Once dismissed as daft or dangerous, some of these "geo-engineering" projects can be of use in fending off the juggernaut of climate change, but only if they go hand-in-hand with cuts in carbon emissions, they warned.

"Geo-engineering" describes large-scale schemes such as erecting sunshades or mirrors in space, sowing the stratosphere with white particles or whitewashing building roofs to reflect sunlight, or scattering iron filings in the ocean to promote carbon-gobbling algae.

None of these projects has been launched on any significant scale.

Green groups are deeply suspicious of them, saying the most ambitious ventures could wreck ecosystems, carry an astronomical price and postpone tough decisions on reducing emissions of fossil-fuel gases that cause the problem.

But promoters of geo-engineering are now getting a closer hearing as political efforts to resolve climate change remain bogged down.

They argue that geo-engineering, by slightly cooling the planet, would buy time for humans to get their carbon pollution under control.

In a paper published on Wednesday in the journal Atmospheric Chemistry and Physics Discussions, researchers at Britain's University of East Anglia make the first attempt at calculating the effectiveness of these schemes.