Sea levels are rising 60 percent faster than U.N. projections, threatening low-lying areas from Miami to the Maldives, a new study says.
The report, issued during U.N. talks in Qatar on combating climate change, also said temperatures were creeping higher in line with U.N. scenarios, rejecting hopes the rate had been exaggerated.
"Global warming has not slowed down, (nor is it) lagging behind the projections," said Stefan Rahmstorf, lead author at the Potsdam Institute for Climate Impact Research that compared U.N. projections to what has actually happened from the early 1990s to 2011.
The study said sea levels had been rising by 3.2mm a year according to satellite data, 60 percent faster than the 2mm annaul rise projected by the U.N.'s Intergovernmental Panel on Climate Change (IPCC) over that period.
"This suggests that IPCC sea-level projections for the future may also be biased low," the authors from Germany, France and the United States wrote in the journal Environmental Research Letters.
The IPCC's latest report in 2007 said seas could rise by between 18cm and 59cm this century, not counting a possible acceleration of the melt of the Greenland and Antarctic ice sheets that could add more still water to the oceans.
In the last century, seas rose by about 17cm.
Rahmstorf told Reuters his best estimate for sea level rise was between 50cm and a metre this century, possibly more if greenhouse gas emissions surged. Higher temperatures would melt more ice on land and expand the water in the oceans.
That would leave low-lying regions - from Pacific island states and Bangladesh to Tokyo and New York - facing a greater risk of storm surges, erosion and, in a worst case scenario, complete swamping by flood waters.
The IPCC was criticised after it had to correct parts of its 2007 report that exaggerated the rate of melt of Himalayan glaciers and wrongly said they might vanish by 2035.
People sceptical that man-made emissions of greenhouse gases are stoking climate change also wonder if warming has flattened out. They note that 1998, 2005 and 2010 are tied as the warmest years since records began in the mid-19th century.
But the study said overall warming was in line with IPCC projections of a gain of 0.16degC a decade from 1990 to 2011, after correcting for natural variations caused by volcanic eruptions, El Nino events that warm the Pacific and shifts in the sun's output.
Almost 200 nations are meeting in Doha from November 26-December 7 as part of floundering efforts to work towards a U.N. deal to curb global emissions of greenhouse gases from 2020.
"Unless we reduce our carbon pollution rapidly, this study clearly shows we are heading for the nightmare world at the top end of the IPCC predictions," said professor Mark Maslin of University College, London.
The IPCC says rising temperatures could cause more floods, droughts, heatwaves, mudslides and desertification that would strain water and food supplies for a rising world population.
"The authors have stressed what many of us have thought for some time - the IPCC is far from alarmist in its projections," said Professor Sir Brian Hoskins, Director of the Grantham Institute for Climate Change at Imperial College, London.