WASHINGTON — Miami would be a memory, Bangkok a soggy shadow of its former self and the Maldive Islands would vanish if melting polar ice keeps fueling a faster-than-expected rise in sea levels, scientists reported Thursday.

In an issue of the journal Science focusing on global warming, climate scientist Jonathan Overpeck of the University of Arizona reported that if global trends continue, Earth could ultimately see sea levels 20 feet higher than they are now.

By the end of this century, Earth would be at least 4 degrees F warmer than now, or about as hot as it was nearly 130,000 years ago.

Back then, significant portions of the Greenland and Antarctic ice sheets melted, pushing the global sea levels to about 20 feet higher than current levels.

A similarly dramatic, and in some cases catastrophic, rise in ocean levels could happen by the year 2500, Overpeck said in a telephone interview, but he noted it could come sooner.

"We know when the sea level was that high in the past, and we know how much warming is necessary to get that amount of sea level rise from both Greenland and Antarctica," Overpeck said.

The Earth will get that hot sometime early in the second half of this century, he said, and once it does, the big ice sheets will start melting "in a more dramatic manner" than they currently are.

A conservative estimate would call for sea level rises of 3 feet per century, he said.

He cautioned, however, that this estimate assumes the Earth will get only as hot as it did 130,000 years ago when the ice sheets melted.

"If we decide to keep on the track we're on now and just keep on warming, because of greenhouse gas pollution, then we could easily cook those ice sheets more rapidly," Overpeck said.

The earlier ice melt was concentrated in the Northern Hemisphere in the summer months, and was due largely to changes in Earth's orbit, he said.

"The climate warming we're in now is global and it's year-round and it's due to human influences on the climate system," he said. "That will be more damaging to the ice sheets than the that warming we had 130,000 years ago."

The ice sheets are already melting, accelerated by relatively warm water that eats away at them, said NASA glacier expert Bob Bindschadler.

"It's not really a debate any more about whether sea level is rising or not. I think the debate has shifted to, how rapidly is sea level rising," Bindschadler said in a telephone briefing.

Overpeck's Web site -- http://www.geo.arizona.edu/dgesl/  -- offers dynamic maps of the projected results of the rise in sea levels.

Source: Reuters