A quarter of the matches in this summer's FIFA World Cup may be played under extremely hot conditions, with temperatures expected to be higher than those during the 1994 tournament in the United States, due to climate change, researchers warned on Thursday.

"Our study shows that climate change has a real and measurable impact on the feasibility of hosting World Cups in the summer in the Northern Hemisphere," said Friederike Otto, a professor of climate science at Imperial College London and co-founder of World Weather Attribution (WWA). This group of scientists will investigate 104 matches taking place from June 11 to July 19 across sixteen stadiums in the United States, Canada, and Mexico.

"Since the last time the United States hosted the World Cup in 1994, the risk of heat waves has doubled due to climate change, threatening players and fans," UN climate chief Simon Stiell responded on Thursday, calling for "faster action to protect the sport we love and all those who watch it through doubling efforts for a decisive shift to clean energy."

The researchers at WWA use a temperature index that accounts for not only temperature but also humidity, sunlight, and cloud cover. This index, which affects the body's ability to cool down, is often used as a starting point for outdoor activities like military operations and sporting events. A seemingly moderate air temperature can become unbearably hot or even fatal when combined with high humidity. A high WBGT index of 28°C is equivalent to 38°C in dry conditions or 30°C in very humid conditions. About a quarter of the matches (26 games) are likely to be played under conditions equivalent to or exceeding 26°C WBGT, which would require additional cooling measures, according to players' unions. Around five matches will be played at 28°C or higher, almost double the temperature in 1994, a dangerous threshold at which players' unions recommend canceling matches, as highlighted by Friederike Otto during a press briefing with journalists.

Scientists have identified several matches with high risk at open-air stadiums, particularly in Miami, Kansas City, and New York/New Jersey, where France will play against Senegal on June 16. Other stadiums are climate-controlled, which reduces risks for players and spectators, but there are also risks for fans who may gather outside. They are exposed to even greater risk because they won't be cared for by many medical professionals, noted Friederike Otto.