January 2015 (Risky Business Project) – This graph shows the mean number of days in a typical Midwest summer when the temperature and humidity could reach Humid Heat Stroke Index (HHSI) category II, III, or even greater. These are days when the heat and humidity could be so high that it will be dangerous for humans to remain outdoors.
Increased heat and humidity and variations in precipitation will have a significant impact on the Midwest economy as a whole and on particular sectors such as energy and agriculture. Because these impacts vary so greatly across the region, we discuss them by metro area below, as well as in a special chapter on climate risk to the Midwest agriculture sector.
As Midwesterners well know, it’s not just the heat, it’s the humidity—or, in this case, a dangerous combination of the two. One of the most striking findings in our analysis is that increasing heat and humidity in some parts of the region could lead to outside conditions that are literally unbearable to humans, who must maintain a skin temperature below 100°F in order to effectively cool down and avoid fatal heat stroke. The Midwest has never yet seen a day exceeding this combination of heat and humidity, which we measure as Category IV HHSI along a “Humid Heat Stroke Index” (see Figure 4)—though Appleton, Wisconsin came very close in 2005 when a combination of an outside temperature of 101°F and dew point of 90°F led to a Category III HHSI day. Our research shows that if we continue on our current path, the average Midwesterner will likely see up to three days at the extraordinarily dangerous Category IV HHSI every year (with a 1-in-20 chance of more than 9 days), and as many as 25 days at Category III HHSI (with a 1-in-20 chance of more than 38 days), by the end of this century.