Crop irrigation could be cooling Midwest
Drop in hot days blamed on moisture from Great Plains
By Sid Perkins
ATLANTA — If summers seem cooler and wetter in parts of the Midwest in recent years, you can thank — or blame — farmers, two new studies contend.
While average global temperatures rose about 0.74 degrees Celsius during the past century, the U.S. Midwest has experienced a noticeable slump in summer temperatures in recent decades, reported David Changnon, a climatologist at Northern Illinois University in DeKalb, on January 19 at the annual meeting of the American Meteorological Society.
On average, daily high temperatures in Chicago rise above 90° Fahrenheit (32.2° Celsius) on 24 days each summer. But from 2000 through 2009, only two years tallied more than 24 days hotter than 90°— the lowest decadal total in 80 years, Changnon noted.
Rather than being just a statistical anomaly, the recent cool temperatures seem to be part of a steady long-term decline in summertime highs in Chicago, Changnon and his colleagues found. The last 10 years have seen a total of only 172 days above 90°; the 1930s saw more than twice as many. And Chicago wasn’t alone. The team noted a comparable decline in unusually hot days at 13 other sites in a swath stretching from western Iowa through Illinois to eastern Indiana.