Rapid acceleration of ice-cover loss from Northern Hemisphere lakes above critical air temperature thresholds

Ice cover is a fundamental regulator of northern lake ecosystems, yet the specific thermal thresholds that trigger abrupt phenological changes have remained elusive. We demonstrate that lake-ice sensitivity to atmospheric warming is not only asymmetrical, with spring thaw being far more volatile than autumn freeze, but also undergoes a rapid nonlinear acceleration. We identify critical winter temperature thresholds that, once breached, cause phenological responsiveness to increase by more than an order of magnitude. Our results reveal that many mid-latitude lakes are currently transitioning into this state of extreme vulnerability. This finding provides a mechanistic framework for predicting threshold-dependent cryospheric loss and underscores the need to incorporate nonlinear thermal responses into climate impact assessments to safeguard global inland water integrity.