Antarctic winter ice area fell between 1989-2015 and 2016-2024, while the compactness change remained uncertain

We tested whether lower Antarctic winter ice area came with lower compactness. We used paired area and extent from five regions in Sea Ice Index Version 4. All 92 June-August days were present in each of 36 winters. Mean area fell by 0.707 million square kilometres between 1989-2015 and 2016-2024. Total compactness, defined as the ratio of era-mean area to era-mean extent, fell by 0.125 percentage points. Its 95% two-year block interval was -0.894 to 0.625 points. An exact symmetric budget assigned 97.3% of the sample area loss to lower extent. Changes within regions supplied -0.146 points of the compactness change; regional weight changes supplied 0.021 points. All planned compactness checks had intervals across zero. Thus the tested area loss does not establish a loss of compactness. This result is not proof of constant concentration. Nine recent winters, one passive microwave product, season choice, and the block assumptions limit inference. The package gives daily used fields, seasonal means, source hashes, code, uncertainty draws, and a separate numerical audit.

Antarctic sea ice · winter compactness · sea ice area · sea ice extent · regional budget · block bootstrap