THE SUMMER OF 2011 WAS A scorcher. It was the hottest summer since the infamous Dust Bowl era of the 1930s, with 42 states reporting above-normal temperatures, and four states breaking records for extreme heat (National Climatic Data Center 2011).

Such weather isn’t just uncomfortable. Extreme heat kills. From 1999 to 2003, 3,442 deaths resulting from exposure to extreme heat were reported (CDC 2006a).

Heat waves in the recent past have struck the Mid-west hard. For example, heat waves in St. Louis and Kansas City, MO, in July 1980 caused a 57 percent and 64 percent increase in deaths, respecticely...

TO UNDERSTAND CHANGES IN weather patterns and what might be driving those changes, we focused on weather systems known as air masses (see “Weather 101”). By evaluating air masses instead of simply temperatures, we were able to provide a more detailed picture of the changes in weather patterns that have actually been experienced by Midwesterners over the last 60 years.

We analyzed some 60 years of data on air masses in five large midwestern cities: Chicago, Cincinnati, Detroit, Minneapolis, and St. Louis. We chose those cities to represent both the northern and southern Midwest, and to reflect proximity to the...

OUR INVESTIGATION UNCOVERED measurable increases in dangerously hot weather in the Midwest. People living in the region have experienced these weather changes during their lifetimes.

Specifically, we found that the air masses associated with harmful health effects—dry tropical and moist tropical+—have become more common in the Midwest over the last six decades, on average. Hot summer air masses lasting three days or longer have also become more common, on average. Meanwhile cool, dry summer days (dry polar) have become less common.

The characteristics of these types of weather have also changed. With some exceptions, hot summer air masses...

HIGH TEMPERATURES AND HUmidity levels pose serious threats to public health. Very hot, humid weather increases the risk of hyperthermia—elevated body temperature—and hot, dry weather raises the risk of dehydration (Vanos et al. 2010; Epstein and Moran 2006; Mairiaux, Malchaire, and Candas 1987). The impact of heat on people’s well-being depends on their exposure to high temperatures or prolonged periods of intense heat, their underlying health, and their economic and social vulnerability, as well as how much their region has invested in measures to help residents cope with heat.

Some groups are especially vulnerable to the dangers of...

A LARGE BODY OF SCIENTIFIC EVIdence indicates that carbon emitted from human activities such as burning fossil fuels and cutting down forests is very likely raising our planet’s temperature (IPCC 2007). As this study shows, hot summer days and heat waves have already become more common across the Midwest. In that region and beyond, communities must find ways to adapt to these dangerous weather conditions.

While several major cities, including Chicago, have implemented climate action plans or emergency-response plans for extreme heat, the latter idea is still very new to most communities. A 2010 survey of 70 communities across the...

EXTREME HEAT EVENTS BECAME more intense and more common in the Midwest over the past six decades. Midwesterners have experienced these changes in weather during their lifetimes. While we did not design our study to determine whether such changes stem from human activities, our findings are consistent with projected warming trends.

A previous UCS report showed that the risk of dangerously hot weather in the Midwest is likely to grow as we continue to release heat-trapping emissions and the climate warms. Under a higher-emissions scenario, nine major cities—including the five in this report—are projected to see at least...