The Antarctic season between 10 million km2 crossings was 15 days shorter in 2016-2024 than in 1989-2015

We compared Antarctic advance and retreat across fixed total ice extent levels in 36 winters. At 10 million km2, the mean time between crossings fell from 220.59 days in 1989-2015 to 205.22 days in 2016-2024. The change was -15.37 days, with a conditional 95% three-year block interval of -20.85 to -9.93. Advance was 9.56 days later and retreat was 5.81 days earlier. Shortening held across four thresholds, three smoothing choices and three block lengths. Evidence that advance contributed more depended on the uncertainty method. The test uses Sea Ice Index Version 4 daily extent and stops before the July 2025 sensor change. It extends known local season changes with a fixed hemispheric timing benchmark. Short records, selected periods, one product and uncertain annual dependence limit inference. The result does not identify a physical cause. Daily evidence, annual dates, code, source hashes, all intervals, a figure and a separate numerical audit are included.

Antarctic sea ice · season duration · advance · retreat · fixed extent · block bootstrap