Scientists are thinking the unthinkable. What can be done to save the planet from global warming if political measures fail? If governments cannot agree on the necessary cuts in carbon-dioxide emissions, can engineering projects such as giant mirrors in space save the world?
This week, more than 2,000 of the world's leading climate scientists will issue their formal assessment of the threat posed by rising temperatures. In its fourth report, the Intergovernmental Panel on Climate Change (IPCC) is expected to say that man-made emissions of greenhouse gases are causing average global temperatures to rise towards a tipping point where climate change will become dangerous, and potentially irreversible.
The document will not look at how to mitigate climate change - that will be dealt with in a later report - but some scientists are already thinking about the kind of large engineering schemes that might have to be deployed if policies for cutting CO2 emissions get nowhere. These range from capturing it at power stations and burying it underground, to launching spacecraft loaded with reflective tinfoil to deflect solar radiation.
Such mega-engineering schemes fall into two broad categories. One is aimed at curbing levels of CO2 in the atmosphere, which would lessen the greenhouse effect that exacerbates global warming. The other focuses on deflecting solar radiation back into space by increasing the albedo, or reflective power, of the Earth, or by using mirrors.
Capturing carbon and storing it underground is already happening in a few pilot projects. There are two ways of doing this. One is to remove carbon from hydrocarbon fuels - namely, oil, gas or coal - before it is burnt. The second way is post-combustion, by removing CO2 from power-station emissions and then burying it underground.
The Norwegian state oil company, Statoil, and BP already remove CO2 from North Sea gas as it emerges from the field. They then pipe it back underground to enhance further gas recovery - and reduce carbon emissions in the process. Removing CO2 post-combustion from power stations requires the fitting of "scrubbers" to chimneys that can absorb the gas.
Both are expensive. A power station with effective scrubbing technology would consume between 10 and 40 per cent more energy than one without, but such a power station could reduce its carbon emissions by up to 90 per cent.
Klaus Lackner of Columbia University has proposed an extension of this idea, by dotting the landscape with windmill-like machines fitted with scrubbers that can remove CO2 from the atmosphere. The advantage of such a scheme is that it could be placed anywhere in the world - a desert rich in solar power, for example, or windy islands in the open ocean - and the technology need not be too efficient provided there are enough scrubbers to offset man-made emissions .
"The trick is to absorb enough carbon dioxide and to get rid of it quickly enough by burying it in long-term deposits," says John Shepherd of the Southampton Oceanography Centre. "This is the only scheme that could potentially reduce global levels of carbon dioxide to pre-industrial levels."
Other schemes focus on improving the efficiency of the world's natural carbon sinks. For instance, scientists have postulated that sprinkling iron filings over the ocean would have a fer-tilising effect on marine plankton. In theory, this could improve the absorption of CO2 by encouraging giant algal blooms that draw down atmospheric carbon. But there are problems.
"I think this is a nonstarter," says Dr Shepherd. "Experiments show that you can initiate a bloom, but the majority of the algae die or are eaten within days, releasing carbon dioxide back into the system."
Another approach is to tinker with the amount of solar radiation that hits the Earth. Changing the planet's albedo by as little as 1 per cent could have a significant impact on global warming. …