Scientists trying to estimate sea level rises the world could face with climate change have come up with a worrying new finding; the last time our world was this warm, sea levels went up at least 20 feet.

Past temperatures similar to or slightly higher than the present global average melted the ice sheets of Antarctica and Greenland, pushing sea levels up least that high, says study lead author Andrea Dutton, a geochemist at the University of Florida.

The finding, reported in the journal Science, suggests the world could face a similar outcome unless a substantial reduction in greenhouse gas emissions is accomplished, she says.

"As the planet warms, the poles warm even faster, raising important questions about how ice sheets in Greenland and Antarctica will respond," she explains. "While this amount of sea-level rise will not happen overnight, it is sobering to realize how sensitive the polar ice sheets are to temperatures that we are on path to reach within decades."

Around 125,000 years ago, sea levels rose 20 to 30 feet above where they are at present when the global average temperature was 1 degree C higher than preindustrial levels - and the average increase we have now reached today, the study team, including American, British and German researchers, says.

Similar sea level rises also occurred during a similar warming period 400,000 years ago, they add.

And there was another period around 3 million years ago, when levels of atmospheric carbon dioxide were as high as they are today.

"We looked at these three different warm periods, because there's no one time period that's going to be a perfect analogue [to today]," says Dutton. "We looked at several of the warmest interglacials, and for each of them, we're finding at least 6 meters worth of sea level rise."

At the present time the melting of the Greenland and Antarctic ice sheet are only pushing sea levels up millimeters or so every year, but the worry is the amount of ice they contain.

If a "tipping" point in climate change were to vastly accelerate their melting, there is enough ice in Greenland to raise global sea levels 20 feet - and enough on the continent of Antarctica to raise them by 200 feet.

At the present those ice sheets are losing ice at rates measured in millimeters, but during past warming periods they would have lost ice by the meters, the researchers point out.

While sea levels are a part of the climate system that responds somewhat slowly to warming - which is why the world has seen just a 20-centimeter rise in the last 100 years - a much larger rise is almost certain to be seen if warming trends continue, they say.

"Because it takes a long time to heat up the oceans into the depths, and also it takes a long time, thousands of years, to melt big ice sheets," says study co-author Stefan Rahmstorf, one of the study authors who is based at the Potsdam Institute for Climate Impact Research in Germany.

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