Burning Questions: What Do Rising Wet-bulb Temperatures Mean for the Midwest?

Wet-bulb temperature, a metric that takes into account both heat and humidity, is on the rise as climate change takes its toll. Once it hits a critical threshold, humans will be unable to cool down outdoors. In the Midwest, where agriculture is a major industry and the Great Lakes provide a large source of moisture, residents could begin feeling the effects of that threshold on their lifestyles and economy by the middle of the century. Cover graphic by Ruth Chang for Midstory.

Broiling, sweltering heat. The muggy kind that wraps around you like a damp, heavy blanket. The unbearable boiling that makes you feel like you’re running on a treadmill in a sauna while wearing your thickest winter coat that’s zipped up to your chin.

When wet-bulb temperatures reach a not-so-distant threshold, that feeling could become a constant reality, and we will — almost literally — be cooking in our skin.

Instead of only looking at heat, wet-bulb temperature is a metric that also looks at “factors related to humidity, wind and radiation,” according to Dr. Jesse Berman, an environmental health sciences professor at the University of Minnesota. This measure allows us to see how well humans can actually cope with heat when factors like humidity are added in.

The wet-bulb temperature index was originally designed by the U.S. Army to understand and lower the risk of heat disorders when troops were doing training exercises, according to Berman. Image courtesy of Master Sgt. Paul Gorman.

In a dry heat, the body has the innate ability to cool itself through sweating.

“But when conditions get too humid, you can’t sweat your heat away,” Dr. Matthew Huber, professor and director for the Institute for a Sustainable Future, Department of Earth, Atmospheric, and Planetary Sciences at Purdue University, said. “There’s simply no way for your skin to cool itself through evaporation at that point … you just slowly cook yourself.”

Physiologically, what this means is a disruption of the body’s internal thermodynamic balance — as the temperature of the body’s core rises beyond its survivable range, the heart stops pumping blood as effectively, and organs fail.

In a report published in 2010, Huber and his colleague put the theoretical human limit for wet-bulb temperatures at 95 degrees Fahrenheit, although it could be lower, depending on factors like water consumption, clothing and age. This limit, which creates conditions “nearing or beyond prolonged human physiological tolerance,” has already been reached for short periods of time in parts of South Asia, the coastal Middle East and southwestern Northern America. Some reports showed an increase in deaths following the recent heat waves across Europe, such as Spain, with over 2,500 more deaths than the five-year average during that time span. Experts cite the heat and lack of infrastructure such as proper air conditioning as related factors.

In the Midwest, where agriculture is a crucial industry and the Great Lakes are a source of moisture, this is a big concern. By NASA’s estimates, states like Arkansas, Missouri and Iowa will likely hit the critical wet-bulb limit within 50 years, making it functionally impossible to be outdoors at certain times.

The Midwest represents one of the “most intense areas of agricultural production in the world,” and had a market value of crop and livestock products sold of over $76 billion in 2007, according to the U.S. Department of Agriculture. Image courtesy of Don Graham.

Even before that threshold is crossed, the outdoors will become more dangerous: a collaboration between ProPublica and The New York Times estimated that by 2060, the Midwest — particularly, Missouri — will experience weeks of wet-bulb temperatures above 82 F, which would make outdoor labor dangerous. By the end of the century, Midwesterners could experience three “extraordinarily dangerous Category IV” days (above 92 degrees Fahrenheit), where heat stroke would be “likely for fit individuals undertaking < 1 hour of moderate activity in the shade.”

The economy will be hard-hit as well. Missouri and Illinois will likely face a 15% average yield loss in the next five to 25 years, and up to a 73% loss by the end of the century, according to a Risky Business report.

Not to mention that, overall, the region could see more heat-related deaths, more expensive electricity bills and a decline in workforce productivity.

(It is important to note that wet-bulb temperature is an imperfect metric. Estimates also vary based on whether researchers assume a low-emissions or high-emissions climate scenario; the predictions above assume either high emissions or no reductions made to current emissions.)

To cope, it is likely that the Midwest will turn to increased air conditioning, which could “exacerbate the situation further if the power demands are met with greenhouse gas-intensive fuel,” Huber said.

“The Midwest would have to make sure that their energy generation isn’t greenhouse gas-intensive,” he added. “Solar power, nuclear — it’s not ideal, but it’s certainly better than powering your AC with coal.”

The people who will likely suffer the most from rising wet-bulb temperatures, however, are those who are constantly outdoors and without air conditioning.

“Farmworkers are doing one of the hardest jobs in some of the most difficult environmental conditions,” Mayra Reiter, project director of occupational safety and health at Farmworker Justice, said. According to the U.S. Centers for Disease Control and Prevention, they are 20 times more likely to die from heat than workers in other industries.

“Heat stress is a really serious health risk that takes place when people are outdoors,” Berman said. “Typically, they’re doing very strenuous activity. Their bodies aren’t cooling themselves down efficiently, and they start to shut down.”

While mild cases may leave people dizzy or exhausted, severe cases could lead to death. Investigate Midwest found that since 2002, at least 65 farmworkers have died from heat-related causes.

“We have reason to believe that the number is actually much higher, because a lot of people don’t experience the deadly effects right on the work site,” Reiter said. “They may go home and have a stroke or a heart attack later that night. The way deaths are classified also differs between countries and jurisdictions.”

Rising wet-bulb temperatures, coupled with a lack of regulatory protections for workers, are causing concern for advocacy groups.

“Historically, agricultural workers have largely been excluded from our labor laws and worker health and safety regulations,” Amy Liebman, chief program officer for workers, environment and climate at the Migrant Clinician’s Network, said. In many states, agricultural workers don’t have a right to unionize, qualify for overtime or have health insurance provided to them.

Today, only four states — California, Washington, Oregon and Colorado — have heat stress standards that protect outdoor workers.

“That leaves most workers all around the country without any legal recourse when they are working in dangerous conditions,” Reiter said.

“What’s needed is a national standard on heat … that’s going to be flexible enough to adjust with this increasing hazard,” Liebman said. “It’s going to outline what it is an employer needs to do to ensure that workers are protected from the heat, what the workers’ responsibilities are, and hopefully create a safer workspace.”

Reiter hopes that the pending Asunción Valdivia Heat Illness and Fatality Prevention Act, which would require a national standard to prevent excessive heat exposure, will be passed before the end of the current Congress.

“The facts don’t support the end of the world and all life, but they do support that things will get very bad if we don’t do something immediately,” Huber said. “I mean, the best solution to all of this is just don’t emit more carbon. This is really all up to us and the decisions we make. That being said, we are committed to a certain amount of warming, and we need to have adaptation measures in place for that.”


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