Monday, October 13, 2008
More on Melting Northern Sea Ice
I sent an e-mail containing my posting about the October 2, 2008 press release from the National Snow and Ice Data Center to Stephanie Renfrow, who works up there, and she was kind enough to write me back and answer the questions I had. I'm still digesting the information she gave about multi-year ice, but I can report on why the NSIDC has a much different measurement of sea ice in the Northern Ocean than the University of Illinois at Urbana-Champaign (The Cryosphere Today). As of today, the NSIDC measurement of sea ice extent is about 6.33 million square kilometers while The Cryosphere Today has the sea ice area at about 4.33 million square kilometers. To me that's a substantial difference.
Here's the explanation from the horses mouth:
So if I understand that properly, 'extent' measurement is better in Summer because the microwave measurer in the satellite has trouble telling melt water on top of sea ice from plain old sea; and the extent numbers used by the NSIDC give one a better 'big picture' then. Likewise, it might be better to use the University's 'area' numbers in Winter to be more precise.
The difference in numbers reported between Cryosphere Today and NSIDC is not a difference in data, but a reflection of a completely different type of measurement drawn from the same data. Cryosphere Today reports sea ice area, while NSIDC reports sea ice extent. Both measurements agree in the larger story: a negative trend in Arctic sea ice over the satellite record. For more details, please read below.
Area and extent are different measures that give scientists slightly different information. Some organizations report ice area; NSIDC primarily reports ice extent. Extent is always a larger number than area, and there are pros and cons associated with each method.
Extent defines a region as "ice-covered" or "not ice-covered." For each satellite data cell, either the cell is said to either have ice or no ice, based on a threshold. The most common threshold (and the one NSIDC uses) is 15 percent, meaning that if the data cell has greater than 15 percent ice concentration, the cell is considered "ice-covered;" less than that and it is said to be ice free. Example: let's say you have three 25 km x 25 km grid cells covered by 16% ice, 2% ice, and 90% ice. Two of the three cells would be considered "ice covered," or 100% ice. Multiply the grid cell are by 100% sea ice and you would get a total extent of 1,250 square kilometers.
Area takes the percentages of sea ice within data cells and adds them up to report how much of the Arctic is covered by ice; area typically uses a threshold of 15%. So in the same example, with three 25 km x 25 km grid cells of 16% ice, 2% ice, and 90% ice, multiply the grid cell area by the percent of sea ice and add it up. You'd have a total area of 562.5 square kilometers.
Scientists at NSIDC prefer to report extent because they are cautious about summertime values of ice concentration and area taken from satellite sensors. To the sensor, surface melt appears to be open water rather than water on top of sea ice. So, while reliable for measuring area most of the year, the microwave sensor is prone to underestimating the actual ice concentration and area when the surface is melting. To account for that potential inaccuracy, NSIDC scientists rely primarily on extent when analyzing melt-season conditions and reporting them to the public. That said, analyzing ice area is still valuable. Given the right circumstances, background knowledge, and scientific information on current conditions, it can potentially provide a better sense of how much ice there really is "on the ground."
I am very grateful that Ms. Renfrow and the unnamed scientist she refers to took the time to respond to my questions. Thank you all.