So far this summer, Arctic sea ice has been melting at a record pace. Satellite data, which go back to 1979, show that ice extent is currently lower than it was at the same time in 2007, the year that went on to shatter all previous records for low ice extent in September, the end of the melt season (Figure 1). It is not yet clear if the ice will hit a new record low this September. But whether or not the ice extent sets another record, Arctic sea ice is continuing its long-term decline, a trend that researchers say is related to warming temperatures in the Arctic.
This time of year, we receive a lot of questions about the upcoming sea ice minimum. What is it and why does it matter?
What is the Arctic sea ice minimum extent?
Each summer, Arctic sea ice melts and contracts, retreating to less than half its winter extent before cooler fall temperatures halt the ice loss, and ice cover again starts to expand. The lowest ice extent of the year, or the Arctic sea ice minimum, occurs in September.
Researchers look to the sea ice minimum because it provides a clear measure of the health of the ice cover. NSIDC scientist and director Mark Serreze said, “After all of the autumn and winter ice growth has occurred, and after all of the melt occurring through summer, what have we got left?”
Scientists measure both the minimum ice extent on the lowest day of the year, typically occurring in the first or second week of September, as well as the average extent for September. While the amount of ice on the lowest day is important, researchers place more weight on monthly averages in calculating long-term trends. The monthly data for September will be available in the beginning of October.
Why does the amount of summer sea ice matter?
Scientists are worried about declining Arctic sea ice because the ice plays an important role in global climate. The bright white sea ice reflects sunlight and heat back into space, so that the Arctic region remains cooler than it otherwise would. The cold temperatures in the Arctic in turn act as a sort of air conditioner for the rest of the world.
As ice cover retreats, additional areas of open water absorb more heat during the summer, leading to a positive feedback effect where the loss of sea ice in turn leads to more sea ice loss and puts more heat into the upper part of the ocean. While ice will grow back during the winter period of polar darkness, come spring it will be thinner than it used to be, melting out all the more easily the next summer.
When will the minimum ice extent happen this year?
Every year, people ask us when Arctic sea ice will hit its minimum extent. In the past, the minimum ice extent has occurred as early as September 3, and as late as September 22. The average date is September 10. There is no way to know exactly when the ice will stop contracting and start expanding, because it depends in part on changeable weather patterns. However, in recent years there has been a trend towards a later minimum ice extent, likely because the increased areas of open ocean absorb more heat during the summer. NSIDC researcher Walt Meier said, “That heat in the ocean needs to dissipate before new ice can grow.”
Once ice extent has been growing for three or more days in a row, NSIDC reports a preliminary extent measurement and minimum date. However, since sea ice extent is strongly influenced by weather and wind, that number will not be final until the complete September data are available in the beginning of October.
What would it look like for sea ice to recover?
After hitting an unprecedented record low in September 2007, Arctic sea ice extent has not fallen near that level again (Figure 2). Some people have asked us if that means sea ice is recovering.
Researchers note that 2007 was not one low year in an otherwise stable system. Arctic sea ice has shown a clear decline since the start of satellite monitoring in 1979. On top of that downward trend, the year-to-year ice extent moves up and down, influenced by variable weather patterns. Serreze said, “The minimum extent is especially sensitive to summer weather patterns and these change from year to year.“
If sea ice were recovering, researchers would expect to see a change in the trajectory of the trend line, with minimum ice extents that fall close to or above the 1979 to 2000 average. Meier said, “Any meaningful recovery would take several years to build up ice thickness. We expect to see ups and downs in the coming years and there may be several years between new record low minimums, but the overall long-term trend will continue downward.”