The study shows current temperatures are “well outside the range of natural variability now,” said Gifford Miller, from the University of Colorado, Boulder, who led the study, in an interview with CBC News Friday.
“And so… there’s really nothing left but greenhouse gases to explain why the warming is occurring.”
Previously, some scientists thought it was possible that current Arctic warming might be within the range of natural variability, and that the Arctic may in fact have been warmer than it is now during the Early Holocene, shortly after the end of the last ice age 11,700 ago. At that time variations in the Earth’s orbit meant the amount of solar energy reaching the Northern Hemisphere was about nine per cent higher than it is now, leading to a 5,000-year warm period that peaked around 6,000 to 8,000 years ago, Miller said.
However, the analysis by Miller and his colleagues suggests that average temperatures never got as high as they are now in the area of Baffin Island that they studied. The study was published this week in the journal Geophysical Research Letters.
The researchers gathered dead moss that had been exposed by melting of the ice caps, and used radiocarbon dating in an effort to find out how long the moss had been buried in the ice before that. Radiocarbon dating can only be used to determine when an organism had been alive within the past 50,000 years. In the case of the moss, the researchers hit the 50,000-year limit, which meant that the moss had been buried since the middle of the last ice age. And since the ice almost certainly didn't melt during the ice age, it had probably been there since the beginning of the ice age, 120,000 years ago.
Miller said he and his colleagues had specifically chosen a flat area for their study so that any ice loss would have to be due to melting and not erosion. The researchers were also able to calculate maximum thickness of the ice based on the local topography. With that information, they calculated that had it been as warm at any point during the Early Holocene as it is today, within 100 years, the ice would have melted enough to expose the moss. The fact that this never happened suggested that it never got that warm.
Ice core evidence
In fact, evidence from ice cores collected in nearby Greenland suggest that summer temperatures in the region haven’t been as warm as they are now for 120,000 years.
Another interesting finding of the new study was that from 5,000 to 500 years ago, average summer temperatures in the region cooled about 2.7 C — about double what most climate models show.
Miller said that suggests the models may underestimate the huge temperature swings in the Arctic relative to other parts of the world when the average global temperature changes. The Arctic is thought to respond more strongly because effects of warming are amplified by the large-scale melting of Arctic ice in forms such as sea ice and ice caps.
“Maybe the future warming estimates for the Arctic are still underestimated,” Miller added.