The latest satellite observations of sea ice in the Arctic shows the ice cover appears to be shrinking: the ice cap is getting smaller, and thinner as well. The ice has been receding more in the summers and not growing back to its previous size and thickness during the winters. Scientists say the ice is profoundly important, as ice is the defining characteristic for the eco-system of the Arctic region. But it is also important for the entire planet, as far as constraining the Earth’s heat budget, and affecting ocean flows and planetary weather.
Arctic sea ice works like an air conditioner for the global climate system. Ice naturally cools air and water masses, plays a key role in ocean circulation, and reflects solar radiation back into space. In recent years, Arctic sea ice has been declining at a surprising rate. As ice melts it is replaced with darker sea water that absorbs more sunlight and heats up the ocean and the planet overall.
According to researchers from the National Snow and Ice Data Center in Boulder, Colo., the maximum sea ice extent for 2008-09, reached on Feb. 28, was 5.85 million square miles (15,151,430 square kilometers). That is 278,000 square miles (720,016 square kilometers) less than the average extent for 1979 to 2000. This is the fifth lowest maximum ice extent on record. The six lowest maximum events since satellite monitoring began in 1979 have all occurred in the past six years (2004-2009).
Until recently, the majority of Arctic sea ice was multi-year ice, which means it survived at least one summer and often several winters. This multi-year ice is thicker and can survive longer than the seasonal ice that melts and re-freezes every year. But things have changed dramatically. According to the scientists, the thin, seasonal ice now makes up about 70 percent of the Arctic sea ice in wintertime, up from 40 to 50 percent in the 1980s and 1990s. Thicker ice, which survives two or more years, now comprises just under 10 percent of wintertime ice cover, down from 30 to 40 percent.
“9.8 percent of the ice is greater than 2 years old,” said Walt Meier, research scientist with NSIDC, at a teleconference with reporters today. “So, it’s about a third of what it used to be in terms of really old thick ice.”
Meier said the thickest and oldest ice has been on a big decline the past couple of years. “Right now, this is the lowest we’ve had,” he said. “Last year, multi-year ice made up 14 percent of the Arctic ice cap. In 2007, it was about the 25% range. That is a pretty sharp decrease. We did see some recovery in 1-2 year old ice, which is up from a low of 5 %. In theory that ice could survive, if it doesn’t get exported out of the Arctic.”
Winds and ocean flows also “flushes” ice out of the Arctic region, Meier said.
Data from NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) has now produced first map of sea ice thickness over the entire Arctic basin.
Ron Kwok from JPL who works with ICEsat said, “This is the first time we’ve had Arctic-wide ice thicknesses at the scale. During the 70’s and 80’s the average ice thickness was about 1.5-2 meters thicker than what we’re seeing at the current time.” Those measurements were taken using submarines and drill holes. Using ICEsat allows for the entire ice cap to be measured from space. ICEsat has been taking data for five years, and only the first two years of data (2005 and 2006) has been fully processed, but preliminary results show the decline is continuing.
During the teleconference, a journalist from northern Canada said their region has been experiencing colder winters the past couple of years, and asked if that was a good sign. “The ice is still in a precarious position,” said Meier, “and we can’t focus on short term trends of one or two years. Long terms trends show a warmer Arctic and thinner sea ice. It will take several cold years in a row to get back to where it was and to get the thick multi-year ice that can survive longer. This is not something that can be turned around in a couple of cool summers and colder winters.”
When asked if they could determine the ice depletion has come from natural or man-made causes, Meier said, “Sea ice certainly varies a lot over time, and we have fairly good records on how it has varied back to the early 1900’s, and we are confident it is much lower than it ever has been in the past half century. It’s clear the sea ice changes we are seeing go hand-in-hand with the warming planet, and the sea ice changes are entirely consistent with that. There isn’t another mechanism that could cause the long term changes we’ve seen.”
Sources: NASA, news conference