New charts - northern hemisphere Arctic sea ice maxima

A recent publication has highlighted the importance of the maximum extent of seasonal ice cover in the Arctic Ocean to the stability of the over all sea-ice system. So I have acquired data from NOAA of the daily sea ice extent since 1979, which I have used to find the maximum annual extent and the date (expressed as a Julian Day - so March 1, 2019 would be day 60, but March 1, 2020 would be day 61). These data sets will be studied in the usual manner, if appropriate. (I used the five-day trailing average, by the way).

The maximum sea-ice coverage has declined since 1979

The graph suggests that the maximum extent of ice coverage has declined more or less constantly over the observation period. There are a few ups and downs. In fact, something popped out at me. In the graph above, what do 1998, 2001, 2008, 2012, and 2020 have in common? They all represent years where sea ice extent increased - presumably due to cooling. They were also years with somewhat trying economic circumstances, at least for some people. Coincidence? Probably--but maybe it means there was an agricultural trigger to some of the economic crises of the past few decades.

There has been an idea that as global warming proceeds, the timing of certain important events will change, becoming either earlier or later. Investigating the above graph for a trend in the timing of the ice extent maximum each year tells me . . . nothing. If there is a trend, it is very weak.

Below we have the 2-d reconstructed state space portrait of the annual maxima sea ice extent, using the time-delay method with a 2-year lag. 

The state space reconstruction shows three regions of stability. The S1 and S2 correspond roughly to the major area of stability in the sea-ice minima phase space, which may actually be two separate such regions, one larger than the other. S3 corresponds with the most recent low-area sea-ice minima.

As with the sea-ice minima plots, there is not enough data to determine the long-term future of this system. It is possible we are in a declining phase of a century-scale cyclical system. Alternatively, we could be on a decline to zero. We may even be in a biased cycle, where the natural cycle is being influenced by global warming. The problem is that the data are not sufficient to tell us which is the correct interpretation.