Winter ice cover on lakes sets the timing and magnitude of most ecological processes in northern freshwaters, from energy balance and nutrient cycling to primary production and fisheries. Yet the exact response of lake ice cycles to rising air temperatures has remained poorly quantified.

Now, an international research team led by scientists from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences (NIGLAS), Nanjing Normal University, Bangor University, and the University of Regina has analyzed ice phenology records and air temperature data from 724 lakes across the Northern Hemisphere between 2000 and 2022.

The study was published in Proceedings of the National Academy of Sciences USA on July 27.

The researchers found that lakes exhibit a pronounced asymmetry in their response to warming. Spring ice-off dates are far more sensitive to rising air temperature than autumn freeze-up dates, meaning the melting process accelerates considerably faster than the freezing process as the climate warms. More critically, the analysis identified a set of winter temperature thresholds ranging from -13.7 to -6.8°C. Once mean winter air temperature climbs above these thresholds, the rate of ice loss increases sharply, with phenological sensitivity rising up to 22-fold.

"Below these thresholds, ice cover responds to warming in a gradual manner," said Dr. ZHOU Jian of Nanjing Normal University, first author of the study. "But once the threshold is crossed, the system enters a fundamentally different regime where each additional degree of warming triggers a disproportionately large loss of ice cover."

The study also found that the thermal thresholds are regulated primarily by broad climatic factors, particularly winter air temperature and surface albedo, rather than by individual lake shape or depth. This indicates that the timing of threshold crossing depends more on large-scale climatic conditions than on local lake morphology.

Looking ahead, the team projected that under a high-emission scenario, the duration of winter ice cover on Northern Hemisphere lakes will shrink by roughly 40 days by the end of this century. The proportion of lakes that have crossed critical thermal thresholds is expected to rise from 23 percent to 70 percent over the same period.

"Our findings suggest that many mid-latitude lakes are approaching a state of heightened vulnerability," said Prof. SHI Kun from the NIGLAS, co-corresponding author of the study. "Even modest additional warming could trigger abrupt and potentially irreversible shifts in lake ice regimes, with cascading consequences for water quality, aquatic ecosystems, and northern communities that depend on stable winter ice cover."

The research provides a new mechanistic framework for predicting threshold-driven ice loss from northern lakes. The authors stress that incorporating such nonlinear thermal sensitivities into next-generation climate models is essential for reliable projections of freshwater ecosystem change.