Analysis of Arctic climate from 2.2 to 3.6 million years ago suggests that with estimated atmospheric carbon dioxide (CO2) similar to today's levels, the Arctic was very warm, with no ice sheets.
The analyses of the longest continental sediment core ever collected in the Arctic, completed by an international team led by Julie Brigham-Grette of the University of Massachusetts Amherst, provide "absolutely new knowledge" of Arctic climate.
"While existing geologic records from the Arctic contain important hints about this time period, what we are presenting is the most continuous archive of information about past climate change from the entire Arctic borderlands.
"As if reading a detective novel, we can go back in time and reconstruct how the Arctic evolved with only a few pages missing here and there," said Brigham-Grette.
Results of analyses that provide "an exceptional window into environmental dynamics" never before possible have "major implications for understanding how the Arctic transitioned from a forested landscape without ice sheets to the ice- and snow-covered land we know today," she added.
Their data comes from analysing sediment cores collected in the winter of 2009 from ice-covered Lake El'gygytgyn, the oldest deep lake in the northeast Russian Arctic, located 100 km north of the Arctic Circle.
"Lake E" was formed 3.6 million years ago when a meteorite, perhaps a kilometre in diameter, hit the Earth and blasted out an 18 km wide crater. It has been collecting sediment layers ever since.
It lies in one of the few Arctic areas not eroded by continental ice sheets during ice ages, so a thick, continuous sediment record was left remarkably undisturbed.
Cores from Lake E reach back in geologic time nearly 25 times farther than Greenland ice cores that span only the past 140,000 years.
"One of our major findings is that the Arctic was very warm in the middle Pliocene and Early Pleistocene (3.6 to 2.2 million years ago) when others have suggested atmospheric CO2 was not much higher than levels we see today.
"This could tell us where we are going in the near future. In other words, the Earth system response to small changes in carbon dioxide is bigger than suggested by earlier climate models," researchers said.
Another significant finding is documentation of sustained warmth in the Middle Pliocene, with summer temperatures of about 15 to 16 degrees C, about eight degrees C warmer than today, and regional precipitation three times higher.
"We show that this exceptional warmth well north of the Arctic Circle occurred throughout both warm and cold orbital cycles and coincides with a long interval of 1.2 million years when other researchers have shown the West Antarctic Ice Sheet did not exist," Brigham-Grette said.
The study was published in the journal Science.