Climate change and Northern Hemisphere lake and river ice phenology

Abstract. At high latitudes and altitudes one of the main controls on hydrological and biogeochemical processes is the breakup and freezeup of lake and river ice. This study uses ~2600 time series from across 644 Northern Hemisphere lakes and river to explore historical patterns in lake and river ice phenology across four time periods (1931–1960, 1961–1990, 1991–2005, and 1931–2005). These time series show later breakup dates by 0.6 days per decade from 1931–2005 across North America and Europe, with trends closely correlating with temperature. Freezeup trends are more spatiotemporally complex with those in Europe negligible compared to later freezeup trends for North America. For the most recent time period (1991–2005) high magnitude trends towards later freezeup that are considerably larger than in other time periods are observed. Freezeup trends show a more limited correlation with climate and this is likely because freezeup is not guaranteed to occur simply by temperatures dropping below 0 °C. Across the Northern Hemisphere the length of the open water season is shown to have increased through time, with the magnitude at its largest in the most recent time period. These results provide an important contribution that can be used to help understand how ice phenology patterns may change in the future with an expected rise in global mean air temperatures. Observations of an acceleration in warming trends through time shows the importance of non-linear responses to climate forcings. This will be crucial because it is probable that lake and river ice phenology changes, brought about by rising air temperatures, may in turn begin to feedback into the climate system. Thus, understanding historical changes, causes, and consequences is required to fully unravel the potential implications of future ice phenology change.