Extreme heat is the biggest weather-related killer in the United States, and dangerously hot weather is already occurring more frequently in the Midwest than it did 60 years ago, according to a new report by the Union of Concerned Scientists.

Heat in the Heartland: 60 Years of Warming in the Midwest, presents an original analysis of weather data for five major urban areas—Chicago, Cincinnati, Detroit, Minneapolis and St. Louis—and five smaller nearby cities.

High temperatures not only can lead to dehydration, heat exhaustion and deadly heatstroke, but can can also aggravate existing medical conditions, such as diabetes, respiratory disease, kidney disease and heart disease. Heat claims, on average, more lives each year than floods, lightning, tornadoes and hurricanes combined. From 1999-2003, exposure to excessive heat killed an estimated 3,442 U.S. residents.

The report examines how the average daytime temperatures, humidity levels and nighttime temperatures within different types of weather systems have changed over time. Key findings include:

  • Heat waves lasting three days or more have become more common over the last six decades. St. Louis has approximately four more three-day heat waves each year than it did in the 1940s.
  • On average, hot humid days have increased more rapidly in frequency, while hot dry days have increased in temperature more rapidly across the Midwest since the 1940s and 1950s.
  • In general, hot air masses have become hotter and more humid during nighttime hours.
  • In some cities, average nighttime temperatures within some air mass types have increased as much as 4-5 degrees Fahrenheit (˚F) over the six decades.
  • Relief from heat is harder to find—all of the cities studied now have fewer cool, dry days in the summer.
  • The results are not due solely to an urban heat island effect on major cities. Less urban neighboring locations showed similar increases in hot summer air masses.

The report sheds light on the importance of city-level efforts to minimize the health risks of future climate change. The findings suggest several consequences for public health, as well as implications for local preparedness and efforts to reduce the effects of a changing climate.

“We need strategies to both build climate-resilient communities and reduce the global warming emissions that are driving climate change,” the study concludes.

It is the third report in a series on Climate Change and Your Health.

1. Perera EM, Sanford T, White-Newsome JL, et al. Heat in the Heartland: 60 Years of Warming in the Midwest. Cambridge, Mass.: Union of Concerned Scientists, 2012.