Contemporary trends and future changes in freshwater ice conditions: inference from temperature indices

摘要

This study examines current trends and future changes in five daily air-temperature derived proxy indices for freshwater ice regimes. The indices are autumn 0 ℃ isotherm date (autumn I_0), spring 0 ℃ isotherm date (spring I_0), winter duration (WD), maximum annual accumulated freezing degree-days (AFDD), and mid-winter thaw frequency. A total of 127 meteorological stations distributed across Fennoscandia (Norway, Sweden, and Finland) have been used to derive the indices for the contemporary period 1961-2010. Results show overall trends towards later autumn I_0 and stronger trends towards earlier spring I_0. Altogether, WD was shortened by a regional average of -5.2 ± 2.9 days per decade, the reduction being significant at 5% level in 71 stations. AFDD decreased in all the 127 stations with a regional average of -57 ± 35 ℃-days per decade. The impact of future warming on the indices was investigated using climate signals from the IPCC's A1B emissions scenario. The results showed that WD will shorten by an average of 36 days in 2041-2070 and by an average of 51 days in 2071-2100. The ecological and socio-economic impacts of the observed and future changes in the indices are qualitatively evaluated.