Scientists studying the geophysical mechanisms behind the periodic cycles of freezing and melting at the polar ends of the Earth reported Wednesday that Earth is headed toward another thaw, though it might take a thousand years or more for it to happen.

The research comes from cores drilled out of the ocean floor in Antarctica in 2006. The drilling project, co-directed by scientists from Northern Illinois University and known as ANDRILL, was one of the largest science projects ever undertaken on the continent.

In a report published on the cover of the research journal Science, the researchers found that during the Pliocene epoch 3 million to 5 million years ago — a time when conditions in Antarctica are similar to today’s — the ice in Antarctica collapsed and melted on a regular basis, raising world sea levels.

Polar ice began melting on a massive scale when atmospheric carbon dioxide levels were up to around 400 parts per million in the Pliocene, said Northern Illinois University geologist Ross Powell, one of the chief ANDRILL scientists.

“We are now at 386 parts per million and rising,” he said, and it grows by one part per million every year, thanks to carbon dioxide that human activity is putting into the atmosphere.

Scientists are particularly concerned about the West Antarctic Ice Sheet, which has collapsed with great regularity about every 40,000 years and is currently in an unstable state. If all the ice atop West Antarctica today melted, it would raise world sea levels 16 feet, inundating major cities and coastal areas where billions of people live.

Two climate modelers, David Pollard of Pennsylvania State University and Robert DeConto of the University of Massachusetts, say the ANDRILL data suggest it probably would take 1,000 or more years from the beginning of a warm-up until the ice sheet would melt away.

Over the Earth’s geologic history, polar freeze/thaw cycles have occurred about every 40,000 years because of a natural shift in the tilt of the Earth’s axis known as the Milankovitch Cycle.

“The tilting changes the amount of radiation absorbed into each hemisphere of the Earth, depending on which hemisphere is tilted closest to the sun,” Powell said. That leads to a gradual buildup of atmospheric carbon dioxide, he said, and eventually destabilizes and melts the ice shelves and ice sheets in Antarctica.

But human activity appears to be having its own effect on the world’s climate. Most climatologists believe that at today’s stage in the cycle, global temperatures should be cooling slightly, not warming, said geologist Ross Scherer, also a member of the ANDRILL team from Northern Illinois University.

“If something is an external cycle,” Scherer said, “it should be predictable. But it is much more complicated than that, and we seem to be throwing the pattern off balance now. It used to be that carbon dioxide rises were driven by the cycle. Now atmospheric carbon dioxide is driving the system.”

In the past, everything known about climate history came from geological data collected in the northern hemisphere and very little scientific data existed from Antarctica, which holds 90 percent of the world’s fresh water as ice atop the continent.

The ANDRILL core, extracted by a rig constructed atop an ice shelf in west Antarctica’s Ross Sea, is the first detailed geological data on Antarctic climate history, giving scientists and climate modelers a far more complete picture of world climate history.

Earth’s average annual temperature has risen 1.4 degrees Fahrenheit in the past 100 years, but over west Antarctica it has risen 4.5 degrees.