New ICESat Data Shows Arctic Ice Cap Thinning

The latest Arctic sea ice data from NASA and the National Snow and Ice Data Center show that the decade-long trend of shrinking sea ice cover is continuing, and new evidence from satellite observations shows that the ice cap is thinning.

Thicker ice, which survives two or more years, now makes up just 10 percent of wintertime ice cover in the Arctic Basin, down from 30 to 40 percent in the 1980s and 1990s, the scientists told reporters on a conference call today.

Since satellite monitoring began in 1979, the six lowest maximum winter ice measurements across the Arctic Basin have all occurred in the past six years, 2004 to 2009, the scientists said.

This data visualization from the AMSR-E instrument on the Aqua satellite show the maximum sea ice extent for 2008-09, which occurred on Feb. 28, 2009. (Image courtesy NASA Goddard's Scientific Visualization Studio)

The maximum sea ice extent for the 2008-09 winter reached on February 28 was 5.85 million square miles. The scientists said that is 278,000 square miles less than the average extent for the years 1979 to 2000 - an area about the size of Texas.

Walter Meier, cryosphere scientist with the National Snow and Ice Data Center at the University of Colorado, Boulder, said the decrease in sea ice "goes hand in hand with warmer temperatures." He said it has been linked to global warming by the Intergovernmental Panel on Climate Change, which reviews and compiles climate science from around the world for its periodic consensus reports.

"There is no other mechanism than climate warming that could cause the sea ice changes we're seeing," Meier said.

Now, arctic sea ice extent has begun its seasonal decline towards the September minimum, and the scientists said this melt season has begun with a substantial amount of thin first-year ice, which is vulnerable to summer melt.

"Ice extent is an important measure of the health of the Arctic, but it only gives us a two-dimensional view of the ice cover," said Meier. "Thickness is important, especially in the winter, because it is the best overall indicator of the health of the ice cover. As the ice cover in the Arctic grows thinner, it grows more vulnerable to melting in the summer."

Until recently, the majority of Arctic sea ice survived at least one summer and often built up over several years. But now, thin seasonal ice that melts and re-freezes every year makes up about 70 percent of the Arctic sea ice in wintertime, up from 40 to 50 percent in the 1980s and 1990s.

Sea ice thickness has been hard to measure directly, so scientists have used estimates of ice age to approximate its thickness. But last year a team of researchers led by Ron Kwok of NASA's Jet Propulsion Laboratory in Pasadena, California produced the first map of sea ice thickness over the entire Arctic Basin.

Using two years of data from ICESat, NASA's Ice, Cloud, and land Elevation Satellite launched in 2003 into an orbit 70 miles high, Kwok's team estimated thickness and volume of the Arctic Ocean ice cover for 2005 and 2006.

The older, thicker sea ice is declining and is being replaced with newer, thinner ice that is more vulnerable to summer melt, says Kwok. The thick, multi-year ice cover has not been able to recover from recent declines, he said.

Arctic ice thickness data shows shrinkage of thicker, older ice. (Image by J. Maslanik and C. Fowler courtesy U. Colorado and NSIDC)

Kwok's team found that seasonal sea ice averages about six feet in thickness, while ice that had lasted through more than one summer averages about nine feet, though it can grow much thicker in some locations near the coast as the ice piles up on itself and against the coast.

Sea ice grows and melts within the ocean so it does not change sea level directly, Meier said, but it has an indirect effect. As the sea ice melts, it exposes the ocean to the Sun, so warming is amplified.

Meier explained that the open ocean is a source of moisture which could get picked up by the atmosphere and be deposited on Greenland as snow, growing the mass of Greenland. But, he said, if temperatures get warm enough, Greenland will lose a lot of land and the melting of land ice does raise sea levels.

To restore the amount of older ice to pre-2000 levels, large amounts of first year ice would need to endure through summers for several years in a row.

But each winter, winds and ocean currents move some sea ice out of the Arctic ocean. This winter, some second-year ice survived the 2008 melt season only to be pushed out of the Arctic by strong winter winds.

Based on sea ice age data from Jim Maslanik and Chuck Fowler at the University of Colorado, since the end of September, 150,000 square miles of second-year ice and 73,000 square miles ice more than two years old moved out of the Arctic.

The prediction of an ice-free summer as early as 2013-2014 is "not totally out of the realm of possibility," Meier said. "Five years ago that suggestion would have been laughed out of the room, now scientists say it's very unlikely but possible."

"We've already lost over one-third of our summer ice cover from the 1980s-1990s, said Meier. "We're already seeing impacts, we don't have to wait for the sea ice disappear during the summer."

The Bering Sea experienced a cool winter, with temperatures 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) below average, but that is not enough to recover the thick multi-year ice cover of the 1980s, the scientists said.

"In terms of long-term health of the climate we're still in a very precarious position," Meier said. "One or two cooler years could slow things down for a while, but long term trends are for a warmer Arctic and thinner sea ice cover."

Copyright Environment News Service (ENS) 2009. All rights reserved.