US daily temperature records past, present, and future.

Observed temperature extremes over the continental United States can be represented by the ratio of daily record high temperatures to daily record low minimum temperatures, and this ratio has increased to a value of about 2 to 1, averaged over the first decade of the 21st century, albeit with large interannual variability. Two different versions of a global coupled climate model (CCSM4), as well as 23 other coupled model intercomparison project phase 5 (CMIP5) models, show larger values of this ratio than observations, mainly as a result of greater numbers of record highs since the 1980s compared with observations. This is partly because of the "warm 1930s" in the observations, which made it more difficult to set record highs later in the century, and partly because of a trend toward less rainfall and reduced evapotranspiration in the model versions compared with observations. We compute future projections of this ratio on the basis of its estimated dependence on mean temperature increase, which we find robustly at play in both observations and simulations. The use of this relation also has the advantage of removing dependence of a projection on a specific scenario. An empirical projection of the ratio of record highs to record lows is obtained from the nonlinear relationship in observations from 1930 to 2015, thus correcting downward the likely biased future projections of the model. For example, for a 3 °C warming in US temperatures, the ratio of record highs to lows is projected to be ∼15 ± 8 compared to the present average ratio of just over 2.