The Arctic sea ice shell game

Winter sea ice extent doesn't vary all that much; summer is where the action is.

Certain skeptical corners of the internet have been abuzz in recent weeks about Arctic sea ice extent, loudly claiming that “Arctic Sea Ice Soars to Highest Level for 21 Years”.

However, highlighting variations in winter Arctic sea ice extent is somewhat misleading; the Arctic largely refreezes in the winter and declines in sea ice in the season have been relatively modest over the past two decades. This means that its not at all unusual for a year have winter sea ice extent comparable to what was seen 20 years ago – though even winter sea ice remains well below the levels seen in the 1980s and 1990s.

Summer, by contrast, shows much stronger declines, with sea ice extent often more than two million square kilometers below where it was in the early 1980s at the start of the satellite record.

The figure below shows daily Arctic sea ice extent anomalies over the full satellite record, from 1979 through the end of January 2024. There is a notable transition after 2005 or so, where the year-to-year variability in the record becomes much larger. This represents the period where the loss of summer sea ice accelerated, while winter sea ice extent has remained relatively flat.

Daily Arctic sea ice extent anomalies from NSIDC. Anomalies are calculated based on day-of-year averages during the 1979-1988 period.

This shows up even more clearly if we separately plot summer and winter extent. The figure below shows summer Arctic sea ice (yellow) and winter (blue), as well as the linear trend over time in both. While winter sea ice extent has flattened over the past two decades, summer sea ice shows a more pronounced continued decline.

Summer (JJA) and Winter (DJF) sea ice extent anomalies from NSIDC. Anomalies are calculated based on day-of-year averages during the 1979-1988 period. Winter sea ice extent counts December values as part of the winter of the following year (e.g. December 2022 is part of winter 2023).

We can see an even greater decline in summer minimum sea ice (the lowest recorded daily value of the year), with a drop of around 3 million square kilometers since the start of the satellite record.

Annual minimum daily sea ice extent anomalies from NSIDC.

Winter is still cold in the Arctic, and winter sea ice is not going away this century even in a scenario where global emissions are not reduced. We only expect to see gradual winter sea ice declines in the future, as warming oceans chip away at the edges of winter sea ice formations. Summer, by contrast, is likely to see ice-free Arctic conditions regularly by mid-to-late century unless emissions rapidly decline.

To put it another way, there is a lot more ice to lose in the summer, and that is already reflected in the historical record. Summer sea ice is where all the action is, and skeptics highlighting year-to-year variations in winter sea ice are just playing a shell game.