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New Scientist: If we don't stop burning oil…

FRED PEARCE
THE 20th century was warmer than any time in the past thousand years, but that is nothing compared with how hot the Earth could become over the next millennium.
If we burn all the fossil fuels that are left underground, the globe will warm by an average of up to 13 °C, according to the first serious assessment of how global warming might progress beyond 2100, the normal time frame of model predictions. That will wipe out most rainforests, destroy the fertility of many soils and leave the Arctic ice-free even in midwinter.
London will be as hot as Cairo – except that, along with many of today's most populous areas, it will have been engulfed by an 11-metre rise in sea levels.
So far humans have released about 400 billion tonnes of carbon into the atmosphere, as carbon dioxide, by burning fossil fuels and destroying forests – enough to have already raised global temperatures by around 0.6 °C. Ten times as much carbon remains underground in reserves of oil, natural gas and coal, according to ateamledbyTim Lenton of the Tyndall Centre for Climate Change Research in Norwich, UK. And unconventional fossil fuels like tar sands, oil shales and methane-rich ices called clathrates beneath the seabed may contain another 10,000 billion tonnes.
Lenton and his colleagues studied the likely impact of burning all these fuels over the coming millennium. He says that the oceans would ultimately absorb most of the carbon dioxide, but even so, air temperatures in AD 3000 would stabilise at some 13 °C warmer than today (Climate Dynamics, DOI: 10.1007/S00382-006-0109-9).
Meanwhile, the thermal expansion of the oceans plus the melting of the Greenland ice cap, which is likely to be irreversible above a 2.6 °C wanning, would raise sea levels by some 11 metres. “Only by starting to reduce carbon dioxide emissions now can we avoid the melting of the Greenland ice cap,” Lenton says. Sea level rise could be even greater if the Antarctic ice sheet starts to disintegrate.
If the world burnt all its conventional fossil fuels but left underground all the unconventional forms, then global warming might not exceed 7 °C, according to Lenton's calculations. That already looks unlikely. George W. Bush's call last month for the US to end its addiction to foreign oil produced a flurry of interest in exploiting unconventional sources of oil in North America.
Shell has announced a breakthrough in extracting oil from shale in Colorado, and the US Department of Energy has reportedly identified oil sands in the Canadian province of Alberta as a vital future resource.
Lenton dismisses the theory that a new ice age may be upon us by 3000, counteracting the warming trend. He says variations in the Earth's orbit mean “the current interglacial era is likely to be exceptionally long”.
Lenton's predictions neatly complement a study published last week that pieced together temperatures in the northern hemisphere over the past 1200 years. Tim Osborn and Keith Briffa of the University of East Anglia, Norwich, assembled sets ^of data, such as the growth of tree rings, that act as a proxy for past temperatures. They show significant natural variation during the medieval warm period from AD 900 to 1200 and in the “little ice age” from 1550 to 1850 (Science, vol 311, p 841). The 20th century, and particularly the past 40 years, emerges as a period of exceptional warming that, unlike previous anomalies, covered virtually the whole northern hemisphere.
Climate sceptics have argued that evidence of this past climate variability, which is probably due to solar cycles, suggests we need not worry about future man-made climate change. But Briffa told New Scientist: “Greater natural climatic variability implies a greater sensitivity of climate to forcing, whether from the sun or greenhouse gases. So greater past climate variations imply greater future climate change.”

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