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Possible silver lining: Can cloud cover slow the pace of global warming? Photo: Ryan Osland

When it comes to global warming, do clouds have a silver lining?

One of the key questions at the heart of climate change science is whether more evaporation and more cloud cover slow the pace of global warming.

The answer, according to the latest report by the Intergovernmental Panel on Climate Change, is probably not. Global warming will probably affect clouds in ways that exacerbate rather than ease the problem.

In what Professor Steven Sherwood deems ''one of the stories of the report'', the IPCC has for the first time stated that it is likely clouds will have a positive heat feedback effect on climate change.

Every cloud has both a greenhouse effect by trapping in heat but also a cooling one because it reflects sunlight back to space. For high clouds, the net effect is small but not so for low clouds, where the albedo, or reflective effect, is relatively important.

If climate change results in fewer and higher clouds, warming will increase as oceans and land will absorb more heat, said Professor Sherwood, a clouds and aerosols expert at the University of NSW.

Uncertainties about the impact of clouds - other than it will enhance warming - remain largely undiminished from previous IPCC reports, he said. Still, getting a better understanding is vital as cloud changes potentially rival the melting at the poles in terms of driving further climate change.

''The ice is up at the poles where there's not a lot of sun anyway'' while much of the clouds are in the tropics, said Professor Sherwood, who was a contributing author to the latest IPCC report.

''There's a lot of low clouds and they cover a lot of the world's oceans, so a small percentage change has a big effect,'' he said.

The trouble is, pinning down how so-called marine low clouds in particular will behave as temperatures rise is proving difficult. ''It's a major source of uncertainty and leaves open the possibility of really large climate changes or more modest ones in the future,'' he said.

As the summary for policymakers notes, the role of aerosol emissions and their interactions with clouds ''continue to contribute to the largest uncertainty to estimates'' of how the Earth's energy budget is changing.

Since every cloud forms on an aerosol particle, changing the amount of particulate matter we send skyward can alter cloud formation.

''Aerosols are a kind of wildcard,'' Professor Sherwood said. ''It makes it hard for us to observe changes and just extrapolate them into the future.''

In recent decades, the rise of China and India's economies, in particular, has caused a surge in aerosol production that has curbed the pace of global warming. More aerosols mean more light reflected back to space and possibly greater cloud cover.

Severe smog over cities such as Beijing, though, has prompted pledges to cut pollution which, while good for the health of residents, will have an impact on climate change.

''If they do that, that will give an extra boost to global warming because it's going to be removing one of the things that have been masking the warming effect from greenhouse gases,'' Professor Sherwood said.