Short ice-season intervals occurred together more often at 30 Finnish lake sites in 1991–2020 than in 1961–1990

We tested whether short ice-season intervals occurred together more often at fixed Finnish lake sites. We used open NSIDC ice dates and selected 30 sites with at least 25 valid paired dates in each of 1961–1990 and 1991–2020. A short interval was below the site's observed early 10th percentile. Across 435 fixed pairs, mean joint frequency rose from 4.41% to 29.56%. The change was +25.15 percentage points, with a nominal 95% common five-year block interval of +7.88 to +43.77. An exact split gave +14.78 points for the product of local event frequencies and +10.37 for the excess in shared years. This excess depends on local event rates. When each site instead supplied its shortest 10% in each period, the joint measure changed by +0.46 points, with an interval of −2.02 to +3.78. Thus, more shared short seasons do not establish stronger alignment of locally rare events. Exact bounds for missing records retain an increase in full-calendar joint frequency at the fixed historical limits. The package preserves source files, code, all results, figures and independent checks. The selected observations do not establish a cause or a species response.

lake ice · joint events · Finland · missing data · ecosystem timing · reproducible analysis