Since 2001 the amount of Arctic Sea ice that has melted during the summer has generally increased. There may have been a long term trend in melting of ice in the northern hemisphere generally, including mountain glaciers, the Greenland glaciers, and seasonally, Arctic Sea Ice. But the seasonal melting of Arctic Sea ice seems to represent a metastable shift unprecedented in available data. There is probably a tipping point followed by positive feedback. From 2001 onwards, the amount of sea ice melted each summer has gone up, and this has resulted in two related effects: 1) The total amount of sunlight sent back into outer space by reflection from ice and snow has gone down and 2) the amount of warming of the Arctic Sea itself by that non-reflected sunlight has gone up. The result is a graph like this one (hat tip Arctic Sea Ice Blog):

Another view shows the numbers somewhat differently. The grey areas show the confidence limits for the 1979-2012 means, so it includes the reduced years, in volume, with the last four years plotted and the present year shown not as an estimate but as the actual measurement. This shows that we are on track to have a lot of melting:

These data include both good news and bad news, depending on how you want to spin it. The good news is that the seasonal reduction in sea ice volume is not lower then, or not a lot lower than, last years, so maybe we are seeing a leveling off in this phenomenon. The bad news comes in two parts. First, the volume of sea ice includes old ice, which tends to be thicker, and much of that has already melted away, so it can’t melt again because it is already gone. Second, being at the extreme low end of a disturbing trend does not mean that the trend is not disturbing. (See more discussion here.)

Let’s look at extent. This graph from the National Snow and Ice Data Center shows extent (not volume):

This shows that the current year is on track to look like last year. Notice the big dip last year’s ice took in just a few days from now. It will be interesting to see what the current year’s ice extend does over this same time frame. One of the differences between last year and this year is winds. There was a lot of wind facilitating the breakup of ice last year, but this years the winds are described as “slack.” Related to this, last year June had warmer temperatures over the ice. The last month this year has been relatively cold.

The next four weeks will be interesting to watch.

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