In a video message played for church leaders gathered in Belem, Brazil during United Nations climate talks, the Pope appealed for concrete action to stop global warming.
"One in three people live in great vulnerability because of these climate changes," Leo said. "To them, climate change is not a distant threat, and to ignore these people is to deny our shared humanity."
His message came as the talks were moving into their second week, with high-level ministers from governments around the world arriving at the edge of the Brazilian Amazon to join negotiations.
Several leaders from Global South nations gave emotional testimony on devastating costs of recent extreme weather and natural disasters.
Vulnerable nations have pressed for more ambition at these talks as world leaders have begun to acknowledge that Earth will almost surely go past a hoped-for limit — 1.5 degrees Celsius in Earth's warming since pre-industrial times. That was the target set at these talks in 2015 in the landmark Paris agreement.
Scientists say in addition to deadly heat, a warming atmosphere leads to more frequent and deadly extreme weather such as flooding, droughts, violent downpours and more powerful hurricanes.
Leo said there's still time to stay within the Paris Agreement, but not much.
"As stewards of God's creation, we are called to act swiftly, with faith and prophecy, to protect the gift He entrusted to us," he said. And he added: "But we must be honest: it is not the Agreement that is failing, we are failing in our response. What is failing is the political will of some."
Leo made history this year by becoming the first American pope, and has embraced Pope Francis' environmental legacy, including dismissing climate sceptics.
The US, the world's second-largest polluter, is skipping the conference. US President Donald Trump called climate change "the greatest con job ever perpetrated on the world" during a speech to the UN General Assembly in September.
UN climate chief Simon Stiell said Leo's words "challenge us to keep choosing hope and action."
Leo "reminds us that the Paris Agreement is delivering progress and remains our strongest tool — but we must work together for more, and that bolder climate action is an investment in stronger and fairer economies, and more stable world," Stiell said.
David Gibson, director of the Centre on Religion and Culture at Fordham University in New York, said Leo is becoming the world's most prominent moral leader against climate change.
"This message does stake Leo out as a voice for the rest of the world, especially the Southern Hemisphere where climate change is wreaking havoc with the vulnerable in Asia, Africa and Latin America," said Gibson.
And he said it shows that Leo, who spent decades working as a missionary in Peru and is a naturalised Peruvian citizen, "has a Latin American heart and voice."
The Laudato Si' Movement, a Catholic climate movement that takes its name from a 2015 encyclical in which Pope Francis called for climate action, called Leo's message "a profound moral intervention."
"He reminds the world that creation is crying out and that vulnerable communities cannot be pushed aside. "His voice cuts through the noise of negotiations and calls leaders back to what truly matters: our shared humanity and the urgent duty to act with courage, compassion, and justice," the group's executive director, Lorna Gold, said.
with Reuters
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