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The Arctic was once warmer, covered by trees

Pliocene epoch featured greenhouse gas levels similar to today's but with higher average temperatures

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The Arctic wasn’t always frozen tundra. About 3.6 million years ago, the far north was blanketed in boreal forests, and summers were 8 degrees Celsius warmer than they are today, geologists report May 9 in Science.

Researchers pieced together that picture from sediments buried beneath Lake El’gygytgyn (pronounced EL-gih-git-gin), about 100 kilometers north of the Arctic Circle in northeastern Russia (SN: 11/20/10, p. 13). The sediments preserve the most complete history of Arctic climate on land over the last 3.6 million years.  

“It’s an unprecedented record,” says study coauthor Julie Brigham-Grette, a geologist at the University of Massachusetts Amherst. “It gives us a way of envisioning what the future may hold.”

The sediments preserve the end of the Pliocene epoch, which lasted from 5.3 million to 2.6 million years ago. The Pliocene was the last time the atmospheric carbon dioxide concentration was roughly 400 parts per million — a benchmark that Earth may soon reach. Because the continents were in about the same locations as they are today, events during the Pliocene may be the best analog for what could happen in the future, says geologist Gifford Miller of the University of Colorado Boulder.

Previous geologic evidence had indicated the Pliocene Arctic was warm, but these terrestrial records are spotty, providing only isolated snapshots of time, Brigham-Grette says. Lake El’gygytgyn, by contrast, provides exceptionally well-preserved sediments over a long time frame because the region is too dry for large glaciers to grow and then erode away the record, she says. The lake formed after a meteorite struck about 3.6 million years ago, creating an impact crater.

To get a more comprehensive look at Arctic climate, Brigham-Grette and colleagues drilled a 318-meter-long core from the lake’s bottom. From 3.6 million to 3.4 million years ago, average summer temperatures were about 15°, the team determined through analyses of the sediments’ chemistry and trapped pollen. The region also received 600 millimeters of precipitation each year, making it three times as wet as today.

From 3.26 million to 2.2 million years ago, temperatures gradually cooled in a series of steps that coincided with the beginnings of a glacial period. Forests gave way to shrubby environments, and the Arctic became more arid.

Despite the cooling, Arctic summers generally stayed 3 to 6 degrees warmer than they are today, until about 2.2 million years ago. Even during periods when changes in Earth’s orbit should have made the Arctic cold, warm summers persisted. “We didn’t expect it to be so consistently warm,” Brigham-Grette says.

These findings hint that the switch to a glacial period may be more complicated than scientists realized. Comparisons with other climate records from the oceans and the tropics may help researchers identify the mechanisms that drove the Earth’s stepped transition from warm to cool, Brigham-Grette says.

The past’s high temperatures also highlight another quandary, says Ashley Ballantyne, a climate scientist at the University of Montana in Missoula. Climate simulations have a hard time replicating the Arctic’s warm Pliocene temperatures. The missing puzzle piece may be sea ice, he says.

Lacking a complete record of sea ice, researchers have assumed that the Pliocene Arctic had no summer sea ice and small amounts of sea ice in winter. In a study accepted for publication in Palaeogeography, Palaeoclimatology, Palaeoecology, Ballantyne, Miller and others demonstrate that without winter sea ice — which normally prevents heat in the ocean from escaping to the atmosphere — simulations can achieve the extremely warm Arctic temperatures.

That sea ice might have been completely absent from the Arctic in the Pliocene is “a bit alarming,” Ballantyne says. If that were to happen in the future, he says, “it doesn’t bode well for animals that coevolved with sea ice.”

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