As negotiators in Sharm el-Sheikh began their figurative marathon on Sunday, about 50,000 people in New York City ran (or tried to) 26.2 miles on the hottest November 6 on record. Sunday’s 75°F at the finish line was also the hottest since moving the race from October to November in 1986. Climate change, mainly caused by the extraction and combustion of fossil fuels, is intensifying heat waves, and warmer air also holds more moisture, increasing humidity. Despite having fewer entrants than the last full-capacity race in 2019, more than double the number of runners did not finish before the cutoff time, including Olympians like Brazil’s Daniel Do Nascimento who collapsed while leading the race at mile 21.

High relative humidity, which reduces the body’s ability to dissipate heat through sweating, also likely contributed to the high number of DNFs. “The main complaint was nausea and I guarantee you that was related to the heat,” NYU physician and professor Lipi Roy, who volunteered in a medical tent near the finish line, told Bloomberg. “Every single runner said, ‘This is the hottest race I’ve ever done.’ But it wasn’t just the heat, it was the humidity.” (Bloomberg $, Gizmodo, Let’sRun, Reuters, AP; Climate Signals background: Extreme heat and heatwaves)