Some 2 million people are expected to travel across the globe to eat, sleep and pray in unison from Wednesday, as the annual Islamic pilgrimage of Hajj gets underway in Mecca.
For billions of Muslims who are physically and financially able, Hajj is a mandatory act of worship. But the religious celebration also has a substantial impact on the environment, said a Thomson Reuters Foundation report on Wednesday.
Environmentally aware worshippers say that should be reduced, while inspiring Muslims to adopt a greener lifestyle.
"Hajj is all about living lightly and centering yourself around God," 28-year-old pilgrim Shanza Ali told the Thomson Reuters Foundation from Mecca in Saudi Arabia. "We make many journeys in our life, and we go to many places, but this is the only journey that's physical, mental and spiritual," said Ali, who is chair of UK-based group Muslim Climate Action.
She has found many similarities between Hajj’s message of simplicity and being environmentally conscious, and has tried to minimize her own carbon footprint and waste during the pilgrimage, which lasts for at least six days and takes worshippers to a series of holy sites in Saudi Arabia.
For Husna Ahmad, author of "The Green Guide for Hajj", Muslims are doctrinally required to be stewards of the Earth.
Tackling climate change is no longer about preserving the planet for future generations as its effects are evident now, she said.
The majority of Muslims live outside Saudi Arabia and could collectively influence the greening of the sacred rituals, she added.
"Consumer power is something that people need to think about in terms of flights, what they take, what they wear, the rubbish they throw, plastic bottles and all those sorts of things. We have to be conscious of that," she told the Thomson Reuters Foundation.
Muslims need to move away from a fast, disposable society, she added, with Hajj being the potential start of that journey.
In recent years, Saudi Arabia has taken steps to green the Hajj, such as setting quotas for pilgrim numbers and developing the Mecca metro system to limit pollution.
The Saudi Green Building Forum, a Riyadh-based non-governmental group recognized by the United Nations, has recently been tasked with auditing green efforts in Medina, the country's second holy city where the Prophet Mohammed is buried and a site visited by millions of pilgrims.
Forum Secretary General Faisal Alfadl said his team will measure the green credentials of the holy cities of Mecca, Medina and others against international guidelines on energy use, waste, water, transport and human well-being.
People now realize it is politically and culturally incorrect not to respect the environment, said Alfadl.
"We have moved forward," he said, noting a shift in the public mood from desert Bedouins to city dwellers on the importance of protecting the environment, with the focus now on action rather than simply raising awareness.
Reviving traditional practices could help - for example, sharing water among pilgrims from a communal source, which was common before plastic bottles became ubiquitous.
And the white marble stones surrounding the central cube-shaped Kaaba building in Mecca naturally prevent the heat-island effect found in other urban areas, Alfadl said.
Recycling may not be at the top of pilgrims' minds, but Muslims have a duty to recognize the creator of the environment and reflect on Islamic teachings not to harm animals, waste water or cut down trees unnecessarily, said Fatima Ragie of Green Deen South Africa, a Muslim environmental network.
Ragie, who completed Hajj in 2009, urged greater efforts once the pilgrimage ends - for instance, ensuring food is not wasted when millions of animals are slaughtered, marking Abraham's near sacrifice of his son and the start of the Eid holiday.
More mosques and Muslim leaders should also speak up about climate change and the environment, she said.
From Bangladesh to North Africa, climate change is a reality for many Muslims, as floods and droughts fuel instability and conflict, said Nana Firman, who participated in the UN climate talks in Morocco last year for the Global Muslim Climate Network.
"A lot of people feel like they don't know what to do, so it's really important that we engage (them)," she said.
Indonesia - which has the world's largest Muslim population, according to the Pew Research Center - has launched initiatives, from a phone app showing pilgrims how to enjoy a green Hajj, to offsetting carbon emissions from flights by planting trees, and limiting the number of times each person can undertake the pilgrimage, said Firman.
She urged Hajj pilgrims to "reflect and make a change in their lives when they go back, and care more for the environment".
As Ali prepares herself to undertake the challenging pilgrimage in the Gulf heat with her husband and mother, the natural environment offers a way for her to draw closer to God.
"I think just reflecting on the fact you're with humanity, you see people from every corner of the world... That really makes you appreciate the idea that we're all sharing the Earth together," she said.