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Trends in Weather Extremes — February 2018 – World Weather Attribution


For cold extremes, the daily average temperature of the coldest day of the year is considered. Figure 2 shows that these cold events heat up even faster than the heat extremes, up to a factor of five times the global mean temperature (see e.g., van Oldenborgh et al, 2015 and the WWA coldwaveanalyses). The strongest increases over land are in Siberia and Canada. Winter temperatures are very low there due to radiative cooling over snow under a clear sky, with strong vertical gradients in the lowest meters of the atmosphere. These stable boundary layers are sensitive to perturbations, probably also to the extra downward longwave radiation due to greenhouse warming. This may explain the strength of the observed trends.