The history only looks back about 30 years, to the beginning of the age of polar satellite coverage.
Before about 1979, no eye in the sky spied on the polar pack. Before that time, any estimate of ice cap size relied on the wind-burned measurements taken by explorers and mariners, or the secretive soundings of covert submarines.
In fact, no one really knew how the ice cap fared in any detail before the early 20th century.
“Although observations show a more or less continuous decline for the past four or five decades, there are few long-term records with which to assess natural sea ice variability,” according to an international team of scientists in a research letter to the current issue of Nature. “Until now, the question of whether or not current trends are potentially anomalous has therefore remained unanswerable.”
To push the reach of this ice horizon as far back as the European Dark Ages, six geographers and glacialogists analyzed 69 “proxies” based on glacial ice cores, tree-rings, lake sediments and historical observations gathered across the region.
What they found underscores the unique climatic downturn that’s gripped Arctic ice during the past few decades. Such a sustained loss has never happened before -- at least, not since about the year 561.
“Although extensive uncertainties remain, especially before the sixteenth century — both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years,” wrote Christophe Kinnard, a geographer with the Center for Advanced Studies in Chile along with five co-authors from Canada, Norway and Ohio, in the new paper.
The details are complex. The rise and fall of Arctic ice over the past centuries didn’t always correspond directly to temperature changes. While ice extent often changed just as fast as the present, it never plunged as far.
“Although their records show that previous sea ice declines occurred at a pace similar to the present trend, none matched the extent of the current decline,” explained a release posted online by Nature.
Over the past 1,400 years, Arctic ice extent appeared to grow or shrink in response to the flow of warmer Atlantic Ocean water into the polar sea, the scientists concluded. The new decline can likely be blamed on the rise of greenhouse gases in the air. (See latest measurements here.)
“Heat transfer to the Arctic by warm Atlantic water has been shown recently to be unprecedented over the past 2,000 years and may be the main driver for the sustained loss of Arctic sea ice over recent decades,” they wrote.
“In the present state of knowledge, anthropogenically forced (‘greenhouse gases’) warming stands out as a very plausible cause of the record atmospheric and oceanic warmth of the recent decades, which may soon lead to an ice-free Arctic Ocean in summer.”
Contact Doug O'Harra at doug(at)alaskadispatch.com.