Based on a unique dataset of more than 50 000 observations of ice phenology from 1213 lakes and 236 rivers in 12 different countries, we show that interannual variations in the timing of ice-on and ice-off on lakes and rivers are not equally pronounced over the entire Northern Hemisphere, but increase strongly towards geographical regions that experience only short periods during which the air temperature falls below 0 °C. We explain our observations by interannual fluctuation patterns of air temperature and suggest that lake and river ecosystems in such geographical regions are particularly vulnerable to global warming, as high interannual variability is known to have important ramifications for ecosystem structure and functioning. We estimate that the standard deviation of the duration of ice cover, viewed as a measure of interannual variability, exceeds 25 days for lakes and rivers located on 7% of the land area of the Northern Hemisphere. Such high variability might be an early warning signal for a critical transition from strictly dimictic, ice-covered systems to monomictic, open-water systems. Using the Global Lake and Wetland Database, we suggest that 3.7% of the world's lakes larger than 0.1 km2 are at high risk of becoming open-water systems in the near future, which will have immediate consequences for global biogeochemical cycles.