The Arctic spring thaw has begun with a bang, with extensive melting of the Greenland ice sheet and sea ice loss that is already several weeks ahead of normal, scientists said.
A stagnant zone of high-pressure air over Greenland last week brought warm air from the south, raising temperatures as much as 40 degrees Fahrenheit above normal. That, coupled with cloudless conditions, led to a pulse of melting across much of the ice sheet surface.
Melting last Wednesday was the most extensive, at 275,000 square miles, or about 45 percent of the surface, said Marco Tedesco, a geophysicist at the Lamont-Doherty Earth Observatory, which is part of Columbia University.
That represents a record early date for such extensive melting, which has been measured by satellites since 1979. In 2012, an exceptionally warm year in the Arctic, there was slightly greater melting a few days later in June.
The early melt is in keeping with the overall trend in the Arctic, where the warming effects of climate change are amplified. Overall, the region is warming about twice as fast as the global average.
But day-to-day conditions in the Arctic can vary, and by Saturday somewhat cooler air led to reduced melting of about 215,000 square miles, according to the National Snow and Ice Data Center, in Boulder, Colo.
In 2012 high-pressure air returned in July and August, leading to record ice-sheet melting for the year — in all, Greenland had a net loss of about 200 billion tons of ice that year.
There is no way of knowing if such a pattern will occur this year, but the early extensive thaw “is setting the island up for more melting as we go on into the summer,” said Ted Scambos, a senior researcher at the Earth Science and Observation Center at the University of Colorado.
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A contributing factor to the early melting this year was the relatively light snowfall last winter, especially in northern Greenland. Thinner snow cover melts more quickly, exposing old snow or ice, both of which are darker than fresh snow and absorb more sunlight, accelerating melting.
“These pulses of melting really change the face of the snow,” Dr. Tedesco said. “It can make the snow melt faster and faster.”
Greenland’s ice sheet is more than a mile thick, and although a recent study showed it was melting much faster than thought, it is in no danger of disappearing anytime soon. If the entire ice sheet melted, it would raise sea levels by about 20 feet; the same study showed that melting since the early 1970s had raised sea levels by about a half inch.
Melting this early in the season generally does not contribute to sea level rise immediately, as most of the water remains near the surface of the ice sheet. As the warm season continues, however, the hydrology of the ice sheet changes and melt water drains down through the ice and eventually out to the ocean. Dr. Tedesco likened it to clearing clogs in a plumbing system.
As for Arctic sea ice conditions, a warm May had reduced ice extent from its maximum of 5.7 million square miles in March to 5 million square miles at the end of the month. (As of Sunday, it was down to 4.2 million square miles.) Sea ice loss contributes to the amplification of Arctic warming, as the darker water of open ocean absorbs more sunlight than ice.
The average sea ice extent in May is nearly half a million square miles below the average for 1981-2010, according to the National Snow and Ice Data Center. Parts of the Arctic, including the Bering Sea off western Alaska, have had exceptionally low ice cover for months.
Dr. Scambos said that Arctic sea ice loss can be linked to temperatures in Siberia. “It’s setting up fairly dramatically right now,” he said. “There’s a lot of warmth in Siberia.”
Still, whether this year will break the record for minimum sea ice extent — 1.3 million square miles, also in 2012 — is anyone’s guess. Conditions can change between now and the end of the summer thaw. “July and August will be pretty interesting,” Dr. Scambos said.
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An earlier version of a picture caption with this article described incorrectly the location of a dog team. The dogs were hauling a sled on coastal ice in northwest Greenland, not on the ice sheet.
Henry Fountain covers climate change, with a focus on the innovations that will be needed to overcome it. He is the author of “The Great Quake,” a book about the 1964 Alaskan earthquake. @henryfountain • Facebook