Rapid decline of ice thickness across Finnish lakes

• A new approach to study future trends of lake ice thickness (LIT) across Finland. • Finnish lakes will lose up to half of their ice thickness by 2100. • Arctic lakes are projected to cross their historic ranges of LIT by the 2050s. The complex relationship between lake ice thickness (LIT) and its environmental drivers, along with the high cost of in-situ measurements and the limited historic resolution of satellite observations—which also carries considerable uncertainties—makes precise regional-scale LIT modeling particularly challenging. To address this, we propose a black-box modeling approach that incorporates a novel parameter: freezing-days (FDs), derived from widely available air temperature data. Using this method, we project future LIT trends across 40 Finnish lakes. We also determine the historical LIT range to which aquatic species have adapted. Compared to the well-established Stefan model, our modeling approach enhances LIT estimation accuracy, achieving a 35% increase in the coefficient of determination and a 38% reduction in the mean absolute error in the validation phase. Our results indicate rapid declines in LIT, with projected reductions of 32% under SSP4.5 and 44% under SSP8.5 by 2100. Under SSP4.5, LIT variability is expected to exceed historical ranges in northern, central, and southern Finland by 2058, 2076, and 2098, respectively. In both emission scenarios, northern lakes are projected to cross this threshold by the 2050s, suggesting an ecological tipping point, regardless of future warming levels. These findings highlight the vulnerability of lake ice ecosystems to climate change and offer critical insights for developing adaptation strategies in a warming Arctic-boreal region. Our proposed model suggests a more accurate, widely applicable alternative for the study of LIT changes across spatial and temporal scales.