warming.

Research and field-testing on what they call "geo-engineering" of the earth's atmosphere to limit risk of climate change must begin quickly, say scientists from the University of Calgary in Canada, and the University of Michigan and Carnegie Mellon University in the US.

Studies on geo-engineering or solar radiation management (SRM) should be undertaken collectively with government funding, rather that unilaterally by nations, argue the scientists.

They say SRM would involve releasing mega-tonnes of light-scattering aerosol particles in the upper atmosphere to reduce earth's absorption of solar energy, thereby cooling the planet.

Another technique would be to release particles of sea salt to make low-altitude clouds reflect more solar energy back into space, the scientists say.

"Collaborative and government-supported studies on solar-radiation management will help identify technologies to combat climate change," writes David Keith of Calgary university in an article in Nature this week.

"Solar-radiation management may be the only human response that can fend off rapid and high-consequence climate change impacts. The risks of not doing research outweigh the risks of doing it," says Keith, who has co-authored the opinion piece with Edward Parson at the University of Michigan and Granger Morgan at Carnegie Mellon University.

However, SRM should not take the place of deep cuts in industrial greenhouse gas emissions and taking action to adapt to climate change, say the scientists.

They say: "We must develop the capability to do SRM in a manner that complements such cuts, while managing the associated environmental and political risks."

The scientists want governments worldwide to establish a global research budget for SRM. This budget should grow about $10 million to $1 billion a year between now and 2020, they say.

They say research results should be made available to every nation and risk assessments be as transparent and international as possible.