Saturday, July 17, 2010

Climate change and the role of clouds


ABC Environment, 12 JULY 2010

Clouds and water vapour play an important part in climate change, but the exact details of their role remains cloudy.

PHILOSOPHERS, artists, politicians and daydreaming kids have long looked to the sky's ungraspable clouds in search of meaning. But those innocuous clumps of water and ice, seen by British poet William Wordsworth as lonely wanderers, are being closely watched by climate scientists for clues to our future.

"It still remains one of the top challenges in climate research - getting a better understanding of clouds,'' says Steve Sherwood, a professor of atmospheric physics at the University of New South Wales Climate Research Centre. "It's the main known unknown in predicting future climate.''

For the average observer gazing wistfully into the troposphere where the vast majority of clouds reside, clouds can be a confusing and complex phenomenon to understand. With the climate too, clouds have a complicated relationship because they both cool and warm the planet, acting like a shade in reflecting the sun's radiation back into space but also stopping heat escaping from the ground.

Water vapour in the atmosphere is invisible and everywhere - it is our most abundant greenhouse gas. The way that water vapour affects the climate is similar to that of carbon dioxide, methane and other gases. They all absorb and re-emit radiation, acting like a blanket enfolding the globe.

"We have quite a good understanding of water vapour,'' says Sherwood. "All our models say water vapour will increase by about seven per cent for every one degree of warming and we have observations to show that's happening."

It is only when the water vapour condenses into a cloud that it can be seen. When we can see it, its bright whiteness also reflects sunlight back into space.

"The problem that we have is predicting how much of that vapour will condense to clouds and how much light they reflect. That is very challenging," explains Sherwood.

"If we had a planet with no clouds then the surface temperature would not be all that different - a little cooler maybe but only by a few degrees. But if you took out the water vapour then it would be significantly cooler - about 15ºC."

What goes up

The race is on to get a better understanding of how clouds will change as the globe continues on a warming track which has seen each decade since the 1940s getting progressively warmer.

While it might seem like common sense that a warmer world means more evaporation and, in turn, means more clouds, it is a common misunderstanding.

A warmer planet will likely speed up the hydrological cycle - the process of evaporation and rainfall - but it won't necessarily give any more or less clouds in the sky.

"All evaporation is eventually matched by rainfall - everything that goes up, must come down,'' says Dr Rob Colman, an expert on climate feedbacks at the Centre for Australian Weather and Climate Research.

"What stays up there as water vapour depends on temperature. If you warm the air by one degree then it can hold more water.

"As greenhouse gases increase, the air is warmer and holds more water vapour and then further reinforces that warming."

Colman says that it is primarily the fault of clouds that predictions of future temperature rises are given with margins of error.

"Very small changes in the way that clouds behave can have a very big impact,'' he says. "One major source of uncertainty of whether we will have warming of 2ºC or 4ºC by the end of the century comes from uncertainty over how clouds will respond."

The future is cloudy

Research published in the journal Science last year looked at the northeast Pacific region and found that low-level clouds seem to thin-out as the oceans get warmer.

A gradual reduction in this cloud cover could already be kick-starting a complex feedback cycle.

Fewer low-level clouds would lead to more sunlight hitting the oceans, causing further warming and further reducing cloud cover.

One US$30 million three-year project, the VAMOS Ocean Cloud Atmosphere Land Study which concludes this year, has seen more than 200 scientists from 10 countries studying the complex feedbacks between clouds, oceans and the land. In particular the team is focussing on the vast decks of clouds, greater in size than the USA, which hover over the Pacific.

These clouds are seen as playing a significant role in the globe's energy balance so understanding their reaction to increased temperatures coming from rising greenhouse gas emissions will be crucial.

Giving clouds a spray

As well as the generation of greenhouse gases there are other emissions which are taking scientists' interest.

Clouds also need aerosols - tiny particles from both natural and human activities - to be able to form. One research project being part-funded by billionaire Bill Gates is trying to find out if spraying sea water containing salt into the atmosphere could generate whiter clouds in the future.

In 2007, an 11-strong team of researchers including five from Australia used computer models to suggest that an increase in clouds and rainfall observed over parts of Australia since the 1950s was, in part, down to pollution drifting across from Asia. When the researchers removed these human-caused aerosols from their calculations, the models failed to recreate the trends.

But being able to set a benchmark to gauge those long-term changes is tricky. Meteorologists have been keeping daily records of rainfall and temperature since the mid 1800s. Cloud observations have been carried out by people on land and sea but the records don't stretch back as far.

Australia's own Bureau of Meteorology is currently compiling records of observations made at 165 surface stations from 1957 onwards.

Colman says preliminary analysis showed they were consistent with more recent satellite observations which, until recently, have only been able to give a flat picture of cloud. When viewed from space, clouds sitting beneath other clouds are obscured. But researchers like Sherwood and Colman have a new weapon, with data from a sophisticated new group of satellites now becoming available.

"For the first time we are now getting hold of very accurate three-dimensional data,'' says Colman. "It's a very exciting time for cloud science."

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