By Angelica Medina and Janina Nuno Rios
June 10 (Reuters) - The World Cup will kick off on Thursday under familiar North American summer threats: extreme heat, suffocating humidity and thunderstorms capable of delaying matches with little warning.
Seasonal forecasts indicate above-normal temperatures across large parts of the United States, while moisture flowing north from the Gulf of Mexico could fuel thunderstorms and severe weather during the opening weeks of the tournament.
While conditions for
individual matches cannot be predicted this far ahead, sports scientists say there are clear weather-related risks facing a summer World Cup spanning Canada, Mexico and the United States.
The key measure is not air temperature alone but wet-bulb globe temperature, which incorporates heat, humidity, sunlight and wind to estimate heat stress on the body.
World Weather Attribution has warned that roughly a quarter of matches could be played in conditions that exceed recommended safety limits.
INTERNAL HEAT CHALLENGE
Chris Minson, a physiology professor and co-director of the Exercise and Environmental Physiology Labs at the University of Oregon, said elite players generate enormous internal heat even before the weather is considered.
"Seventy-five percent of all the energy that we utilise during exercise gets converted to heat," Minson told Reuters. "Only about 25% goes to actually doing the exercise."
In hot, sunny or humid conditions, the body's normal cooling system begins to struggle. Humidity is a particular concern, since sweat cools the body only when it evaporates.
"One of the hardest things for us is when the humidity is very high," Minson said.
High-humidity World Cup venues include Houston, Miami, Dallas and Monterrey.
CLIMATE CHANGE IMPACT ON PERFORMANCE
Climate change has increased the likelihood of temperatures high enough to affect player performance at 97 of the 104 tournament matches, according to new research from Climate Central.
The biggest such increase is projected for the June 26 group-stage match between Uruguay and Spain in Guadalajara, where researchers estimated a 70% chance of performance-impairing heat - 37 percentage points higher than it would have been without climate change.
Ryan Calsbeek, a biological sciences professor at Dartmouth College who studies how body type affects athletic performance in different climates, said heat and humidity could influence not only player welfare but the pace and style of matches.
"Higher temperature, higher humidity is likely to slow games down," he said. "When athletes have to perform for a very long time, they're just not going to be able to balance the explosive power of their fast-twitch efforts with the more aerobic long-term efforts of a 90-plus minute game in the heat and humidity."
Nearly half of all matches face at least a 50% chance of temperatures exceeding 28 degrees Celsius (82.4 Fahrenheit) — a threshold linked to declines in sprinting, distance covered and recovery time.
Calsbeek said Mexico City's altitude - some 2,240 meters (7,350 ft) above sea level - could also prove significant, particularly for those arriving from lower elevations without time to acclimatise. The city is set to host five matches.
FIFA has said every match at the World Cup will include a three-minute hydration break in each half, while scheduling decisions took into account factors including average temperatures, travel, rest days, medical planning and cooling infrastructure.
SAFETY PROTOCOLS QUESTIONED
Several venues feature retractable roofs or climate-control systems, and tournament regulations allow matches to be delayed, suspended, rescheduled or relocated for health, safety or security reasons, including severe weather.
Minson said FIFA should mandate interventions when the wet-bulb globe temperature reaches 26C and should consider postponing matches around 28C to 30C.
Minson also called for six-minute cooling breaks, shaded cooling areas, emergency ice baths and longer halftimes when conditions warranted.
"If you have a player who seems to be having some delirium or not thinking straight, or collapses on the field, you need to cool them down immediately," he said.
For FIFA, the tournament is a logistical showpiece. For players, coaches and scientists, it may also be a test of how football adapts to a hotter future.
(Reporting by Angelica Medina and Janina Nuno Rios, Editing by Rosalba O'Brien)
