This year’s World Cup is already historic for being the biggest in the competition’s history, stretching across Mexico, the US, and Canada — but it could also become the hottest and most polluting tournament ever.

Heat is a central concern, measured not just by air temperature but by the wet-bulb index, which factors in humidity, wind, and sunshine. At 26°C, water breaks are mandatory; at 28°C, players’ unions want games halted; at 32°C, play stops entirely. The first two thresholds are very likely, The Nature Conservancy’s Chief Scientist Katharine Hayhoe said, the third is a possibility. For football fans, the quality of the spectacle is at risk too: Climate change has increased the likelihood of temperatures high enough to affect player performance in 97 of the 104 tournament matches, Climate Central found.

FIFA’s decision to expand from 32 to 48 teams also means more host countries, more fans, longer travel distances, and a much larger carbon footprint. According to one estimate, this year’s tournament could generate 7.8 million metric tons of CO2 — for reference, the 2022 World Cup in Qatar produced roughly 3.8 million tons.