Much of the nation was transfixed this summer by waves of extreme heat that hammered the American west and midwest states from June into September.
At weather stations nationwide during that period, record highs crashed in multiple waves of heat: over 7,000 daily highs (the highest temperature recorded at that weather station on a particular date on the calendar), more than 400 record average high temperatures for a calendar month, and 27 all-time highs (the highest temperature ever recorded at that weather station).
The effects of these daytime highs were often dramatic: tinder-dry forests burning in out-of-control wildfires, power grid overloads and failures, thousands of vulnerable people falling ill (or even dying) from heat stress. The highs included what meteorologists are calling the most extreme September heat wave ever in the western United States. (Across the nation, tens of thousands of weather stations are tied into a network monitored by the National Oceanographic and Atmospheric Administration (NOAA). This permits reliable monitoring and analysis of trends over time.)
As dramatic as those daytime highs could be and were, they overshadowed attention to another creeping sign of the climate crisis with insidious effects. Sustained periods of rising night-time heat are stressing many regions that have thus far been less obviously hit by the recent daytime highs.
When the summer night heat doesn’t fall below 70 degrees for an extended period, it makes it harder for those stressed by daytime highs to recover. This is especially the case for those who work outside during the day and do not have (or cannot afford to use) air conditioning during the evening. The economic stress of higher bills and the stress on the electric grid and power production also increase.
In North Carolina’s Piedmont Triad region, for example, average nighttime temperatures are rising significantly more quickly than daytime highs over the past 60 years. The average summer lows have risen 3.5 degrees over that period, while the average summer highs have risen less than a half-degree.