By George Webster, CNN

(CNN) – An estimated 2.5 million pilgrims have descended on the city of Mecca for the Islamic Hajj, said to be the largest annual gathering of people in the world.

Every fit and able Muslim is obliged by their faith to make the journey at least once in their lifetime. But with the rising threat of climate change, there are now calls for both pilgrims and authorities in Mecca to reduce the environmental damage wrought by this yearly influx of travelers.

"Everyone arrives at the same time, at exactly the same place, and every year there are more and more people," said Dr Husna Ahmed, principal author of "The Green Guide for Hajj," a booklet promoting ecologically-sustainable practices among Hajj pilgrims, released earlier this week.

Read the full story on the environmental impact of the Hajj.