An international team of scientists said Thursday that the Arctic went through ice-free periods of extreme warmth over the past 2.8 million years, based on a new analysis of deep sediment in Russia.

The team led by Martin Melles of the University of Cologne, Germany, drilled into an iced-over lake formed by a meteorite impact on the Chukchi Peninsula in Siberia for the longest sediment core ever collected in the terrestrial Arctic.

Since the meteorite struck an area of Lake El'gygytgyn that was not eroded by glaciers, the sediment record reaches back nearly 30 times further in time than ice cores from Greenland that cover the past 110,000 years.

The sediment reveals periods of extreme warmth that show the polar regions are much more vulnerable to change than previously thought, and are difficult to explain by greenhouse gases alone, said the study in the journal Science.

Scientists have long known that the Arctic went through climate cycles, but the latest research shows some of these warm phases were "exceptional," with temperatures four to five degrees Celsius (7.2 to nine degrees Fahrenheit) warmer and 12 inches (30 centimeters) wetter than during normal interglacials, the study said.

Two of these "super-interglacials" happened about 400,000 years ago and 1.1 million years ago, and the data suggests it was virtually impossible for Greenland's ice sheet to have existed in its present form at those times.

But just what caused these extreme changes remains a mystery.

Since some of the Arctic changes mirror variations in the Antarctic discovered by previous studies, events at one pole may have triggered events at the other, the researchers said.

One possibility is that reduced ice cover in Antarctica led to less cold bottom water mass in the northern Pacific, triggering warmer surface waters, higher temperatures and more precipitation.

Another is that the dissolving Antarctic ice sheet led to global sea level rise that sent warm water rushing into the Arctic Ocean, the study said.

Co-authors of the study included experts from the University of Massachusetts and the Far East Branch of the Russian Academy of Sciences.