HOBART (Reuters) - Rising sea levels and melting polar ice-sheets are at upper limits of projections, leaving some human population centers already unable to cope, top world scientists say as they analyze latest satellite data.
A United Nations report by the Intergovernmental Panel on Climate Change (IPCC) in February projected sea level gains of 18-59 centimeters (7-23 inches) this century from temperature rises of 1.8-4.0 Celsius (3.2-7.8 Fahrenheit).
"Observations are in the very upper edge of the projections," leading Australian marine scientist John Church told Reuters.
"I feel that we're getting uncomfortably close to threshold," said Church, of Australia's CSIRO Marine and Atmospheric Research said.
Past this level, parts of the Antarctic and Greenland would approach a virtually irreversible melting that would produce sea level rises of meters, he said.
There has been no repeat in the Antarctic of the 2002 break-up of part of the Larsen ice shelf that created a 500 billion ton iceberg as big as Luxembourg.
But the Antarctic Peninsula is warming faster than anywhere else on Earth, and glaciers are in massive retreat.
"There have been doomsday scenarios that west Antarctica could collapse quite quickly. And there's six meters of sea level in west Antarctica," says Tas van Ommen, a glaciologist at the Hobart-based Australian Antarctic Division.
Doomsday has not yet arrived.
But even in east Antarctica, which is insulated from global warming by extreme cold temperatures and high-altitudes, new information shows the height of the Tottenham Glacier near Australia's Casey Base has fallen by 10 meters over 15-16 years.
Scientists say massive glacier retreat at Heard Island, 1,000 km (620 miles) north of Antarctica, is an example of how fringe areas of the polar region are melting.
The break-up of ice in Antarctica to create icebergs is also opening pathways for accelerated flows to the sea by glaciers.
Church pointed out that sea levels were 4-6 meters higher more than 100,000 years ago when temperatures were at levels expected to be reached at the end of this century.
Dynamic ice-flows could add 25 percent to IPCC forecasts of sea level rise, van Ommen said.
Australian scientist John Hunter, who has focused on historical sea level information, said that to keep the sea water out, communities would need to begin raising sea walls.
"There's lots of places where you can't do that and where you'll have to put up with actual flooding," he said.
This was already happening in the south of England, where local councils and governments could not afford to protect all areas from sea water erosion as land continued to sink.
About 100 million people around the world live within a meter of the present-day sea level, CSIRO Marine Research senior principal research scientist Steve Rintoul said. "Those 100 million people will need to go somewhere," he said.
Worse, every meter of sea level rise causes an inland recession of around 100 meters (300 feet) and more erosion occurs with every storm.
"You can't just say we'll just put sea walls," Hunter said.