22 days ago, the extent of the sea ice in the Arctic Ocean crossed below the mark set in 2007, the year when record sea ice extent reached its lowest observed point. For each of the last 22 days, sea ice extent has been lower than on corresponding days in 2007, and the difference in extent between now and then has been growing daily. As of yesterday, the Arctic sea ice extent sat four standard deviations below the mean observed from 1979-2000.
Don’t just trip over that last sentence and move on. An observation four standard deviations below the mean is a big deal, an astoundingly strong indication that we’re not seeing a typical year. How astoundingly strong? Well, in a normal distribution centered on a mean, 99.997% of the time you make an observation of a randomly varying event you should expect to see conditions that fit within +/- four standard deviations; this is simply what the standard deviation means as a statistic. Applied to the Arctic sea ice data, this means if Arctic sea ice extent were varying randomly around a tendency still centered on that 1979-2000 mean, 99.997% of the time we took a reading of the Arctic Ocean’s ice extent, we’d find that the ice extent sat within plus or minus four standard deviations of the mean. The likelihood of seeing what we’re seeing now as just a regular part of the 1979-2000 climate is 0.003%.
It’s pretty fair, actually far beyond pretty fair, to say that there’s been a substantial deviation in the Arctic climate even from the relatively recent 1979-2000 conditions. It’s pretty fair to say that what’s going on is a big climatic shift.