Frequent summer temperature extremes reflect changes in the mean, not the variance

Hansen et al. (1) demonstrated that the probability of extremely hot summers has markedly increased because the mean of the distribution of seasonally averaged temperatures has increased. However, the authors also implied that the variance of the distribution has increased, a result that differs from regional studies that show changes in the extremes are consistent with a simple shift in the mean (2, 3). Here we extend the spatially aggregated distribution analysis of Hansen et al. to show that once issues related to normalizations, trends, and reductions in surface-station density are accounted for, there is no indication that variability about the mean has increased, at least using these methods and data. Hansen et al. reported that the variance of summer surface temperature between 1981 and 2010 is greater than that in the 1951–1980 …