<!--intro-->Global warming could be on the verge of triggering a rise in sea levels that would flood huge swathes of the Earth's most densely populated regions, says an unpublished report from the world's top climate scientists. Caused in large part by the melting of Greenland's ice sheet, this process would take a thousand years or more but would be "irreversible" once under way.<!--/intro--> <center ><a target="_new" HREF='http://www.exosci.com/portal.php?url=http%3A%2F%2Fwww.newscientist.com%2F&what=link&item=20001115202740695'><img SRC='http://www.newscientist.com/ads/people_why.gif' BORDER=0></a></center> The report, due to be published next May by the UN's Intergovernmental Panel on Climate Change (IPCC), is being read by the world's governments. The final draft seen by New Scientist suggests that dozens of the countries meeting this week to agree on global warming limits through the Kyoto Protocol may face being wiped off the world map. Four years ago, the IPCC forecast that sea levels could rise by half a metre in this century and by a maximum of between 1.5 and 3 metres over the coming 500 years. The new assessment suggests an eventual rise of 7 to 13 metres is more likely. This is enough to drown immense areas of land and many major cities. These rises will occur even if governments succeed in halting global warming within the next few decades, the report says. Two factors are causing the rise: the slow spread of heat to the ocean depths and the destabilising of major ice sheets. It will take about a thousand years for warming in the atmosphere to reach the bottom of the oceans. The resulting thermal expansion "would continue to raise sea levels for many centuries after stabilisation of greenhouse gas concentrations". Even if global warming is halted within a century, thermal expansion will eventually raise the oceans by between 0.5 and 4 metres. <center ><img src="http://www.newscientist.com/ns_images/2266/22664F2.JPG"></center> Even more alarming is the fate of the ice that covers Greenland. Among all of the world's ice sheets, this is now thought to be "the most vulnerable to climatic warming". It contains enough snow and ice to raise sea levels by about 7 metres if it melts. And this looks increasingly likely to happen. Models show that after any warming above 2.7 °C, "the Greenland ice sheet eventually disappears". Nearly all predictions show Greenland warming more than this, says the report, and the faster the warming, the faster the melting. An extra 5.5 °C would cause sea levels to rise by 3 metres over a thousand years. An 8 °C warming would cause a 6-metre rise in sea levels in the same time. The report's authors are not allowed to discuss their findings until publication. But Jonathan Gregory of Britain's Hadley Centre for Climate Prediction and Research in Bracknell, who co-authored the chapter on sea level, told New Scientist recently that once under way, the disintegration of the Greenland ice sheet would be "irreversible this side of a new ice age". The fate of the West Antarctic ice sheet, which is perched on submerged islands, remains controversial, says the report. If it melted, it would raise sea levels by a further 6 metres. Some experts quoted in the report predict that the sheet could entirely disappear within 700 years. Others, supported by the authors, expect that the sheet will contribute "no more than 3 metres" to sea level in that time. If sea levels were 10 metres higher than today by the year 3000, it would cause the inundation of a total area larger than the US, with a population of more than a billion people and most of the world's most fertile farmland. Fred Pearce From New Scientist magazine, 25 November 2000.