Last month, a strange atmospheric phenomenon spread over the central United States: a brutal, self-perpetuating “heat dome.” Hot air descended onto the region, sucking the moisture out of soils and plants, and raising ground temperatures higher and higher. On August 23, Chicago hit a heat index (temperature combined with humidity) of 116 degrees Fahrenheit.
Stranger still? This was really out of character for the central US. Unlike the western and eastern parts of the country, daytime summer temperatures haven’t really warmed here since the mid-20th century. Scientists call this a “warming hole”—a blip in the overall heating trend across the US. But that doesn’t mean global warming has somehow skipped the central US: In a weird twist, climate change may be partly responsible for this gap.
“There’s a significant population that lives and works in this part of the US that scratches its head and says, ‘What’s all this fuss about climate change?’” says Martin Hoerling, a climate scientist at the National Oceanic and Atmospheric Administration (NOAA). “It’s very counterintuitive, because global warming has accelerated while the warming hole has continued.”
You can see the warming hole in the top map above, which shows maximum temperatures between May and August in the years 2001 to 2020, compared to the years 1957 to 2000. White areas show where there has been no change. Illinois, Missouri, and Arkansas are solid white. There are even some splotches of blue in parts of the Dakotas, Minnesota, and Iowa, where temperatures have actually fallen. Meanwhile, red signifies higher temperatures—and that covers basically the entirety of the American West.
Scientists have several theories for why this warming hole has persisted. Maybe aerosols in the region’s atmosphere reflect some of the sun’s energy back into space. Perhaps agricultural land, and its accompanying irrigation, water-cool the area. The landscape “sweats,” much like your body would.
In a new paper, Hoerling and his colleagues argue for something similar: Summertime precipitation across the central US has dramatically increased over the past two decades compared to the years 1957 to 2000, and that has boosted the amount of water on the landscape, acting as an evaporative cooler. In the bottom map of the trio above, the green spattered across the central US indicates up to a 30 percent rise in precipitation between the two time frames.
“There still potentially could be effects from agriculture, but we found this warming hole to be bigger than the agricultural change,” says climate scientist Zachary Labe of Princeton University and NOAA, and coauthor of the new paper. “We think it’s more likely linked to some sort of atmospheric condition.”