Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 1:33 PM GMT on April 20, 2009
March 2009 Northern Hemisphere sea ice extent was the 6th lowest since 1979, according to the National Snow and Ice Data Center. The record March low was set in 2006, and it has now been nearly two years since we have set any record monthly minimums for Arctic sea ice. While it is good news that the area of sea ice coverage has not been reaching record lows recently, there is concern about the recent record loss of thick, multi-year ice in the Arctic. Strong winter winds pushed a considerable amount of multi-year-old ice out of the Arctic this year, leaving the Arctic Ocean with its lowest amount of old sea ice on record (Figure 1). Sea ice more than two years old fell below 10% for the first time since satellites began observing the ice in 1979. This is a factor of three lower than the 30% coverage for the period 1981 through 2000.
Figure 2. These images show declining sea ice age, which indicates a thinning Arctic sea ice cover more vulnerable to melting in summer. Ice older than two years now accounts for less than 10% of the ice cover. Image credit: National Snow and Ice Data Center, courtesy J. Maslanik and C. Fowler, University of Colorado.
As the ice melting season begins this year, the exceptionally low amount of old, thick ice leaves the Arctic very vulnerable to melting. Air temperatures over the Arctic Ocean this past winter were 1 - 2°C (1.8 - 3.6°F) above average, and a continuation of conditions this warm would probably cause record melting of the Arctic ice cap this summer. On the other hand, the amount of 1 - 2 year old in the Arctic increased in 2008 compared to 2007, so if the Arctic experiences below average temperatures this summer and throughout 2010, the potential exists for old ice to make a comeback. However, the Arctic has experienced very warm temperatures in excess of 1°C above average for most of the past decade (Figure 2). The latest Arctic Report Card 2008 concludes that "it is becoming increasingly likely that the Arctic will change from a perennially ice-covered to an ice-free ocean in the summer". The best hope I see for the Arctic sea ice to recover in the next few years is for a major volcanic eruption in the tropics to create a "Volcanic Winter" cooling effect for a year or two. Such an eruption would probably not allow for a complete recovery of Arctic sea ice, but might delay the transition to a summertime ice-free state by several years.
Figure 2. Arctic-wide annual averaged surface air temperature anomalies (60°-90°N) based from 1900 - 2007 on land stations north of 60°N relative to the 1961 - 1990 mean. From the CRUTEM 3v dataset, (available online at www.cru.uea.ac.uk/cru/data/temperature/). Note this curve does not include ship observations. The year 2007 was the warmest year on record in the Arctic. The year 2008 (not plotted) was cooler than 2007. Image credit: Arctic Report Card 2008.
As usual, I'm saying little about Antarctic sea ice, since the ice at the bottom of the world is not changing much and has a small impact on global climate compared to Arctic sea ice. Antarctic sea ice is currently quite a bit above average in extent, making total global sea ice above average this month. However, as I explained in a post earlier this year, drawing attention to this statistic is not a very intelligent thing to do, and hides the important fact that Arctic sea ice is in serious danger.
My next post will be on Wednesday--Earth Day--when I'll pick my top wunderphotos from the past year. I'll talk more about "Volcanic Winter" on Friday.
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