US scientists say...

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 New analysis of satellite data for the 1982-2009 period by California's Scripps Institution of Oceanography has resolved what the researchers say is one of the biggest uncertainties involving climate science.

Clouds play contradictory roles in the climate. They have a cooling effect because they reflect solar radiation back to space but they also have a warming role by restricting the thermal infrared radiation from the Earth.

Clouds play contradictory roles in climate change.

Clouds play contradictory roles in climate change. Photo: Shannon Hall

"A small cloud change can have a substantial effect on Earth's radiation budget," Joel Norris, Professor of Climate and Atmospheric Sciences at Scripps, told Fairfax Media.

While climate models have generally predicted clouds would change as greenhouse gas levels rise in ways that added to global warming, observations have not matched the projections - until now.

"It was known that there was robust agreement among climate models that global warming would lead to expansion of the subtropical dry zone, retreat of mid-latitude storm track clouds, and rise of the highest cloud tops," Professor Norris said.

The world's dry zones are expanding as storm tracks move towards the poles.

The world's dry zones are expanding as storm tracks move towards the poles. Photo: Amy Paton

"What was lacking was observational confirmation due to artefacts present in the satellite cloud record that overwhelmed any real signal."

By removing those distortions - caused by altered satellite orbits, sensor degradation and other factors - the researchers found large-scale cloud patterns were in line with model predictions for the period studied.

The warming mechanisms themselves were well understood, such as the reasons why cloudy nights are typically warmer than clear ones.

Clouds are rising, adding to their 'blanket' effect.

Clouds are rising, adding to their 'blanket' effect. Photo: Nick Moir

"As cloud top grows higher, the 'cloud blanket' grows thicker, and becomes more effective at keeping the Earth warm," Professor Norris said.

The biggest cloud effect on the reflection of solar radiation occurs in ocean regions where having fewer clouds means more heat is absorbed by the relatively dark surface of the seas.

"Even if there is no change in the overall coverage of clouds on the earth, clouds closer to the pole reflect less solar radiation because there is less solar radiation coming in closer to the pole," he said.

For people and ecosystems living on the fringe of the world's dry zones - such as parts of southern Australia or California - one impact of reduced cloud cover may be less rain.

"Less cloudiness can [also] be an add-on factor increasing surface temperature and evaporation," Professor Norris said.

(See Nature chart below showing the areas where clouds are increasing or decreasing.)

The research also took into account the planet-wide recovery from two major volcanic eruptions - the 1982 El Chichón eruption in Mexico and 1991's eruption of Mt Pinatubo in the Philippines - which had a large net cooling effect for several years because of the aerosols produced.

'Much more serious'

Professor Steven Sherwood, a climate researcher at the University of NSW specialising in clouds, said the Scripps research is the "first confident measurement" of the rising altitude of clouds. It also confirms a 2012 study by Bender et al in Science that indicated a drop in cloud cover over subtropical oceans.

The cloud changes may not have big direct impacts but their importance lies in confirming positive feedbacks that have long been anticipated by researchers, he said. 

These feedbacks "make global warming a much more serious problem than it otherwise would be", Professor Sherwood said.

"Unfortunately we are not finding anything that will reduce the danger of climate change — instead, results are pointing to things being as bad as feared or maybe a bit worse," he said.

The Nature paper concludes that the trends in cloud changes will continue if greenhouse gas levels in the atmosphere keep rising, "unless offset by unpredictable large volcanic eruptions".

by: Peter Hannam  July 12, 2016

Follow Peter Hannam on Twitter and Facebook.


original story HERE

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