The Muslim pilgrimage to Mecca, the Hajj, attracted no fewer than two million pilgrims in 2023. But this pilgrim boom has an environmental downside: climate scientists are warning that the five-day Hajj alone, with its bargain flights, hotels, catering and local transport, produces over 1.8 million tonnes of greenhouse gases, roughly the amount New York City emits every two weeks. Yet the Saudi government has plans to go much bigger still: by 2030, they want 30 million pilgrims a year to take part in the Hajj and Umrah. Zubeida Malik asks what the Saudi authorities, local groups and campaigners, religious scholars and the pilgrims themselves can do to reduce the environmental footprint of one of the largest religious gatherings on the planet.