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."

Man-made 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 -- with the exception of a minority of skeptics who doubt man-made global warming exists and that if it does, it's not a big deal -- the universal consensus 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, and 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 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.

earlier related report
Climate toolbox: The options for tackling global warming
Paris (AFP) April 28 - Here are the main options for reducing emissions of greenhouse gases, as sketched in a draft report by the UN's Intergovernmental Panel on Climate Change (IPCC).

The IPCC's "summary for policymakers" on how to mitigate climate change is to be issued in Bangkok next Friday after a five-day meeting.

The draft says an overarching goal is to establish a "price for carbon," a reference to carbon dioxide (CO2) and other greenhouse-gases emitted by burning fossil fuels and by farming.

If carbon pollution carries a significant price, producers and consumers are encouraged to switch to low-carbon products, technologies and processes -- and the higher the price, the faster the change and the deeper the cut in emissions.

To achieve a reasonable price, a basket of policies and technologies is needed and the mix can include regulatory, fiscal and voluntary measures, says the report.

The document does not make any recommendations, but highlights the following options as being proven as "environmentally effective":

TARGET FOSSIL FUELS
Reduce subsidies for fossil fuels and impose a carbon tax. Resistance by the fossil-fuel industry and by the public may make these measures politically hard to implement. Carbon storage (capturing CO2 from burning fossil fuels at power plants and other big sites and then storing the gas in chambers underground) has "significant mitigation potential" over the next two decades.

ENCOURAGE RENEWABLE ENERGIES
Help wind, solar, geothermal and other "clean" energies with subsidies, require the purchase of a given amount of the electricity they produce, or set tariffs that make them competitive against fossil-fuel rivals. A price of 20-100 dollars per tonne of CO2 would give renewables 30-35 percent of the total electricity market by 2030.

REDUCE ROAD POLLUTION
Fuel-economy and CO2 standards for cars, trucks and buses can be toughened. Countries can also invest in public transport and non-motorised forms of transport to wean people off the road. Higher taxes on car purchase, fuel and parking likewise discourage vehicle ownership, but become less effective with people on higher incomes. If CO2 is priced at 25 dollars per tonne, biofuels would get a tenth of the market for petrol and diesel.

MAKE BUILDINGS ENERGY EFFICIENT
Homes and offices are indirectly one of the biggest sources of carbon dioxide (CO2) as they are heated, cooled and lit chiefly by fossil fuels. Countries can reduce the emissions by regularly updating building codes and energy-efficiency standards for lighting, boilers, air-conditioning and other appliances. Thirty percent of projected emissions from buildings could be avoided by 2020 at "negative cost," meaning there would be a net gain because of lower energy bills. Solar panels, smart metering and "intelligent controls" over building conditions also have big emissions-cutting potential.

CUT CO2 FROM INDUSTRY
Options include subsidies and tax credits, as well as tradable permits, such as the so-called "carbon market" already underway in Europe under the UN's Kyoto Protocol. Voluntary agreements between industry and government are "politically attractive" and raise awareness about carbon pollution but most of these deals have not achieved significant reductions.

AGRICULTURE, FORESTRY AND WASTE
Provide financial incentives to improve land management to avoid CO2 and methane being released from soil and to maintain and manage forests. Trees capture CO2 when they grow, although the carbon is released back into the atmosphere when they die and decay. Landfills offer big potential in recovering methane from rotting rubbish, but they may need financial incentives and regulatory support to get the technology established.

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