Fossil-fuel hangover may block ice ages
The New Scientist, Fred Pearce, Aug. 22, 2007
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
"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
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."