Saudi Arabia has ambitious plans to welcome millions more pilgrims to Islam's holiest sites. But as climate change heats up an already scorching region, the annual Hajj pilgrimage — much of which takes place outdoors in the desert — could prove even more daunting.

The increased number of pilgrims, with the associated surge in international air travel and infrastructure expansion, also raises sustainability concerns, even as the oil giant pursues the goal of getting half its energy from renewable resources by 2030.