CHICAGO — An abrupt warming of the Arctic has reversed a cooling trend that began about 8,000 ago, according to a study published Thursday which sheds light on the threat of rising sea levels and climate change.
Increased greenhouse gas emissions appear to have overridden the natural cooling caused by a wobble in the Earth's axis which has been gradually pulling the planet away from the Sun.
This wobble has cooled summer temperatures by an average of about 0.2 degrees Celsius per thousand years, the study published in the journal Science found.
But Arctic temperatures began to rise at the beginning of the 20th century even though the orbital cycle that produced the cooling continued.
The result was summer temperatures that were about 1.4 degrees Celsius warmer than they should have been by the year 2000, according to the study which mapped Arctic temperatures for every decade of the past 2,000 years.
Temperatures for four of the last five decades were among the highest on record.
"This study provides us with a long-term record that reveals how greenhouse gases from human activities are overwhelming the Arctic's natural climate system," said co-author David Schneider of the Colorado-based National Center for Atmospheric Research.
"This result is particularly important because the Arctic, perhaps more than any other region on Earth, is facing dramatic impacts from climate change."
The Arctic tends to warm about three times faster than elsewhere in the Northern Hemisphere because of a phenomenon called Arctic amplification.
When highly reflective ice and snow melt, the exposed dark land and ocean absorb more sunlight. The warmer temperatures accelerate melting, which then accelerates warming.
"This has consequences globally because, as the Arctic warms, glacier ice will melt, contributing to (a) sea-level rise and impacting coastal communities around the globe," said lead author Darrell Kaufman of Northern Arizona University.
"Thawing permafrost will release methane adding to the global greenhouse effect," Kaufman added.
The researchers used glacial ice, tree rings and lake sediments to supplement a complex computer model of global climate and generate a reconstruction of temperatures for the past 2,000 years.
Previous studies had only pinpointed Arctic temperatures for the past 400 years.
"Scientists have known for a while that the current period of warming was preceded by a long-term cooling trend," Kaufman said.
"But our reconstruction quantifies the cooling with greater certainty than before."
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