Changing freeze-thaw seasons in northern high latitudes and associated influences on evapotranspiration

Seasonal frozen states in the northern terrestrial cryosphere limit vegetation photosynthetic activities and evapotranspiration (ET) through cold temperature constraints to biological processes and chemical unavailability of water as a result of being frozen. Seasonal transitions of the landscape between predominantly frozen and thawed conditions are analogous to a biospheric and hydrological on/off switch, with marked differences in ET, vegetation productivity and other biological activity between largely dormant winter and active summer conditions. We investigated changes in freeze-thaw (FT) seasons and ET from 1983 to 2006 and their connections in the northern cryosphere by analyzing independent satellite remote sensing derived FT and ET records. Our findings show that the northern cryosphere (≥40°N) has experienced advancing (-2.5days/decade; P=0.005) and lengthening (3.5days/decade; P=0.007) non-frozen season trends over the 24-year period, coinciding with an upward trend (6.4mm/year/decade; P=0.014) in regional mean annual ET over the same period. Regional average timing of spring primary thaw and the annual non-frozen period are highly correlated with regional annual ET (|r|≥0.75; P<0.001), with corresponding impacts to annual ET of approximately 0.6 and 0.5% per day, respectively. The impact of primary fall freeze timing on ET is relatively minor compared with primary spring thaw timing. Earlier onset of the non-frozen season generally promotes annual ET in colder areas but appears to suppress summer ET by increasing drought stress in the southernmost parts of the domain where water supply is the leading constraint to ET. The cumulative effect of future freeze-thaw changes on ET in the region will largely depend on future changes of large-scale atmosphere circulations and rates of vegetation disturbance and adaptation to continued warming.