The Antarctic winter extent shift is uncertain after summer adjustment in 1979–2025 data

We tested whether Antarctic September sea ice extent was lower in 2017–2025 than in 1979–2016 after adjustment for February extent. We used 47 annual pairs from Sea Ice Index Version 4. The adjusted shift was -0.395 million square kilometres, with a 95% interval from -0.983 to +0.192. The three-year block interval also included zero. Six of nine recent February values were below the earlier observed range. A model with different slopes gave a difference of -0.090 million square kilometres at a February extent of 2.7 million square kilometres, with an interval from -0.339 to +0.158. The raw mean seasonal gain rose slightly while mean winter extent fell. Thus, the test does not resolve a change in seasonal recovery. It shows limited range overlap and sensitivity to the assumed summer-winter relation. Nine recent years, one extent product, sensor changes, and approximate uncertainty limit the result. No physical cause is identified. Code, annual inputs, source hashes, model checks, an independent numerical audit, and a figure are included.

Antarctic sea ice · seasonal recovery · range overlap · conditional shift · uncertainty