(Bloomberg) — A fleet of sail-powered, ocean-going vessels spraying seawater in the air could save billions of dollars and allow the world to continue emitting carbon dioxide like it does by burning oil and coal.

Marine cloud whitening, which allows solar radiation to bounce off water vapour and cool the planet, is 2,000 times more cost-effective than reducing CO2 emissions.

That's according to a study by the think tank headed by Bjoern Lomborg, an adjunct professor at the Copenhagen Business School who wrote 'The Skeptical Environmentalist' (Cambridge University Press, 2001).

Geo-engineering projects like cloud whitening, sea fertilisation and painting roofs white to reflect heat are attracting attention among some policymakers as the world looks at alternative ways to combat global warming.

Most plans to cut CO2 emissions involve using more wind and solar power, boosting energy efficiency and constructing airtight buildings.

"People are only hearing about one solution to climate change and that's to cut carbon-dioxide emissions," Lomborg said in a telephone interview. "In a recession, I'm sure people see the point in finding the cheapest and most effective ways of fighting climate change."

The study on alternatives to reducing CO2 emissions comes four months before negotiators from more than 180 countries meet in Copenhagen as part of United Nations-sponsored talks to find ways to reduce greenhouse gases and limit global temperature gains to two degrees Celsius (3.6 degrees Fahrenheit) after an increase of about 0.8 degrees since industrialisation.

The world should try to have "white roofs everywhere" to help fight climate change, US Energy Secretary Steven Chu said on May 26.

Painting flat roofs of homes and commercial buildings white would reflect more of the sun's heat back to space and reduce electricity used for air conditioning by as much as 15 percent, Chu said.

"Some of these geo-engineering solutions are like Frankenstein ideas," said Kim Carstensen, head of the World Wildlife Federation's global climate initiative. "There are consequences that we only half know about. And it doesn't offer solutions on the scale we require."

Humans need to reduce greenhouse-gas emissions by 50 percent to 85 percent by 2050 in order to limit the risk of exceeding the two-degree target, which would mean rising sea levels that swamp coastal cities, drought and desertification elsewhere, the United Nations Environment Programme says.

"The biggest problem I see with geo-engineering is that it invites us to take our eye of the ball," said Chris Henschel, a policy manager at the Canadian Parks and Wilderness Society. "The world's attention needs to be focused on getting a strong agreement in Copenhagen for major reductions in emissions."

Tweaking Mother Nature's ways may come with a price.

Research still needs to be done to determine the effects of geo- engineering and how rain patterns, cloud formations and other atmospheric processes may be altered, Lomborg said.

"It's true that everything comes with a risk," he said. "But there are also risks connected with carbon-dioxide cuts."