Another arctic melt season is over, so it’s time to check progress. This year it’s more good news. The recovery over the last couple of years has continued, though it remains but a bump in a long term decline.
Sea ice is ice floating on sea water and formed from that water (mostlty; there’s some frozen snow). It excludes the great floating ice shelves formed by glacial outflow from land and the icebergs that break from them, and it’s quite separate from the giant ice sheets covering Greenland and Antarctica. Our planet has permanent sea ice covering much of the Arctic Ocean in the north and on the Southern Ocean surrounding Antarctica. It has had for a very long time — probably since at least the Eemian, the brief warm period about 120,000 years ago, before the last ice ‘age’*. Summer sea ice is much more extensive in the north than the south — currently by about a factor of two, previously nearer three. (The summers are of course out of phase, six months apart.)
Those paying attention will know that while the lesser southern summer sea ice area has been stable to slightly increasing, northern sea ice has been undergoing dramatic and largely unexpected collapse; so much so that some have predicted that we would see a seasonally ice free northern polar ocean by early next decade. We may still; timing depends on whether the apparent accelerating decline continues all the way to zero, or commences an S-curve tail-out. The last few years data now favour the second scenario.
My preferred measure — as always — is PIOMAS, because it tracks total ice volume rather than the easier to measure ice area (actual frozen surface) or ice extent (sea surface with at least some floating ice). It therefore also considers ice thickness (volume being area times average thickness). PIOMAS is just a data assimilation model, but one that now has demonstrated skill. It’s a product of the Polar Science Centre at the University of Washington. Here’s how it looks:
PIOMAS northern sea ice volume
Notes:
* More correctly, “last glacial”. Technically “ice age” refers to the whole interval — the 2 million years of the Pleistocene with its periodic glaciations, not just the individual glacials.
