During the European heat wave of 2003 that killed tens of thousands, the temperature in parts of France hit 104 degrees. Nearly 15,000 people died in that country alone.
During the Chicago heat wave of 1995, the mercury spiked at 106 and about 600 people died.
In a few decades, people will look back at those heat waves "and we will laugh," said Andreas Sterl, author of a new study. "We will find [those temperatures] lovely and cool."
Sterl's computer model shows that by the end of the century, high temperatures for once-in-a-generation heat waves will rise twice as fast as everyday average temperatures.
Chicago, for example, would reach 115 degrees in such an event by 2100. Paris heat waves could near 109 with Lyon coming closer to 114.
Sterl, who is with the Royal Netherlands Meteorological Institute, projects temperatures for rare heat waves around the world in a study soon to be published in the journal Geophysical Research Letters.
His numbers are blistering because of the drying-out effect of a warming world. Most global warming research focuses on average daily temperatures instead of these extremes, which cause greater damage.
His study projects a peak of 117 for Los Angeles and 110 for Atlanta by 2100; that's 5 degrees higher than the current records for those cities.
Kansas City faces the prospect of a 116-degree heat wave, with its current all-time high at 109, according to the National Climactic Data Center.
A few cities, such as Phoenix, which once hit 122 degrees and is projected to have heat waves of 120, have already reached these extreme temperatures once or twice. But they would be hitting those numbers a little more often as the world heats up over time.
For New York, it would only be a slight jump from the all-time record of 104 at John F. Kennedy Airport to the projected 106.
It could be worse. Delhi, India is expected to hit 120 degrees; Belem, Brazil, 121, and Baghdad, 122.
Those figures make sense, Ken Kunkel, a top Midwestern climate scientist and interim director of the Illinois Water Survey.
These are temperatures that are dangerous, said University of Wisconsin environmental health professor Dr. Jonathan Patz.
"Extreme temperature puts a huge demand on the body, especially anyone with heart problems," Patz said. "The elderly are the most vulnerable because they don't sense temperature as well."
And it's not just at the end of the century. By 2050, heat waves will be 3 to 5 degrees hotter than now "and probably be longer-lasting," Sterl said.
By mid-century, southern France's extreme heat waves should be around 111 degrees and then near 118 by the end of the century, Sterl's climate models predict.
In the 1990s, that region's extreme heat wave peaked at 104 degrees; in the 1950s, the worst heat wave peaked around 91 degrees, according to Sterl.