Ratios of Record High to Record Low Temperatures in Europe Show an Accelerating Trend Since 2000 Despite a Slowdown in Mean Temperature Trends

Abstract:

The present study has examined the behavior of extreme high and low temperatures in Euirope since the mid-20th century, in terms of the ratio of number of days per year with record Tmax and record Tmin. The investigations have shown that there has been a clear and massive increase in the number of high Tmax: low Tmin ratios in the most recent decade of the 1951-2013 temperature record for 30 selected observation stations in Europe. This sharp increase is seen to occur despite an apparent hiatus – or at least a reduction in the rate of warming – since the early 2000s, as observed not only in Europe but on a hemispheric basis too. The «explosion» of record high:record low temperature ratios since 2000, despite relatively small increases in mean temperatures in the last 10-15 years of the observational record, can be explained by a non-linear (quadratic) relation between mean temperatures and the Tmax:Tmin record ratios. It is suggested here that the increases are probably a consequence of increasing dryness during the summer in the Mediterranean region (where today there are on average 30 more dry days than in the 1950s), and a reduction in the cold season conducive to snow days in Arctic summers and Northern European springs (with up to 40 days less freezing days than 60 years previously). Both effects can serve to amplify positive temperature feedbacks in the lower atmosphere that result in strong increases in the number of Tmax record high temperatures and correspondingly strong reductions in the number of Tmin record low temperatures.