New information on Antarctica's regularly melting distant past is giving scientists a glimpse into what may be a flooded future as the planet warms up.
The West Antarctic ice sheet collapsed periodically between 3 million and 5 million years ago, adding more than 16 feet to global sea level, according to the first examination of soil cores far below the surface of the Ross ice shelf.
Also, new computer models suggest that warmer waters nearby attacked the ice from below, triggering those collapses.
Both findings appear in studies published Thursday in the journal Nature.
"What we're seeing in the past would lead us to believe that we are on track for losing parts of the West Antarctic ice sheet," said Tim Naish, director of the Antarctic Research Centre at Victoria University in New Zealand and leader of the study that looked at dirt cores.
The West Antarctic ice sheet is roughly the size of the American Southwest. In recent years, nearby smaller chunks have disintegrated and others are close to collapsing. They are being monitored. But those events are nothing compared to the massive melt of the entire region that the researchers studied.
There is some hope, or at least breathing room. The complete collapse of west Antarctica won't happen too quickly — it will be hundreds if not a thousand years from now — slower than some of the most dire predictions, the studies' authors said.
One paper suggested that temperatures would have to rise 9 degrees to trigger a complete collapse in West Antarctica, although the study authors said that was a rough estimate. Leading global scientists have suggested a 9-degree temperature rise is a possibility by 2100.
The type of melting triggered by warmer ocean water is incredibly difficult to stop or reverse, said University of Massachusetts geoscience professor Robert DeConto, co-author of the modeling study. And the climate around West Antarctica is warming faster than anywhere else in the world, Naish said, referring to earlier research.
The studies have a "take home message that unless something is done to curb emissions soon...this would commit us to an inevitable 5-meter (16.4 foot) sea level rise over the next several centuries," said Andrew Weaver, a climate scientist at the University of Victoria in Canada, who wasn't part of either study.
Weaver and four other outside scientists praised the research, which are the first results of a $30 million multinational Antarctic drilling program. Two-thirds of the money came from the U.S. National Science Foundation.
Scientists said the core samples indicated the massive ice sheet on West Antarctica regularly melted about every 40,000 years during a period when the climate was about 5 degrees warmer than now and carbon dioxide levels were slightly higher.
The past melts coincided with regular changes in Earth's tilt, something that isn't occurring now.