Wednesday, 31 January 2007

Giant mirrors in space that deflect the sun's heat

Apocalypse never? Science could yet save the day

Giant mirrors in space that deflect the sun's heat, carbon 'scrubbers' that clean up the atmosphere - where governments have failed to tackle global warming, science could yet save the day, says Steve Connor

Published: 31 January 2007

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 fertilising 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 non-starter," 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. An advantage of this approach is that CO2 levels could be allowed to rise to perhaps four or even eight times pre-industrial levels, which would boost agricultural output with improved carbon fertilisation.
John Latham, of the National Centre for Atmospheric Research in Boulder, Colorado, is working with Stephen Salter of Edinburgh University to increase the albedo of low-level clouds over the oceans, by atomising sea water to produce droplets that enter the clouds and make them whiter and more reflective.
"The scheme could produce a global cooling, sufficient to balance the warming resulting from a doubling of the atmospheric carbon-dioxide concentration, by seeding clouds in three oceanic regions which together cover about 3 per cent of the Earth's surface," Dr Latham says. "The only raw material required is seawater, the amount of global cooling could be controlled and, if necessary, the system could be switched off, with conditions returning to normal within a few days."
Another way of altering atmospheric albedo has been proposed by Paul Crutzen, a Nobel Prize-winning chemist at the Max Planck Institute for Chemistry in Germany. He suggests that the release of sulphate particles in the upper atmosphere would reflect sunlight and heat back into space. The idea is based on the way that a volcanic eruption releases sulphur into the air, resulting in discernible global cooling. Weather balloons, aircraft or even artillery shells could be used to put the sulphate aerosol into the atmosphere, Dr Crutzen suggests. "Such a modification could also be stopped on short notice, if undesirable and unforeseen side effects become apparent," he says.
Perhaps the most ambitious plan of all involves for solar reflectors. A fleet of tiny aluminium balloons could be released into the upper atmosphere, or a giant mirror could be placed in space at the point of lowest gravitational pull between the Earth and the sun.
The idea has been proposed by Lowell Wood of the University of California Lawrence Livermore National Laboratory. He is a disciple of Edward Teller, the military scientist who first suggested the idea. Dr Wood believes that removing less than 2 per cent of solar radiation would avert dangerous global warming - although it would cost billions.
Many unanswered questions remain about these mega-technological fixes, but perhaps the biggest of all is whether they are safe. James Lovelock, the inventor of the Gaia theory of Earth systems, has suggested that the mega-engineers should take the equivalent of the Hippocratic oath. Their first priority, he says, should be to do no harm to the Earth and the life on it. Otherwise the cure for global warming could end up being worse than the illness.
The big ideas
Capturing carbon
Carbon is removed from hydrocarbon fuels before they are burnt, or from power station emissions, and then buried.
Carbon scrubbers
Windmill-like machines that can remove carbon dioxide from the atmosphere. Such machines could be located anywhere in the world.
Carbon sinks
It is hoped that sprinkling the ocean with iron filings would have a fertilising effect on marine plankton, a natural carbon sink, thus improving the absorption of atmospheric carbon.
Reflective clouds
The albedo (reflective power) of clouds could be enhanced by atomising sea water to produce tiny droplets that enter the clouds and make them more reflective, thus deflecting heat from the Earth.
Sulphate aerosols
Weather balloons, commercial aircraft or artillery shells would be used to release sulphate particles into the upper atmosphere, to reflect sunlight and heat back into space.
Mirrors in space
A giant folding mirror or a fleet of tiny aluminium balloons would be released into the upper atmosphere to deflect solar radiation before it reaches the Earth.





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