Heat Waves in the United States
EPA Climate Change Indicators in the United States from the NOAA CCSP U.S. Climate Change Science Program. 2009. (Updated version of the U.S. Climate Change Science Program's 2008 report: Synthesis and Assessment Product 3.3: Weather and climate extremes in a changing climate), page 24 (http://www.epa.gov/climatechange/indicators/pdfs/ClimateIndicators_full.pdf).
The CCSP devised a Heat Wave Indicator Index to measure the changes since 1895 for the United States. The data for this indicator were provided by the National Oceanic and Atmospheric Administration's National Climatic Data Center. Surface temperature anomalies were calculated based on monthly values from a network of long-term monitoring stations. Satellite data were analyzed by two independent groups, resulting in the slightly different "UAH" and "RSS" trend lines.
The indicator showed very clearly that heat waves occurred with high frequency in the 1930s, and these remain the most severe heat waves in the U.S. historical record (see Figure 1).
Many years of intense drought (the "Dust Bowl") contributed to these heat waves by depleting soil moisture and reducing the moderating effects of evaporation. There is no clear trend over the entire period tracked by the index. Although it is hard to see in Figure 1 (because of the extreme events of the 1930s), heat wave frequency decreased in the 1960s and 1970s but has risen since then.
The areal coverage of heat has increased but like the Index, the recent run-up falls far short of the peak in the 1930s. The recent period of increasing heat is distinguished mostly by an increase in high nighttime temperatures with elevated moisture levels.
These results in figure 1 were generally consistent with an analysis of monthly record highs and lows for the states since 1895 (source Bruce Hall from NOAA NCDC) although the 2000s was unusually benign with fewer state records than any decade since the 1880s.
Figure 2. State Monthly heat and cold records 1895-2010. Note when new records are set, the count is adjusted down accordingly in the prior record decade.
One must conclude from the data that heat waves are not increasing at an alarming rate as reported by the IPCC. Indeed, elevated nighttime temperatures which show up in the data are actually better correlated with urban heat island contamination. Greenhouse warming should result in elevated daytime and nighttime temperature and logically more record highs.
Meehl, G.A. and Tebaldi, C. 2004. More intense, more frequent, and longer lasting heat waves in the 21st century. Science 305: 994-997.