By Christa Marshall and ClimateWire, Source: Scientific American

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Scientists have long known that the Arctic is warming faster than the rest of the globe, even as they had less of a grasp of how recent trends compare to thousands of years ago.

Now, a new study aims to fill the knowledge gap by concluding that recent summer warming in the eastern Canadian Arctic is unprecedented in more than 44,000 years. Prior research documented melt and temperature dynamics going back about 2,000 to 4,000 years in comparison, said study lead author Gifford Miller, associate director of the Institute of Arctic and Alpine Research at the University of Colorado, Boulder.

The findings, published online in Geophysical Research Letters this week, counter the conclusions of some prior studies suggesting that natural forces — along with greenhouse gases — may be contributing to some of the extensive Arctic warming. The study also suggests that climate models are underestimating Arctic changes, as their past predictions were off by more than 2 degrees Celsius.

“Our study pushes the clock way back,” said Scott Lehman, a research professor at the institute and co-author of the paper.

The scientists concluded that the level of warming now matches or goes beyond what occurred during a natural warm period about 5,000 to 10,000 years ago, known as the Holocene Thermal Maximum. The study provides the first “direct” evidence that Canadian Arctic temperatures in the last century exceeded the peak warmth of that earlier thermal maximum, the scientists said.

Discovery linked to ancient vegetation
The fact that certain ice caps did not melt during the Holocene Thermal Maximum, despite the extreme warmth at the time, suggests that today’s unusual warming period can only be caused by greenhouse gases, Miller said.

“Nothing else out there can explain it,” Miller said. Based on the Earth’s current position in relation to the sun, the region should be cooling in the summer, not warming, he said.

The scientists benefited from a discovery of vegetation on Baffin Island in the Canadian territory of Nunavut. When ice caps receded on the island in recent decades, they revealed mosses long entombed in the ice.

The mosses became exposed recently, in the past year or so. A longer period would have eroded or blown them away, according to the study. Therefore, the scientists determined that the last time the vegetation appeared was during melting of the ice caps.

Via radiocarbon dating of 365 vegetation samples, they determined that some of the newly exposed mosses from four of the ice caps were at least 40,000 years old. “We never expected to find plants that old,” Miller said.

Their old age means that the ice caps entombing them had not melted for at least that long, staying colder than the present day through the peak warmth of the Holocene thermal maximum.

During that time, about 5,000 to 10,000 years ago, the eastern Canadian Arctic was closer to the sun in the summer than now, because of natural variabilities in Earth’s orbit. The amount of solar radiation hitting the area was about 9 percent higher than now.

Some of the tested vegetation samples were younger, indicating that their ice cap resting places on Baffin Island melted during the peak Holocene warmth 5,000 to 10,000 years ago. However, the melted caps were very close to the ones that remained intact for at least 44,000 years, Miller said.

Are models underestimating warming?
“Those ice caps that didn’t melt, you can throw a stone at a slightly lower ice cap that did melt. They are all mixed in together,” Miller said. That means that average summer temperatures now are unprecedented in the region in comparison to the past 44,000 years, the study said. The scientists studied 110 ice caps on Baffin Island in total.

Via analysis of the snow-line elevation in the region — a measure strongly tied to temperature — the scientists also concluded that summer cooling occurring in Arctic Canada after the peak heat of the Holocene was much greater than predicted by climate models.

As natural variations in Earth’s orbit cooled the region between 5,000 years ago and 100 years ago, summer temperatures declined by about 2.7 degrees Celsius, about twice the level predicted by global climate models known as Coupled Model Intercomparison Project Phase 5 (CMIP5).

The discrepancy between the data and the models indicates that scientists also may be underestimating how much the region will warm in the future, Miller said. Many models suggest that the Arctic Ocean will be ice-free by the end of the century, for example, while some scientists say that the ice-free threshold could be reached much sooner (ClimateWire, July 16).

Early results from additional vegetation samples gathered from sites in Greenland and Norway indicate a similar warming trend, Lehman said.

The findings are a “big deal” for showing that current temperatures are higher than the Holocene maximum, even though solar forcing was greater back then, said James Overland, an oceanographer at the National Oceanic and Atmospheric Administration who did not participate in the study.

David Pompeani, a paleoclimateologist and doctorate candidate at the University of Pittsburgh, added that the study’s strength was the number of samples and the extensiveness of the radiocarbon dating.

“That is the real big kicker for me,” he said.