Global Climate Change Impacts in the United States 2009 Report Legacy site

Weather extremes have profound societal impacts, and their characteristics are expected to change in the future due to greenhouse forcing. In Chicago, heat waves and cold waves cause more than 100 fatalities per year, while extremely heavy rainfall can trigger disease outbreaks via contaminant discharge of storm water and sewage overflows. Here we analyze statistically downscaled climate model projections of extreme heat, cold, and precipitation in Chicago, based on higher (SRES A1FI) and lower (SRES B1) greenhouse gas emissions scenarios. The frequency, duration, and intensity of heat waves in Chicago are likely to increase substantially, and the heat-wave "season" extended (time during the year when extreme heat occurs). The simulated frequency of hot days--daily maximum temperature (Tmax) > 32 °C--increases from 15 days/year in the late 20th century to 36 days (B1) to 72 days (A1FI) by the end of this century. Proportionally, a much larger increase (a factor of 4 to 15) is projected in very hot days (Tmax > 38 °C). Conversely, the frequency and intensity of extreme cold is likely to decline considerably during this century. The coldest night of the year is projected to warm by 4-8 °C, while the simulated occurrence of very cold conditions (daily minimum temperature < - 18 °C) declines by ~ 50% (5 days, B1) to nearly 90% (8 days, A1F1) relative to the late 20th century. Simulated extreme precipitation events generally increase, especially during winter and spring, consistent with the seasonal changes in total precipitation. The projected seasonal changes in atmospheric circulation generally resemble the synoptic weather patterns associated with current extreme events, particularly during spring and summer, suggesting that some of the modeled response of extremes may be driven by mean dynamical changes.