Americans east of the Rockies are sweltering as daytime temperatures soar toward 100 degrees or more. It is now customary for journalists covering big weather events to speculate on how man-made climate change may be affecting them, and the current heat wave is no exception. Take this headline in The New York Times: “Heat Waves in the Age of Climate Change: Longer, More Frequent and More Dangerous.”
As evidence, the Times cites the U.S. Global Change Research Program, reporting that “since the 1960s the average number of heat waves—defined as two or more consecutive days where daily lows exceeded historical July and August temperatures—in 50 major American cities has tripled.” That is indeed what the numbers show. But it seems odd to highlight the trend in daily low temperatures rather than daily high temperatures.
As it happens, chapter six of 2017’s Fourth National Climate Assessment reports that heat waves measured as high daily temperatures are becoming less common in the contiguous U.S., not more frequent.
Here, from the report, are the “observed changes in the coldest and warmest daily temperatures (°F) of the year for each National Climate Assessment region in the contiguous United States.” The “changes,” it explains, “are the difference between the average for present-day (1986–2016) and the average for the first half of the last century (1901–1960).”