A group of scientists has warned the Fédération Internationale de Football Association (FIFA) about extreme weather conditions, including heat and humidity, expected during this year’s World Cup in North America. The warning comes from two networks of climate, health, and sports experts: World Weather Attribution (WWA) on one side and 20 scientists who have signed an "open letter to FIFA" on the other side.

According to WWA, up to a quarter of the matches at the upcoming World Cup may take place in extremely hot conditions. Due to climate change, the problem is likely to be more severe than during the previous tournament held in the United States in 1994.

"Our research shows that climate change has a real and measurable impact on the sustainability of World Cups held in summer in the Northern Hemisphere," emphasizes Friederike Otto, professor of climate science at Imperial College London and co-founder of World Weather Attribution (WWA).

This group of scientists analyzed 104 matches to be played on 16 stadiums in the US, Canada, and Mexico. They referred to the wet bulb globe temperature (WBGT) index, which takes into account not only temperature but also humidity, sunlight, and cloud cover.

This index is commonly used to measure outdoor activities, such as by the military or for sports events. A seemingly moderate air temperature can become unbearable and even deadly 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 matches) are likely to be played in conditions equal to or greater than 26°C WBGT, which would require specific and rigorous cooling measures, according to the players' union (FIFPro). About five matches could be played at temperatures of 28°C WBGT, nearly double the level in 1994, "at a dangerous threshold where the players' union recommends canceling matches," Ms. Otto emphasized during a press briefing.

Scientists have particularly highlighted matches of "high risk" that will be played on open-air stadiums during the day, especially in Miami, Kansas City, and New Jersey. Other stadiums are air-conditioned, which reduces the risks for players and spectators, but "there are also risks for fans who may gather outside the stadiums."

The "open letter to FIFA," signed by researchers from numerous countries, including the United States and Canada, relies on the same temperature index.

Above 26°C WBGT, "even well-prepared and acclimatized athletes often cannot maintain thermal balance during intermittent high-intensity physical activities," they emphasize.

"It is clear that the three-minute breaks that FIFA has adopted for all matches are too short to have a significant effect on rehydration and body cooling. They should last at least six minutes," they noted.

Two three-minute "cooling breaks" are scheduled in the middle of the first and second halves of each match, and this will be the first World Cup where FIFA has made them mandatory.

"The risk of heat waves has doubled since 1994, threatening players and fans," Simon Stiell, UN chief for climate change, reacted on Thursday.

He called for "faster action to protect the sport we love and everyone who watches it," by "doubling our efforts for a decisive transition to clean energy."