Declining number of northern hemisphere land-surface frozen days under global warming and thinner snowpacks

Freeze–thaw processes shape ecosystems, hydrology, and infrastructure across northern high latitudes. Here we use satellite-based observations from 1979–2021 across 47 northern hemisphere ecoregions to examine changes in the number of frozen land-surface days per year. We find widespread declines, with 70% of ecoregions showing significant reductions, primarily linked to rising air temperatures and thinning snowpacks. Causal analysis demonstrates that air temperature and snow depth exert consistent controls on the number of frozen days. A trend-informed assessment based on historical observations suggests a potential average loss of more than 30 frozen days per year by the end of the century, with the steepest decreases in Alaska, northern Canada, northern Europe, and eastern Russia. Scenario-based analysis indicates that each 1 °C increase in air temperature reduces frozen days by ~6-days, while each 1 cm decrease in snow depth leads to a ~ 3-day reduction. These shifts carry major ecological and socio-economic implications.