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04-Nov-2017 06:00

A simplified way to think of extent versus area is to imagine a slice of swiss cheese.Extent would be a measure of the edges of the slice of cheese and all of the space inside it. Is wintertime Antarctic sea ice increasing or decreasing?Our originally published value was 4.13 million 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 (16 miles x 16 miles) grid cells of 16% ice, 2% ice, and 90% ice, multiply the grid cell areas that are over the 15% threshold by the percent of sea ice in those grid cells, and add it up.Why are the daily minimum values that you are currently quoting for past years slightly different than what you originally stated? The new calculations show, for example, that the record minimum occurred on September 18, 2007, which was two days later than we originally reported (September 16).Why is the monthly average extent not equal to the average of the daily extent values? Why do you use the 1981–2010 average for comparisons? In addition, NSIDC updates extent values, calculated initially with near-real-time data, when final processed data becomes available.

Scientists at NSIDC report extent because they are cautious about summertime values of ice concentration and area taken from satellite sensors.In the final data, the date of the minimum may also change for some years.For more information on calculating daily sea ice extent values, see the Sea Ice Index documentation.This lets us compare “apples to apples” and see change on a year over year time scale.

Created October 7, 2013 Area and extent are different measures and give scientists slightly different information.Some organizations, including Cryosphere Today, report ice area; NSIDC primarily reports ice extent.