Nonlinear Transformation of Temperate River Ice Regimes Under Climate Warming: Historical Trends and Projections Through 2100

River ice is a characteristic feature of the temperate zone, yet its future decline remains poorly quantified. This study analyses ice regimes of two Central European rivers, the Pilica and Węgorapa, using 1965–2022 observations and projections to 2100 under SSP1-2.6, SSP3-7.0 and SSP5-8.5. Water temperature was estimated from seven-day mean air temperature with a simplified Mohseni model, and ice occurrence with an autologistic model. Over 1965–2022, the ice season shortened by 10.95 and 10.91 days per decade. By 2071–2100, mean ice-phenomena days are projected to decline to 10.9, 5.8 and 4.3 on the Pilica, and to 28.4, 14.9 and 10.2 on the Węgorapa under the three scenarios. The projections assume a stationary air–water temperature relationship and no continuation of the non-climatic decline beyond 2022; continuing the observed drift of that relationship would reduce ice-phenomena days by a further 4–10%. Because air–water coupling also depends on hydrological and physical factors not represented, the results indicate the direction and broad magnitude of change rather than future water temperature dynamics, and process-based thermal models are a priority for future research. Such losses may affect river ecosystems, water-resource management, river-valley infrastructure and energy production.