Unconventional schemes for tackling global warming by installing a giant sunshade in orbit, sowing the seas with iron and scattering sulphur into the upper atmosphere are set to be bluntly rejected by UN experts this week.
The oddball initiatives are being fostered by "geo-engineers" -- scientists who say headway to reduce the fossil-fuel pollution which drives global warming is so ludicrously slow that bold new ideas are needed to avert climate catastrophe.
Among solutions they sketch is a giant network of tilted mirrors, deployed in orbit, that would deflect some of the sunlight Earth receives.
One idea is to sow particles of sulphur dioxide (SO2) particles in the upper atmosphere to reflect sunlight back into space. Another is to "fertilise" the seas with iron so that surface algae sucks up carbon dioxide (CO2) from the air.
The goal of these and other schemes is to offset the warming effect of greenhouse gases and, at the very least, buy time for an effective deal for slashing carbon pollution.
In a report due to be delivered in Bangkok on Friday, a top UN scientific authority will agree that the window of opportunity is narrowing.
What happens over the next two to three decades "will determine to a large extent the long-term global mean temperature increase and the corresponding climate-change impacts that can be avoided," according to a draft of the Intergovernmental Panel on Climate Change (IPCC) document.
But it pours scorn on geo-engineering as a means of tackling the problem, branding its approaches as hypothetical, tarred with risk and carrying unknown economic costs.
"Geo-engineering options... remain largely speculative and with the risk of unknown side effects," it says witheringly. "Reliable cost estimates for these options have not been published."
Mainstream scientists who assess geo-engineering schemes say they get a fair hearing but are typically simplistic.
A study published in the latest issue of Nature, the British weekly journal, looked into ocean fertilisation.
Under this, iron particles would be scattered into the sea in certain areas to spur the growth of surface phytoplankton, or algae.
As they grow, these micro-organisms would absorb CO2 from the air by photosynthesis, eventually sinking to the ocean floor after their death, thus "sequestering" the carbon in the briny depths.
he champion of fertilisation was US oceanographer John Martin, who once declared ringingly: "Give me a half tanker of iron, and I will give you an ice age."
But research published on Thursday in the British journal Nature says phytoplankton blooms need far more nurturing than that.
Scientists led by Stephane Blain of the University of the Mediterranean in Marseille, southern France, studied a huge, 45,000-square-kilometre (17,300-square-mile) naturally-occurring bloom in the Southern Ocean, near the French islands of Kerguelen.
They found that iron which welled up to the bloom from the ocean depths did indeed spur CO2 uptake -- and far more than previous studies have suggested.
But macronutrients such as nitrates, phosphates and silic acid, brought from surrounding waters and from below, are also vital. Without them, the carbon-munchers cannot be sustained for long.
Also unclear are the side effects.
Edouard Bard, a professor at the College de France in Paris, says there could be mechanisms by which the CO2, instead of being durably locked on the ocean floor, could be released back into the ocean, encouraging acidity and oxygen starvation.
Nitrate-loving bacteria thrive in such conditions, releasing nitrous oxide -- an even more powerful greenhouse gas than CO2.