Actual observations of September Arctic sea ice,
in red, show a more severe decline than any of
the 18 computer models, averaged in a dashed line,
reported by IPCC in the 2007 reports.
NSIDC
The frozen cap covering the Arctic Ocean has been disappearing much faster than predicted by the world’s top climate models — pushing the late-summer meltdown up to 30 years ahead of forecasts used for recent IPCC reports, according to a new analysis by scientists at the National Snow and Ice Data Center and the National Center for Atmospheric Research.
The extent of sea ice observed each September — the time of year when it shrinks to its smallest coverage — has been significantly lower than what has been predicted by the 18 different supercomputing models deployed by the Intergovernmental Panel on Climate Change.
The Arctic, long considered the harbinger of global warming, may be far more sensitive to greenhouse gas “forcing” than scientists realize.
“This suggests that current model projections may in fact provide a conservative estimate of future Arctic change, and that the summer Arctic sea ice may disappear considerably earlier than IPCC projections,” said lead author Julienne Stroeve, in a NSIDC news story.
The new analysis arrives amid more alarming sea ice news. After producing the second smallest ice extent for March on record, the Arctic meltdown sped up in April.
Satellite images taken over the past month show the extent of ice has retracted to about 13.9 million square kilometers — slightly below the record minimum of 14 million square kilometers seen in April 2006.
“It looks to be another record low,” said NSIDC researcher Walt Meier, one of the co-authors on the overall study, in an email message. “The long-term average (1979-2000) is about 15.0 million sq. km. (5.8 million sq. mi.), so we’re about 1.1 million sq. km. (425,000 sq. mi.) below the long-term average” for April.
Polar bears lose habitat
when ice shrinks
Credit: USFWS
Such a loss replaces white reflective floes with a dark sea that absorbs more solar radiation and makes warming go even faster. It also leaves marine mammals like seals and polar bears in the drink. A frozen habitat about 1.5 times the size of Texas — or perhaps about two-thirds the size of Alaska — has simply disappeared from the Arctic for this time of year.
The study Arctic Sea Ice Decline: Faster Than Forecast? was published May 1 in the online edition of Geophysical Research Letters.
“While the ice is disappearing faster than the computer models indicate, both observations and the models point in the same direction: the Arctic is losing ice at an increasingly rapid pace and the impact of greenhouse gases is growing,” added co-author Marika Holland of NCAR in news release.
From the NSIDC story:
When the authors analyzed the IPCC computer model runs, they found that, on average, the models simulated a loss in September ice cover of 2.5 percent per decade from 1953 to 2006. … But newly available data sets, blending early aircraft and ship reports with more recent satellite measurements, show that the September ice actually declined at a rate of about 7.8 percent per decade during the 1953 to 2006 period.
“Because of this disparity, the shrinking of summertime ice is about thirty years ahead of the climate model projections,” said NSIDC scientist and co-author Ted Scambos. This suggests that the Arctic could be seasonally free of sea ice earlier than the IPCC projected range of 2050 to well beyond 2100.
What does this mean? The computer models used to calculate climate change assume that half of the ice loss during the past 27 years was triggered by rising concentrations of greenhouse gases and half by natural shifts in year-to-year weather and climate. This new analysis suggests that the gases play a bigger role than scientists realized.
The scientists are still sorting out what explains the disconnect. Some computer models may overestimate the thickness of the sea ice. Accurately reproducing how the Earth ocean and air currents really behave is also extremely difficult.
The loss in March — the climax of the floe-forming seasons — was off as well. Sea ice extent has shrunk about 1.8 percent per decade between 1953 and 2006, about three times faster than predicted by the models, Stroeve said.
Add it all up — changes in currents, increased greenhouse gas concentrations, warmer and warmer ocean absorbing ever more sunshine, skinnier and more fragile floes — and Arctic ice loss has accelerated about 9.1 percent per decade from 1979 to 2006, according to satellite observations.
“The Arctic has often been viewed as a region where the effects of (greenhouse gas) loading will be manifested early on, especially through loss of sea ice,” the authors concluded. “The sensitivity of this region may well be greater than the models suggest.”
Want more bad news? The trend continues this winter and spring, Meier said.
In March, the climax of the floe-building season throughout the Arctic, the area covered by sea ice extent was the second lowest ever measured by satellites.
“We’ve been consistently running about 1 million square kilometers (about 386,100 square miles) below the long-term mean throughout the fall and winter,” Meier said in an email message. “The melt seems to be occurring a bit faster this April and for the past three weeks we’ve been pretty consistently below the 2006 extent values by about 100,000 sq. km.”
March 2007 was not a record, Meier said, “but April 2007 will be.”
[...] dramatic. Earlier this spring, researchers at the National Snow and Ice Data Center reported that Arctic ice shrink may be 30 years ahead of worst-case [...]
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