(Climate Central) - by Michael D. Lemonick
The National Snow and Ice Data Center announced Monday that Arctic sea ice is at its lowest extent since satellite observations began in the 1970s. The previous record, set in 2007, saw the Arctic Ocean covered with 1.61 million square miles of ice, but on Sunday, the coverage was just 1.58 million square miles. What makes this milestone especially significant is that there are still two or three weeks left to go in the melt season.
"We still have a little time left," said NSIDC's Julienne Stroeve in an interview, "but the rate of melting in August has been really fast, the fastest we've ever seen." It's starting to slow down, she said, "but I doubt we've seen the true minimum."
Arctic sea ice extent for August 26, 2012 (right) was 27,000 square miles below the September 18, 2007 daily extent (left).
Credit: National Snow and Ice Data Center (NSIDC)
It's not just a new Guinness record that's at stake here. The rapid decline in Arctic sea ice during the past several decades is one of the most visible signs of manmade global warming.
Arctic ice, whether on land or on the sea, is a powerful reflector that bounces a lot of sunlight back into space rather than letting it warm the Earth. When that ice melts, it exposes the darker ground or water underneath, turning the region into an energy absorber rather than a reflector. Sea ice is especially vulnerable to melting, and over the past 30 years or so there's been a downward trend in sea ice coverage in summer. The result is a feedback loop that accelerates global warming, with melting ice leading to more warming of the water below leading to more melting
The Arctic is warming at nearly twice the rate of the rest of the globe, largely due to those feedback loops. In addition, recent research shows that the loss of sea ice cover may be contributing to extreme weather events throughout the Northern Hemisphere, and may be partly responsible for major cold air outbreaks and paralyzing snowstorms in the northeastern U.S. and western Europe during the past few years.
This trend isn't perfectly steady, since year-to-year weather conditions can affect the melt for a particular season. In 2007, for example, Stroeve said, wind patterns brought warm air into the region and pushed significant amounts of ice out of the Arctic ocean. "This year," she said, "conditions were kind of chaotic. We did have a big storm in August that broke the ice into smaller pieces, but the breakup was easier because the ice was already thin."
The ice was thin in large part because of the meltbacks in previous summers. In places where the ice doesn't melt, any additional freezing that happens during the winter can make the so-called multi-year ice even thicker. In places where it does melt, by contrast, the winter refreezing creates thinner first-year ice, which can melt with relative ease the following summer. Since 1980, the average thickness of Arctic sea ice has been cut nearly in half.
That trend is virtually certain to continue, until at some point the Arctic Ocean will be completely ice free for at least part of the summer, turning the planet's greatest heat shield into a global-warming accelerator. Exactly when that will take place isn't clear. "Current models show this happening before 2050," Stroeve said. "But the trends we observe make me think it could happen earlier than that."