The great Chicago heat wave was of the worst US disasters ever recorded
Between July 14 and July 20 of 1995, an unusually hot and humid air mass settled over the upper midwest region of the US, causing thousands of Chicago residents to fall ill; power transformers to fail; ambulance services and hospitals to become overwhelmed; and 739 people to die. It represents the 17th worst US disaster on record of any kind in terms of overall fatalities.
The event was the most intense short-duration heat wave in at least the last 48 years at many locations in the southern Great Lakes region and Upper Mississippi River Basin, a finding that comes as no surprise to scientists have long said that one of the clearest findings of climate science is that global warming amplifies the intensity, duration and frequency of extreme heat events.
Ingredients of an extreme heat event—there's more to it than record high temperatures
The severity of the great Chicago heat wave was not due only to the extreme high temperatures, but also the extreme minimum temperatures, humidity and the urban heat island effect, all of which increase under global warming.
During the event, the combination of high air and dewpoint temperatures resulted in daily average apparent temperatures exceeding 96.8°F (36°C) over a large area on some days. The urban heat island effect is estimated to have exacerbated the impacts in Chicago by raising nighttime temperatures more than 2°C. And, most severe of all, there was a period of 48 hours where the minimum temperature failed to go below 89°F (31.5°C).
Exposure to extreme heat is already a significant public health problem and the primary cause of weather-related mortality in the US
Extreme heat events have long threatened public health in the United States, and the risk to human health is understandably amplified as globally averaged temperatures increase. Deaths result from heat stroke and related conditions, but also from cardiovascular disease, respiratory disease, and cerebrovascular disease.
Heat waves are also associated with increased hospital admissions for cardiovascular, kidney, and respiratory disorders. Extreme summer heat is increasing in the United States, and climate projections indicate that extreme heat events will be more frequent and intense in coming decades.
So how unusual was the Chicago heat wave?
One study estimates that extreme high temperatures like those observed in the 1995 heat wave are likely to occur once every 23 years (what is known as a 23-year return period). The study's authors, however, are quick to point out that the severity of the Chicago heat wave depended more on the unrelenting nature of the heat and extreme nighttime temperatures. For two days, the minimum temperature failed to go below 89°F (31.5°C), an event so rare it has a probability of occurring only once every 1,000 years.
This, however, assumes the absence of global warming. Scientists have since investigated the likely influence of fossil fuel emissions on extreme heat waves, showing that heat waves will become more frequent, intense and long-lived as average and extreme temperatures increase over the coming century. One study finds that under a high future emissions scenario (known as the A1F1 scenario), 1995-like heat waves could occur as frequently as three times per year by 2100, or once every other year under a lower (B1) emissions scenario.