An international team of scientists, including a researcher from the University of Connecticut, have determined that Greenland's ice sheet retreated hundreds of thousands of years ago, a discovery that will help forecast what will happen to sea levels as climate change brings a new threat of melting ice.

By examining sediment samples gathered from beneath the Greenland ice sheet, scientists have been able to make new estimates about the impact of melting ice and rising sea levels.

The scientists estimate that because of this melting period thousands of years ago, the seas around the world would have been approximately five feet higher.

Today, an increase in sea level of five feet could be catastrophic, inundating low-lying cities across the world, including the Connecticut shoreline.

"It's the first time we've been able to put our finger on which of these interglacial warm periods caused a major retreat of the ice," said Kurt Cuffey, professor of geography, earth and planetary science at University of California, Berkeley.

Greenland is like a sensitive global thermometer that straddles warm air and currents in the south and the cold of the polar north, scientists explain. What happens to Greenland's glaciers affects the entire world. And understanding how they changed over time is critical for our future, scientists say.

A discovery beneath the ice

The new research pinpoints Greenland's glacial melt thousands of years ago to a long, warm period of time, like unearthing a historical photo from an archive.

"For us humans it's another flashing red light for us to very quickly decarbonize our society, abate further carbon emissions and prevent additional warming," said study lead Dr. Andrew Christ, who recently finished a post-doc appointment at the University of Vermont.

Greenland's mass of ice is the second-largest ice sheet in the world after Antarctica. If the entire ice sheet were to melt, sea levels would rise by about 24 feet.

This warning light was discovered through a bit of luck. The samples the scientists analyzed were taken during the 1960s during the Cold War at a remote, US airstrip called Camp Century. The camp was a proof-of-concept for Project Iceworm, a U.S. plan to build a vast network of nuclear missile launch sites that could survive a first strike.

Scientists working at the camp took ice core samples through the glaciers. In one of the core samples they went through the ice sheet and struck the sediment below.

Back then they didn't have the tools to analyze the sediment. It was put into an archive of ice core samples in Denmark. There it remained forgotten until re-discovered during an ice core archive reorganization.

Danish scientists called their American colleagues. An international phone tree grew. Geologists and climate scientists globally realized they had struck gold.

"It's a game of bringing in who can help understand this awesome opportunity," said Dr. Julie Fosdick, an associate professor of geoscience at UConn. Fosdick was brought in to figure out where the sediment came from.

Modeling the future

The team carefully parceled out tiny sections of their sample. Analysis revealed tiny plant fossils. But the plants were too old for carbon dating, so the scientists turned to another technique that measures traces of radiation in the sediment samples.

"Those fossil plants immediately told us this land currently covered by a mile of ice must have been ice-free," said Christ. "There was a green tundra ecosystem there in the (geologically) recent past."

By measuring these traces, scientists are able to estimate how long something has been underground.

Using a different radiometric technique, Fosdick was able to determine that all of the sediment was roughly the same age and had come from the same place. This means that it's likely that everything in the sample was ground up by the same glaciers over millions of years of carving, and that nothing else, like earthquakes, floods, volcanism or tectonic movement, had changed that spot of Greenland.

"What this tells us is that the landscape was very stable," said Fosdick. "The landscape itself wasn't changing dramatically during that time."

As Greenland's southern end breaks down the rate at which the ice melts accelerates. The top of the ice sheet also warms and becomes more prone to melting. Cuffey said that this almost certainly occurred during the period identified by the study.

"Knowing that is a great step forward that will allow climate and glaciological modelers to figure out exactly what is going on," said Cuffey. He explained this would help us see what might happen with Greenland's ice in the future. "Ultimately that's the goal. It's once piece in a very large puzzle."