After a false alarm earlier this month, the 2010 Arctic sea ice melt season has come to a close, with sea ice extent reaching the third-lowest in the satellite record. This continues the steady and steep decline in sea ice cover during the past few decades, which scientists have traced mainly to emissions of greenhouse gases, as well as natural climate variability. Underscoring the rapid changes sweeping the Arctic, both the Northwest and Northeast Passages were open for a time, and two sailboats set new records for transiting both of them in just one season — a feat that would have been impossible throughout modern history.

Yesterday, the National Snow and Ice Data Center (NSIDC) in Boulder, Colo. announced the end of the melt season, with sea ice extent dropping very close to the level reached in 2008. (Earlier this month, NSIDC had pronounced the sea ice melt season over, only to retract that five days later after sea ice began declining again. Further declines at this point are not anticipated). 

According to NSIDC, sea ice extent fell to the third-lowest on record at 1.78 million square miles, which was more ice than in the record melt season of 2007, but only about 14,000 square miles greater than the second lowest melt season on record in 2008. The 2010 minimum ice extent was 672,000 square miles below the 1979 to 2009 average. The minimum occurred on Sept. 19, which was eight days later than the 1979 to 2009 average.

The chart below shows sea ice extent this year compared to the last several years. Note how close 2010 came to dropping below the 2008 minimum.

Daily Arctic sea ice extent as of Sept. 26, 2010, along with daily ice extents for years with the previous four lowest minimum extents. Solid gray indicates average extent from 1979 to 2000.
The gray area around the average line shows the two standard deviation range of the data.
Credit: NSIDC

Another key metric — sea ice volume — tells a similar, albeit more alarming story (for information on the differences between sea ice volume and extent, see NSIDC's FAQ page). According to the University of Washington's Polar Science Center, sea ice volume plunged to a new record low this year. The chart of sea ice volume shows a stunning drop compared to the overall rate of decline in the past several decades, and indicates that this summer, the Arctic Ocean contained unusually sparse and thin sea ice. 

Arctic Sea Ice Volume Anomaly from the PIOMAS model. Daily sea ice volume anomalies for each day are computed relative to the 1979 to 2009 average for that day. The trend for the 1979-present period is shown in blue.
Shaded areas show one and two standard deviations from the trend.
Credit: Polar Science Center.

Here's how the Polar Science Center describes the relevance of sea ice volume in relation to the NSIDC's reporting of sea ice extent:

"Arctic Sea Ice Volume is an important indicator of climate change because it accounts for variations in sea ice thickness as well as sea ice extent. Total Arctic sea ice volume cannot currently be observed continuously. Observations from satellites, Navy submarines, moorings, and field measurements are limited in space or time. The assimilation of observations into numerical models, currently provides one way of estimating sea ice volume changes on a continuing basis."

More ice monitoring products are also available at the National Ice Center, which is a federal agency that tracks land and sea ice.

As Joe Romm has reported over at Climate Progress, the openness of the Arctic Ocean this summer allowed two sailing vessels to set a new milestone by transiting both the Northwest and Northeast Passages in the same season. (Check out the captain's log from one of the vessels, which belongs to a Norwegian polar explorer, as the ship exited the Northwest Passage). That alone should be enough to drive home the implications of sea ice loss for marine shipping, natural resources extraction, and military activities.

Sea ice decline will be very difficult to reverse — even if greenhouse gas emissions are rapidly and significantly cut, a scenario that appears increasingly unlikely — since feedback effects help ensure that sea ice loss in the summer leads to warmer water and air temperatures and thinner ice in the fall and winter, which leaves more vulnerable ice heading into the next melt season. The realclimate blog has an excellent post on climate feedbacks that explains the ice-albedo feedback, which is one of the key ways in which a warming climate may propel a virtually self-sustaining loss of sea ice.

Some climate change contrarians, such as meteorologist Anthony Watts, instead promote the idea that sea ice decline (along with the majority of recent warming in general) is primarily driven by natural ocean cycles, and that a cooler North Atlantic and Pacific Ocean could allow Arctic sea ice to recover. I find that argument to be highly dubious, based on a reading of the scientific literature and interviews with Arctic sea ice researchers.