As you have likely guessed, there is more to this story than first meets the eye. First, the chart begins in 1961, near the end of a 33-year cooling period that had some of the coolest temperatures since the 1930s and conveniently omits the high temperatures of that decade of the "Dust Bowl".
Second, and more importantly, the charts above are not representative of America but, rather, are heat measurements in the 50 largest metropolitan areas in the Lower 48 states. As you may be aware, the Urban Heat Island Effect has artificially increased urban temperatures greatly over the last several decades.
This fact is revealed only deep within the fine print of the description of the methodology used. The authors DO admit that "Urban growth since 1961 may have contributed to part of the increase in heat waves..." Indeed.
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