Some of the suggested methods involve deflecting the Sun's rays with sulphur dioxide particles or giant mirrors.

Credit: iStockphoto

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PARIS: Unconventional schemes for tackling global warming – such as installing a giant sunshade in orbit – are set to be bluntly rejected by U.N. experts this week.

The oddball initiatives are being fostered by geo-engineers: scientists who say 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, Thailand on Friday, the U.N.'s Intergovernmental Panel on Climate Change (IPCC) will agree that the window of opportunity is narrowing. The report will lay out ways to mitigate global warming.

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 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," the report states witheringly. "Reliable cost estimates for these options have not been published."

Current global warming is caused by emissions of carbon gases, chiefly from the burning of oil, coal and gas. These gases trap solar heat, warming Earth's surface and inflicting change to the climate system.

The commonsense approach would be to address the emissions that cause the problem in the first place. But the universal consensus is that this effort is not advancing nearly fast enough. It is being braked in large part by fears about the economic cost of reducing emissions and the switch to cleaner energy. Vested interests, opposed to a move out of fossil fuels are also influential.

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.

The champion of fertilisation was U.S. oceanographer John Martin, who once declared ringingly: "Give me a half tanker of iron, and I will give you an ice age."

But the Nature study says that 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 km² 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, an oceanographer at the College of 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.