By one measure, summers in the middle of the country have actually gotten cooler.
The Midwest and Great Plains are in what some scientists call a “warming hole” — summertime high temperatures have gotten measurably cooler in recent decades.
“Our climate is changing, but just a little bit differently than the general trend that we talk about nationally or globally,” said Missouri’s state climatologist Zack Leasor. “We're still seeing these warming temperatures, but it looks different than other locations.”
From eastern Texas and Louisiana up through the Dakotas and Minnesota, the average high daily temperatures have been cooler in recent decades than they were in the middle of the 1900s.
But these same areas have still seen an average warming trend, mainly driven by hotter overnight temperatures and winters. Local climate scientists say this is an example of the Midwest’s unique flavor of climate change.
What’s behind it
Globally, average temperatures have spiked in recent decades and scientists say the increase in greenhouse gases in the atmosphere is to blame.
Last year was the warmest year on record, but warming has not happened evenly across Earth. The Midwest stands out.
“There's this area of the United States that hasn't seen a lot of warming during the summertime, particularly during the day,” said Zachary Labe, a climate scientist at the nonprofit Climate Central.
Labe and a group of other scientists looked into why that might be happening in an article published in the American Meteorological Society Journal of Climate.
“Admittedly, there are still some uncertainties, but I think the clues are starting to come together,” Labe said. “Like most things in Earth's weather and climate, you know, it's never a simple answer.”
There are a few factors that seem most likely to create this “warming hole” in the Midwest, Labe said. The main one is a change in moisture.
In recent decades, there has been an increase in cloud cover, rainfall and moisture in the Midwest, which are all coming together to keep daytime high temperatures lower.
Scientists think one ingredient in the warming hole recipe has to do with the ocean.
“It's likely that the cause of some of the increases in precipitation and cloud cover might be related to changes in ocean temperatures in the Pacific Ocean,” Labe said.
Those ocean temperature changes might be a mix of both climate change and natural variability, Labe said.
Increases in agricultural land could also play a part through a phenomenon that has gotten a lot of attention lately — corn sweat or evapotranspiration.
“There's so many articles written about corn sweat these days that I think probably people can kind of get this idea,” said Illinois state climatologist Trent Ford. “The large-scale conversion of Savannah forest, or, in the northern part of the Midwest, just straight up mixed hardwood forests, or prairie, to cropland agriculture has increased the local contributions to humidity.”
Large swaths of land that are now irrigated crops like soybeans and corn can make things more humid as the plants release water vapor, and humidity has a leveling effect on temperatures. When the air is thick with moisture, temperatures can’t spike as high.
“We have seen in the summertime more humid conditions,” Ford said. “What that humidity does is it makes it a little bit harder for the actual air temperature to heat up once the sun comes out, and it makes it a little bit harder for that air temperature to cool down once the sun sets.”
Sticky humidity
Despite the cooler daytime highs, the Midwest’s summers are still getting warmer on average. That has been driven by dramatic increases in nighttime temperatures.
“We're seeing more and more of these really hot nights where sometimes the low temperature doesn't even fall below 80 degrees,” Labe said. “That really stresses the body, causing a multitude of health issues in people.”
The same humidity that keeps afternoon temperatures lower also holds onto heat in the air overnight.
“Just as we've seen a decrease in those 90 degree days, we've seen an increase in the number of nights where the temperature doesn't cool below 70,” Leasor said about Missouri.
Those warm nights have offset the cooler afternoons, Leasor said, leading to slightly increasing average summer temperatures in Missouri overall.
Last month, multiple states also broke their records for the most humid July, including Missouri, Illinois and Michigan.
Humid heat has also led to summers feeling warmer in the Midwest. Climate Central has found a pattern of more humid, hot summers in the middle of the country when considering equivalent temperature, a measure of humidity and temperatures that is different than the heat index.
Rethinking heat
These hot, muggy days and abnormally warm nights can actually be more dangerous for human health. That’s why scientists think the Midwest needs to rethink its approach to heat.
“How we think about heat, how we communicate that … and then how we build out those systems, has to be more nuanced than just, ‘How hot is it during the day?’” Ford said.
Extreme heat communication like warnings from the National Weather Service are often built around the highest possible temperature and highest possible heat index. But in the Midwest, researchers have found heat-related health issues actually happen at lower temperatures.
Scientists say factors like humidity, total hours of consecutive extreme heat and overnight lows need to be taken into account to protect the public.
“Often the records are what captures people's attention, both in the media and just day to day conversation,” Labe said. “But really, I think things like nighttime temperatures, to me, are extremely concerning.”
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.
