Step or Trend? The 20th Century Evolution of Arctic Ice Area

September ice area in the Arctic is analyzed using the satellite era record of ice concentration (1979-2014) along with longer records, including NIC charts and the Hadley SST2.2 dataset. Evidence is presented that breakpoints (statistically significant shifts in the mean) occurred in the Pacific sector, with higher amounts of open water starting in 1989 for example. Models with these breakpoints, as well as models with breakpoints in the Atlantic, Canadian and Russian sectors and the Arctic as a whole outperform linear trend models in fitting the data. An analysis of the NCAR CESM Large Ensemble demonstrates the same behavior. From a physical system standpoint, the results support the thesis that Arctic sea ice has critical states at which a return to the previous state is less likely. From an analysis standpoint, the findings imply that demeaning the data using the breakpoint means is less likely to induce errors or cause spurious signals than employing the traditional linear detrend.