Global warming is liable to become an even more acute problem than anticipated, according to the new assessment of the Intergovernmental Panel on Climate Change (IPCC).
Whereas global average-surface temperature has increased by 0.6 °C during the past century, it is set to rise by between 1.4 °C and 5.8 °C by 2100, according to the report. There is “new and stronger evidence” that global warming is caused by human activities, it adds.
“The projected rate of warming is much larger than the observed changes during the twentieth century and is very likely to be without precedent during at least the last 10,000 years, based on palaeoclimate data,” it says.
The report was finalized and unanimously accepted last week in Shanghai by 150 scientists and government representatives of the IPCC's working group on the science of climate change. The working group's third assessment report (the first two were published in 1990 and 1995) was three years in the making, involving 123 lead authors from around the world, and more than 500 contributing authors.
The new estimation of the probable range of global warming differs considerably from that in the 1995 report, which had projected a maximum temperature rise of 3.5 °C. The new projection — almost the same as the figures leaked to the press last October — is based on advances in climate modelling and on data that have become available from empirical work since 1995.
But the authors of the report remained cautious about detailed predictions of possible climate instabilities on a more regional scale. Frequency and intensity of extreme local weather events, such as hurricanes and heavy rainfall, have been excluded from the projection, owing to “scientific uncertainties”. New research on possible discontinuities in the ocean circulation, such as a shut-off of the thermohaline circulation of the North Atlantic (see Nature 409, 153–158; 2001), was also left unconsidered.
The report arrives after international talks last November failed to reach agreement on implementing the 1997 Kyoto Protocol to reduce greenhouse-gas emissions.
John Houghton, co-chair of the IPCC working group on the scientific assessment of climate change, is happy that the “serious news” of the report was accepted by the representatives of governments, including the United States and Saudi Arabia, which, in the past, had been reluctant to accept evidence for man-made climate change.
“It is now widely undisputed that the increased intensity of the hydrological cycle will lead to a rise in sea levels, and, in some regions, to more frequent floods and droughts,” Houghton says. “I hope that this insight will do a great deal to convince governments that actions further to the Kyoto Protocol are urgently needed.”