[url=http://www.agu.org/journals/gl/gl0923/2009GL040736/]Relative increase of record high maximum temperatures compared to record low minimum temperatures [/url]
Analysis of observed annual U.S. record high maximum compared to record low minimum daily temperatures shows that the recent values of the ratio of about 2 to 1 are just the transient values of a ratio that has been increasing with mean annual mean temperature over the U.S. since the late 1970s. B. C. Trewin and H. Vermont (Changes in the frequency of record temperatures in Australia 1957–2007 submitted to Australian Meteorological and Oceanographic Journal 2009) have also documented a similar recent ratio of about 2 to 1 over Australia for monthly temperature records. As noted in other studies (Portmann et al. manuscript in preparation 2009) there are geographic and seasonal characteristics to these records and it is shown that the greater values of the ratio for the western U.S. (where mean warming has been greater) compared to eastern U.S. are simulated with reduced contrast in the model with greater values of the ratio simulated in the model likely indicative of the greater mean warming in the model over the U.S. compared to observations (about 30%) by the early 21st century. Additionally while observations seem to indicate that record lows have been declining in larger measure than record highs have been increasing the model simulates a more symmetric behavior between minimum and maximum record temperatures. For later in the 21st century the model indicates that as warming continues (following the A1B scenario) the ratio of record highs to record lows will continue to increase with values of about 20 to 1 by mid-century and roughly 50 to 1 by late century.
Two factors contribute to this increase as noted by Portman et al. (manuscript in preparation 2009): 1) increases in temperature variance in a future warmer climate (as noted in the model by Meehl and Tebaldi [2004]) and 2) a future increasing trend of temperatures over the U.S. (model projections given by Meehl et al. [2006]).