The relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S

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Tuesday, 19 January 2010: 3:30 PM
B216 (GWCC)
Gerald Meehl, NCAR, Boulder, CO; and C. Tebaldi, G. Walton, D. R. Easterling, and L. R. McDaniel

Summed over the first decade of the 21st century and averaged over the U.S., the observed ratio of record high maximum temperatures to record low minimum temperatures is about two to one. That there are more record highs than lows is not surprising since observed U.S. average temperatures have been increasing over this time period, and a shift in the distribution of temperatures would dictate a greater number of record high temperatures being set than record low minimum temperatures. But is the two to one ratio particularly unique or characteristic of a warming climate? It is shown that the two to one ratio of record highs to record lows is not unique but happens to be a product of the recent time period in the non-stationary warming climate. Records that were declining uniformly earlier in the 20th century following roughly a decay of 1/n have been declining less slowly for record highs than record lows since the late 1970s, and for the recent decade the ratio has grown to about two to one. A multi-member ensemble of model simulations of U.S. climate of the 20th and early 21st centuries shows a greater ratio of about four to one for this recent time period due to more uniform warming across the U.S. in contrast to observations that have been characterized by relatively greater warming in the western U.S. compared to the eastern U.S. Following an A1B emission scenario for the 21st century, the ratio of record high maximum to record low minimum temperatures, averaged over the U.S., is projected to continue to increase, with ratios of about 20 to 1 by mid-century, and roughly 50 to 1 by the end of the century.