Arctic sea ice continues rebound

When we last checked in to the Nansen Sea Ice Graphs, it looked like they were heading towards the “normal” line in a hurry. Ice area seems to still be on that trend, while extent seems to be leveling off it’s growth rate. Area appears to be within about 200,000 square kilometers of the 1979-2007 monthly average and still climbing.

Of course the fact that the 2007 data is included in the average line, means the average is a lower than usual target than one might expect. If we compare to ice area over at Cryopshere today, they use a 1979-2000 mean, which is higher.  Still the rebound we are seeing is impressive.

Sea ice extent looks like this:

These graphs will automatically update, so check back often.

For those of you wondering, here is the difference between area and extent, as described in the NSIDC FAQ’s page:

What is the difference between sea ice area and extent? Why does NSIDC use extent measurements?

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. Extent is always a larger number than area, and there are pros and cons associated with each method.

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. Area would be the measure of where there’s cheese only, not including the holes. That’s why if you compare extent and area in the same time period, extent is always bigger. A more precise explanation of extent versus area gets more complicated.

Extent defines a region as “ice-covered” or “not ice-covered.” For each satellite data cell, the cell is said to either have ice or to have 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 kilometer (km) x 25 km (16 miles x 16 miles) 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 area by 100% sea ice and you would get a total extent of 1,250 square km (482 square miles).

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 area by the percent of sea ice and add it up. You’d have a total area of 675 square km (261 square miles).