Dangerous heat and humidity conditions are likely to occur every year during the Muslim pilgrimage, or Hajj, near the end of the century if high carbon emissions continue. A new analysis of temperature predictions made by three computer climate models for the area around Mecca, Saudi Arabia shows how climate change could reshape the religion practiced by 1.8 billion people around the world.

All Muslims are obligated to undertake Hajj at some point during their life, if health and finances permit. The ritual takes place in Mecca and the surrounding desert over the course of five days. It involves standing, walking, and praying along with millions of other pilgrims at a series of holy sites – about 20 to 30 hours outdoors overall.