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Fossil-fuel hangover may block ice ages
22 August 2007
NewScientist.com news service
Fred Pearce

Enlarge Shifting balance in the CO2 sink

The fossil fuels we burn today may leave an atmospheric "hangover" lasting hundreds of thousands of years, which may cause enough residual warming to prevent the onset of the next ice age. This is the most far-reaching disruption of long-term planetary processes yet suggested for human activity.

The UN's Intergovernmental Panel on Climate Change describes carbon dioxide as having a lifetime in the atmosphere of between five and 200 years before it is ultimately absorbed by the oceans. In fact, as much as one-tenth of the CO₂ we are emitting now will linger in the air for at least 100,000 years, and perhaps much longer, says Toby Tyrrell of the UK's National Oceanography Centre in Southampton.

"It is often assumed that the Earth will always recover from perturbations. But our research shows that it doesn't necessarily behave like this," says Tyrrell. "It isn't always inherently self-rectifying."

Tyrrell and his colleagues used mathematical models to study what would happen to marine chemistry in a greenhouse world. As the ocean absorbs ever more CO₂ from the atmosphere, it becomes more acid and so dissolves more calcium carbonate from the shells of marine organisms. This in turn reduces the oceans' ability to absorb more CO₂ (see Diagram), leaving more greenhouse gas in the atmosphere.

This complication has been suggested before, notably by David Archer of the University of Chicago. Tyrrell's analysis substantiates Archer's suspicions, providing a firm estimate of just how big, and how long-lasting, the fossil-fuel hangover is likely to be (Tellus B, vol 59, p 664).

The effect may be great enough to prevent the next ice age, Tyrrell found. Ice ages occur roughly every 100,000 years. The chill begins when wobbles in the planet's orbit marginally change where solar radiation hits the Earth. This is enough to trigger the growth of ice caps. But for reasons that are not yet clear, this initial cooling also causes the oceans to draw CO₂ out of the air. Starved of this greenhouse gas, the atmosphere's temperature nosedives until much of the planet is covered in ice.

Atmospheric CO₂ is now at 380 parts per million, up from a pre-industrial level of 280 ppm. An analysis by Archer two years ago, using models linking climate and ice sheets, suggested that atmospheric CO₂ levels above 560 ppm would almost certainly be enough to prevent the global cooling that now triggers an ice age every 100,000 years or so. Even levels of 400 ppm would make such cooling less likely.

Tyrrell's new analysis of ocean chemistry suggests that if CO₂ levels in the air rise to 900 ppm by 2100, as predicted by the IPCC's "business as usual" scenario, there would be little chance they would fall below 560 ppm in time for the next ice age to appear on schedule or, possibly, at all. While that might sound to some like a good thing, the short-term warming caused by that much carbon dioxide is likely to cause such severe disruption that it would not be good policy.

Further CO₂ releases, from the burning of all known fossil fuels, for example, could postpone the next ice age for at least half a million years. Only by then could nature reabsorb the excess carbon - mainly because it would be used up as part of the slow chemical weathering of rock.

Tyrrell's findings add further reason to act now on global warming, says Archer. "People care, quite rightly, about the ultimate fate of nuclear waste that can remain radioactive for 10,000 years," he told New Scientist. "I think we should also care about global warming lasting 100,000 years."

From issue 2618 of New Scientist magazine, 22 August 2007, page 16

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