Arctic extent reached 11 million square kilometres later despite faster 9-to-11 expansion in 1989-2024

We tested whether faster autumn sea ice expansion meant earlier return to a fixed extent. We used Sea Ice Index Version 4 daily Arctic extent for 36 years from 1989 to 2024. With a five-day mean, the dates at 9 and 11 million square kilometres moved later by 8.41 and 6.73 days per decade. The transition duration fell by 1.68 days per decade, with conditional 95% three-year block limits of -3.04 to -0.30. The mean annual expansion rate rose, but its uncertainty depended on smoothing and error assumptions. At 8 to 10 million square kilometres, the duration interval included zero. Faster expansion over the main range thus coexisted with later arrival at its upper level. This is a fixed-range timing benchmark, not the first report of faster autumn growth. One extent product, selected thresholds, short records and trend assumptions limit inference. The data do not identify a physical growth feedback. Code, daily inputs, provenance, all checks and a figure are included.

Arctic · sea ice · autumn expansion · recovery timing · fixed thresholds