Sea ice extent measures the area of the sea where there is at least some minimum concentration of sea ice, typically 15% coverage. Sea ice area is a measure of the actual area covered by the sea ice. Because extent counts grid cells which only have partial coverage, extent will always be larger than area, but when things are in the deep freeze and there is little breakage at the edges and even in the interior of the ice pack, they will approach each other. Thus the difference/ratio of the two is a measure of compactness
Everybunny who owns the keys to a blog has been showing Winipus' scary global sea ice extent graph, which if anything as iconic of the mess that we are in of our own doing as any hockey stick.
The dive at the end indicates the continuing breakup of the Antarctic ice pack while growth of the ice in the Arctic is historically low. However Winipus has now produced a sea ice area graph which is beyond scary
In previous years the November peak is well above the June one. Not this year.
Sea ice is crashing. The clause is probably human driven climate change imposed on natural variability, but the reticence of scientists can dangerously go the more study is needed route too easily. Mark Serreze from the National Snow and Ice Data Center has gone the full Al Gore is an alarmist route
The combined number, while easy to derive from our online posted data, is not useful as an analysis tool or indicator of climate trends. Looking at each region's ice extent trends and its processes separately provides more insight into how and why ice extent is changing. Sea ice in the Arctic is governed by somewhat different processes than the sea ice around Antarctica, and the very different geography of the two poles plays a large role. Sea ice in the Arctic exists in a small ocean surrounded by land masses, with greater input of dust, aerosols, and soot than in the Southern Hemisphere. Sea ice in the Southern Hemisphere fringes an ice-covered continent, Antarctica, surrounded by open oceans. While both regions are affected by air, wind, and ocean, the systems and their patterns are inherently very different. Moreover, at any point in time, the two poles are in opposite seasons, and so a combined number would conflate summer and winter trends, or spring and autumn trends, for the two regions.The detailed mechanisms may differ, but the cause is the same.