Mean ice-off advanced by six days at 45 Finnish lake sites, while the change in annual correlation was uncertain between 1961–1990 and 1991–2020

We tested a change in shared annual ice-off timing at 45 Finnish lake sites on 42 lakes. Each site had at least 27 records in both 1961–1990 and 1991–2020. We removed a separate linear trend at each site in each period. Mean Pearson correlation across the 990 fixed site pairs fell from 0.694 to 0.521. The change was −0.174. A bootstrap that sampled the same 3-year blocks for all sites gave a 95% interval of −0.372 to +0.044. The interval includes zero and a large decrease. All tested data and block-length checks gave negative point estimates, but their intervals also included zero. Mean ice-off was 6.25 days earlier, with a change interval of −9.62 to −2.69 days. The mean absolute gap between site dates rose by 1.37 days, with an interval of −0.21 to +2.95 days. These estimates apply to the fixed monitoring network. They do not establish a regional change in annual correlation or its cause. The package has the original Syke data, source hashes, code, all estimates, a local plan with corrections, a figure, and independent numerical checks. A separate source-to-result run matched all numerical files exactly.

lake ice · Finland · ice-off · annual synchrony · shared years · bootstrap · environmental monitoring · reproducible data