The terms "Extent", "Area", "Thickness", and "Concentration" are central to ASIF discussions, but the glossary does not have definitions for those terms. Here are suggested versions. They are longer than the other glossary entries. But given the centrality of these terms, I think the word counts are justified. If shorter definitions are required, so be it. Less complete but more concise versions would be better than nothing.
Extent = area of the Arcitc ocean where at least 15 percent of the surface is frozen. This threshold was chosen because scientists have found that it gives the best approximation of the edge of the ice.
Source:
https://www.epa.gov/climate-indicators/climate-change-indicators-arctic-sea-ice#:~:text=Sea%20ice%20extent%20is%20defined,the%20edge%20of%20the%20ice.Area = area covered by each grid cell by the percentage of sea ice it contains, provided it has at least 15 percent concentration.
Here is an example using three equal-sized grid cells of satellite observation. The three cells have ice cover on 5%, 50%, and 100% of their respective area.
Area = 0 for the 1st cell (since it is <15%).
0.5 * 1 km2 for the 2nd cell = 0.5
1 * 1 km2 for the 3rd cell = 1
Total AREA for the 3 cells = 0 + 0.5 + 1 = 1.5 km2 By contrast, the Extent values for these same cells would be
0 for the 1st cell (since it is <15%).
1 * 1 km2 for the 2nd cell = 1 (because 0.5 is >15% it counts as 1 for Extent)
1 * 1 km2 for the 3rd cell = 1
Total EXTENT for the 3 cells = 0 + 1 + 1 = 2 km2 Because of the way Exent and Area are defined, Extent is always greater than Area.
Extent describes the size of the ocean area defined by the perimeter of the Arctic sea ice.
Area describes the area within that perimeter covered by ice.
Source:
https://nsidc.org/learn/ask-scientist/what-difference-between-sea-ice-area-and-extent#:~:text=Sea%20ice%20area%20is%20the,gives%20higher%20values%20than%20area.Thickness - Freshly formed sea ice can be as thin as a few sheets of paper or as thick as a one-story house, depending on how long it has existed. Measuring sea ice thickness is harder than measuring how much sea ice covers the ocean surface. No current technology enables scientists to directly and continuously measure sea ice thickness over a broad area. Instead, they use multiple approaches to estimate sea ice thickness: surface measurements, submarine surveys, remote sensing, and computer models. Every method currently employed to measure sea ice thickness has its limitations, and methods differ in the details.
Source:
https://nsidc.org/learn/ask-scientist/how-thick-is-sea-iceConcentration - Area divided by Extent.