THE warning is of 鈥渁brupt and irreversible鈥� climate change. They are words we have heard often enough 鈥� but never before from the UN鈥檚 Intergovernmental Panel on Climate Change. Its 鈥渟ynthesis report鈥� published last week 鈥� which is intended mainly as a summary of findings presented in three detailed studies released earlier this year 鈥� has in fact gone further than those reports.
IPCC chiefs headed by chairman Rajendra Pachauri were stung by criticisms from scientists that their report on the physical science of climate change, agreed in February, had painted too rosy a picture. The charge was that their efforts to concentrate on findings with a 90 per cent certainty or better had resulted in them leaving out scarier but less certain scenarios. The synthesis report tries to make amends.
For instance, the February report predicted that sea levels will rise 鈥渂etween 18 and 59 centimetres鈥� by 2100. Many glaciologists say that growing evidence of the instability of major ice sheets on Greenland and Antarctica, plus a recent doubling in the rate of sea-level rise, has made this an improbably low estimate, and the new report has responded to this. 鈥淭his report does not assess the likelihood, nor provide a best estimate or an upper bound for sea level rise,鈥� it says.
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At the launch in Valencia, Spain, Pachauri explained what had changed since February. 鈥淚t became apparent that, concerning the melting of the Greenland and Antarctic ice sheets, we really don鈥檛 know enough. Given the uncertainty it was prudent, and scientifically correct, to remove the upper boundary,鈥� he said.
The changing view is especially marked on the prognosis for Antarctica. In February, the IPCC said that 鈥渃urrent model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain mass due to increased snowfall鈥�. This would, if anything, lead to lower sea levels.
The new report is much more cautious about those modelling studies, and acknowledges that while the West Antarctic ice sheet is not likely to melt any time soon, it may physically break up. As the report itself puts it: 鈥淭he risk of additional contributions to sea level rise [from Antarctica] may be larger than projected by ice sheet models鈥� because ice dynamic processes seen in recent observations [are] not fully included in ice sheet models and could increase the rate of ice loss.鈥�
鈥淲hile the West Antarctic ice sheet is not likely to melt any time soon, it may physically break up鈥�
The changes follow fierce debate within the IPCC. 杏吧原创s writing a report on the impacts of climate change, which came out in April, were upset that they could not explore the effects of more severe rises in sea level that had been ruled out in the earlier report.
The more urgent tone is reflected elsewhere. The new report says that 鈥渁nthropogenic warming could lead to some impacts that are abrupt and irreversible鈥�, and for the first time highlights the risk of 鈥渧ery large impacts鈥� and 鈥渓arge-scale singularities鈥�. Such events, it suggests, could include collapsing ice sheets, a shutdown of the Gulf Stream and runaway warming.
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