Backers of extreme technologies to curb global warming advocate dumping iron dust into the seas or placing smoke and mirrors in the sky to dim the sun.

But, even though they are seen by some as cheap fixes for climate change when many nations are worried about economic recession, such "geo-engineering" proposals have to overcome wide criticism that they are fanciful and could have unforeseen side effects.

"We are at the boundaries, treading in areas that we are not normally dealing with," said Rene Coenen, head of the Office for the London Convention, an international organization that regulates dumping at sea.

The London Convention, part of the International Maritime Organization, will review ocean fertilization at a meeting this week.

Among those hoping for approval for tests is Margaret Leinin, chief science officer of California-based Climos, a company that is looking at ways to use the oceans to soak up greenhouse gases.

"The world has not been able to get carbon emissions under control." Leinin said. "We should look at other options."

Climos is seeking to raise money to test adding iron dust to the southern ocean to spur growth of algae that grow by absorbing heat-trapping carbon dioxide from the air. When algae die, they fall to the seabed and so remove carbon.

'Chemical soup'

Other short-cut ideas include spraying a smoke of tiny particles of pollutants into the sky to dim sunlight, or even deploying a vast thin metallic barrier in space, with 100 space shuttle flights, to deflect the sun's rays.

The UN Climate Panel has said world greenhouse gas emissions from human activities, mainly burning fossil fuels, rose 70 percent between 1970 and 2004.

But it said that fertilizing the oceans or dimming the sun "remain largely speculative and unproven, and with the risk of unknown side-effects."

"More evidence has been coming in since then, but it's far from making a reliable case for geo-engineering," said Terry Barker, head of the Cambridge Centre for Climate Change Mitigation Research and one of the leading authors of the UN panel report.

The seas are already suffering enough from a "chemical soup" of pollution from humans, he said. "There's no need to add to the mess."

With fears of recession and amid the deepest financial crisis since the 1930s, some governments may find cheap geo-engineering attractive compared with reducing carbon emissions. "It would be shortsighted," Baker said.