Guest post by David Middleton
We keep hearing about the Arctic being ice-free anytime from next month up until a continuously rolling forward decade or so. One question that has to be answered is:
What does ice-free mean?
When does ice-free mean ice-free?
First, we need to clarify what exactly an “ice-free” Arctic summer is.
By “ice-free”, scientists usually mean a sea ice extent of less than one million square kilometres, rather than zero sea ice cover.
–Dr Alexandra Jahn, Assistant Professor in the Department of Atmospheric and Oceanic Sciences and Fellow at the Institute of Arctic and Alpine Research at the University of Colorado. Carbon Brief, August 25, 2016.
Why use extent rather than area?
What is the difference between sea ice area and extent?
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 is cheese only, not including the holes. That is 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 areas that are over the 15% threshold by the percent of sea ice in those grid cells, and add it up. You would have a total area of 662 square km (255.8 square miles).
Scientists at NSIDC 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 quite valuable. Given the right circumstances, background knowledge, and scientific information on current conditions, it can provide an excellent sense of how much ice there really is “on the ground.”
Arctic sea ice as a percentage of the area of the Arctic Ocean
The Arctic Ocean has a surface area of approximately 14,056,000 km2. An ice-free Arctic would be less than 1,000,000 km2 of sea ice extent during summet. This would equate to less than 7% of the Arctic Ocean’s surface area.
During the era of satellite measurements of Arctic sea ice, the minimum ice extent has always occurred in September and the maximum extent has almost always occurred in March, occasionally in February.
|March (1979-2008 Avg)||Min||Max|
|Sept (1979-2008 Avg)||Min||Max|
Try to process logically process this:
The 1970’s Arctic sea ice extent range of 55-116% of the area of the Arctic Ocean gave us this:
Whereas, the 2017-2018 Arctic sea ice range of 35-102% of the area of the Arctic Ocean gave us this:
The stupid literally could not burn any brighter.
Here’s the graph without funny magazine covers…
And here’s my spreadsheet:
Arctic sea ice in the context of the Holocene
The Arctic was probably ice-free during summer for most of the Holocene up until about 1,000 years ago. McKay et al., 2008 demonstrated that the modern Arctic sea ice cover is anomalously high and the Arctic summer sea surface temperature is anomalously low relative to the rest of the Holocene.
Over most of the Holocene, >50% sea ice coverage occurred from 5.5 to 9 months each year. During the “Anthropocene”, >50% sea ice coverage has ranged from 9 to 12 months each year.
Fetterer, F., K. Knowles, W. Meier, M. Savoie, and A. K. Windnagel. 2017, updated daily. Sea Ice Index, Version 3. [Indicate subset used]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: https://doi.org/10.7265/N5K072F8. [Accessed September 26, 2018].
McKay, J.L., A. de Vernal, C. Hillaire-Marcel, C. Not, L. Polyak, and D. Darby. 2008. Holocene fluctuations in Arctic sea-ice cover: dinocyst-based reconstructions for the eastern Chukchi Sea. Can. J. Earth Sci. 45: 1377–1397