Many of us have seen graphs of the dramatic decline of Arctic sea ice over the last 30+ years (since we’ve been monitoring it via satellites), a decline which is unmatched for at least a century and almost certainly for several thousand years. The graph of sea ice extent anomaly (monthly average data from the National Snow and Ice Data Center) shows the change plainly:



Both the extent and area of sea ice coverage in the arctic has declined at an alarming rate — the phrase “death spiral” comes to mind.

There’s far less data about the thickness or the arctic ice cap, but there is a growing body of evidence from satellites, moorings, aircraft, and from submarine measurements of ice draft. It reveals that not only is Arctic ice covering less of the sea, its thickness is also declining.

The good people at the Polar Science Center combine all the available data to form an estimate of Arctic sea ice volume. They have recently made their data available for download; let’s take a look.

Here’s their raw data, estimates of Arctic sea ice volume in thousands of cubic kilometers:

As expected, there’s an obvious seasonal cycle with more ice in winter and less in summer. In addition there’s a downward trend which is rather obvious even with the seasonal changes intact. The annual cycle shows the greatest volume in April, the least in September:

I’ve highlighted both 2010 and 2011 in red, with connecting lines. A clear sign of the continued loss of Arctic ice is that both 2010 and 2011 showed the lowest values on record, for every month of the year. The phrase “death spiral” comes to mind.

We can remove the seasonal cycle by computing anomaly, the difference between a given value and the average for that same time of year. For that purpose, I translated daily values of sea ice volume into monthly averages, then computed anomalies, giving this:

Except for a slight difference in the zero point (they use the 1979-2010 average as a baseline to define anomaly, wherease I’ve used the entire time span), this is essentially the same as the anomaly graph given on the PIOMAS website:

As most of you know, the annual minimum in sea ice extent occurs in September, as does the minimum of sea ice volume. The September extent is in sharp decline and the decrease has actually accelerated:

The September volume is also in sharp decline, one which has also accelerated, and shows its lowest value just last year:

We can divide the volume of sea ice by the area to get an estimate of the average thickness of the sea ice. This too shows both a decline and an annual cycle, with ice thinnest in December and thickest in July:

The reason it’s thinnest in early winter is that new ice has formed but it’s mostly thin, first-year ice, and the dead of winter hasn’t yet thickened it. Even so, the December thickness has declined steadily:

The thickness at its annual maximum in July has also declined:

Meanwhile, thickness during the September volume/extent minimum has declined sharply with the last two years showing a dramatic drop:

The decline in Arctic sea ice extent and area is truly alarming, and the decline in sea ice volume is even more so. The last two years have plumbed new depths in ice cover; the outlook for the polar ice cap is grim indeed.

The phrase “death spiral” comes to mind.