The heat is on
Rapid, widespread increases in instances of extreme heat are projected to occur across the USA as a direct consequence of climate change, according to ‘Killer Heat’, a 2019 report compiled by the Union of Concerned Scientists. Disturbingly, the analysis includes four different heat index thresholds, each of which brings increasingly dangerous health risks: above 32.2°C, above 37.8°C, above 40.6°C, and ‘off the charts’. (Off-the-charts days are so extreme they exceed the upper limits of the National Weather Service heat index scale, which starts topping out at or above a heat index of 52.8°C, depending on the combination of temperature and humidity). At a heat index of 40.6°C, even healthy adults are at risk of heatrelated illness with prolonged exposure.
The results show that, with no action to reduce heattrapping emissions, by mid-century (2036–65), the average number of days per year with a heat index above 37.8°C will more than double from the 1971–2000 average, while the number of days per year above 40.6°C will quadruple. On average, more than one-third of the area of the USA will experience heat conditions once per year that are so extreme that they exceed the current heat index range – that is, they are literally off the charts. Up until now, the only place in the contiguous USA that has had off-the-charts days in an average year is the Sonoran Desert, where southern California meets Arizona. Nearly one-third of the nation’s 481 urban areas with a population of 50,000 people or more will experience an average of 30 or more days per year with a heat index above 40.6°C, a rise from just three cities historically (El Centro and Indio in California, and Yuma in Arizona).
The report offers a morsel of hope, stressing that it’s not too late to take action and that ‘the intensity of the coming heat depends heavily on how quickly we act now to reduce heat-trapping emissions’. The authors describe ‘a stark choice: We can continue on our current path, where we fail to reduce emissions and extreme heat soars. Or we can take bold action now to dramatically reduce emissions and prevent the worst from becoming reality.’ However, the authors add: ‘There is little time to do so and little room for half measures.’