France, May 29 -- The hajj pilgrimage to Mecca in western Saudi Arabia, one of the most arid places on the planet, is one of the world's largest religious gatherings - at which hundreds of thousands of pilgrims are now being exposed to extreme heat.

This year's gathering, which ends on Friday, was again held in temperatures exceeding 40C.

Global heating has "fundamentally altered" the climate of Mecca, the study by the United Kingdom-based group World Weather Attribution said.

Temperatures recorded in Mecca in May "are now typical of summer in the 1980s, as climate change shrinks the safe window" for the hajj, the study found.

Researchers said that by the end of the century, the hajj would take place amid dangerous heat almost all yea...