Climate change and Northern Hemisphere lake and river ice phenology from 1931–2005

Abstract. At high latitudes and altitudes one of the main controls on hydrological and biogeochemical processes is the breakup and freeze-up of lake and river ice. This study uses 3510 time series from across 678 Northern Hemisphere lakes and rivers to explore historical patterns in lake and river ice phenology across five overlapping time periods (1931–1960, 1946–1975, 1961–1990, 1976–2005, and 1931–2005). These time series show that the number of annual open-water days increased by 0.63 d per decade from 1931–2005 across the Northern Hemisphere, with trends for breakup and, to a lesser extent, freeze-up closely correlating with regionally averaged temperature. Breakup and freeze-up trends display a spatiotemporally complex evolution and reveal considerable caveats with interpreting the implications of ice phenology changes at lake and river sites that may only have breakup or freeze-up data, rather than both. These results provide an important contribution by showing regional variation in ice phenology trends through time that can be hidden by longer-term trends. The overlapping 30-year time periods also show evidence for an acceleration in warming trends through time. Understanding the changes on both long- and short-term timescales will be important for determining the causes of this change, the underlying biogeochemical processes associated with it, and the wider climatological significance as global temperatures rise.