Yahoo Sports reports: Extreme heat could be a key factor at the World Cup tournament, set to kick off in Mexico on Thursday, the UN climate secretariat warned on Monday. "The 2026 FIFA World Cup, the biggest tournament in football history, will be played at a time of rising levels of extreme heat, driven by worsening climate change hitting billions of people around the world," the climate secretariat said at the start of a 10-day climate conference in the German city of Bonn.