Dangerously hot summer days have become more common across the U.S. Midwest in the last 60 years, and the region will face more potentially deadly weather as the climate warms, according to a report issued by the Union of Concerned Scientists on Wednesday.
The report looked at weather trends in five major urban areas - Chicago, Cincinnati, Detroit, Minneapolis and St. Louis - along with weather in nearby smaller cities such as Peoria, Illinois, and Toledo, Ohio. The report focused on the Midwest because of its numerous major population centers, and because it is projected to face more heat waves with climate change.
The report found that the number of hot, humid days has increased, on average, across the Midwest since the 1940s and 1950s, while hot, dry days have become hotter.
Finding relief from the heat during the summer has become more difficult, as all the cities studied now have fewer cool, dry days in the summer, and nighttime temperatures during hot periods have risen.
"Nighttime is typically when people get relief, especially those who don't have air conditioning," said Steve Frenkel, Union of Concerned Scientists' Midwest office director. "The risks of heat-related illness and death increase with high nighttime temperatures."
The report found that heat waves lasting three days or longer have become more common. St. Louis, for example, has more than doubled its number of three-day heat waves since the 1940s. Studies have linked at least three consecutive days of high temperature and humidity to more deaths.
Extreme heat and humidity can be lethal. In Chicago, more than 700 deaths were attributed to a heat wave in July 1995. More recently, extreme heat in Russia in 2010 led to an estimated 55,000 deaths.
With more extreme summertime heat, annual deaths in Chicago are projected to increase from 143 from 2020-2029 to 300 between 2090-2099, according to the report.
The report warned that conditions could get much worse if emissions of gases believed to cause global warming continue at their current pace, or at a higher pace.
Chicago, for example, could see more than 70 days with temperatures of 90 Fahrenheit (32 Celsius) each year toward the end of the century, on average, if emissions continue at the current pace. Under a higher-emissions scenario, dangerously hot days over 100 F (38 C) in Chicago could increase dramatically, producing a month of such days, the report said.
Hot, humid temperatures are tougher on the elderly, whose percentage in the population is increasing. About 20 percent of U.S. residents are projected to be over age 65 by 2030, up from about 13 percent now, according to the report.
"We must take preventative measures to protect public health during extreme heat events, but the only way to ensure these heat waves are not a threat in the future is by reducing the harmful emissions that are driving them in the first place," said Frenkel.
Though the study focused on weather in the last six decades through 2011, the summer of 2012 has so far reflected a continued warming trend.
June temperatures contributed to a record-warm first half of the year and the warmest 12-month period the nation has experienced since recordkeeping began in 1895, according to the National Oceanic and Atmospheric Administration.
In the Midwest, the U.S. corn yield is seen at a 10-year low due to an expanding drought.