Extreme Cold Days and Spells in Northern Europe at 0.5°C–2.0°C Global Warming Levels

ABSTRACT Projections for cold days and spells in Northern Europe at global warming levels of 0.5°C–2.0°C are derived from bias‐corrected output data from 60 runs performed with 25 global climate models. The threshold temperature of a cold day is defined as the 10th percentile of December–February daily mean temperatures at the 0.5°C warming level. A transition from the 0.5°C to the 2.0°C warming level reduces the multi‐model mean annual number of cold days by 60%–80%. The total cold day extremity index, consisting of the sum of anomalies below the threshold temperature during the cold season, is reduced even more, by 70%–90%. The largest changes occur in the Kola Peninsula and regions surrounding the White and Baltic Seas. This pattern of change is consistent with the signal‐to‐noise ratio of the winter mean temperature increase. Nevertheless, inter‐model differences in the response are substantial, being largely attributable to the manner in which global warming affects regional winter temperatures. To assess the occurrence of severe cold spells, a 1200‐year sample was established by concatenating the bias‐corrected output of all 60 model runs. From this extensive sample, it is feasible to robustly determine the return levels of even very extreme cold spells. A severe cold spell occurring once in 10 years on average at the 0.5°C warming level would be experienced approximately once in 30 (70) years at the 1.5°C (2.0°C) level. Correspondingly, a baseline‐climate 100‐year cold spell would occur at the 2°C warming level less frequently than once in a 1000 years.