In a video message played for religious leaders gathered at the UN COP30 climate summit in Brazil's Amazon city of Belem, Leo said nations had made progress, "but not enough".
"One in three people live in great vulnerability because of these climate changes," he said.
"To them, climate change is not a distant threat, and to ignore these people is to deny our shared humanity."
The Pope's message on Monday reflected mounting concern about flagging international ambition and rising greenhouse gas emissions a full decade after the 2015 Paris Agreement, a landmark deal at which countries for the first time agreed to limit global warming to well within two degrees Celsius above pre-industrial levels.
Scientists say the earth is destined now to overshoot that threshold, opening the door to devastating impacts.
The talks at COP30 are 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.
The day was dominated by speeches, with several leaders from Global South nations giving 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.
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.
"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 Center 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".
with Reuters
