Eight years after his landmark thesis outlined the devastation of manmade climate change, the 86-year-old pontiff published a follow-up that warned that some damage was "already irreversible".
"With the passage of time, I have realised that our responses have not been adequate, while the world in which we live is collapsing and may be nearing the breaking point," he wrote in the 12-page letter.
But he said the next round of UN climate talks opening in Dubai on November 30 "can represent a change of direction", if participants make binding agreements on moving from fossil fuels to clean energy sources such as wind and solar.
Only a real commitment to change "can enable international politics to recover its credibility", he wrote.
With his 2015 text, the pope had put himself -- and the worldwide Catholic Church -- firmly behind the science on blaming human behaviour for climate change.
But he acknowledged Wednesday that there were still those, including inside the Church, who had "certain dismissive and scarcely reasonable opinions".
"Despite all attempts to deny, conceal, gloss over or relativise the issue, the signs of climate change are here and increasingly evident," wrote Francis.
- Energy transition -
The 200-page encyclical in 2015, entitled "Laudato Si" ("Praise Be To You"), was an global call to arms to protect the Earth, and sparked debate unprecedented for a religious text, including commentaries in scientific journals.
Months later, there was a breakthrough in UN climate talks in Paris, with nearly every nation on Earth committing to limit warming to "well below" two degrees Celsius above pre-industrial levels.
But the UN warned last month the world is not on track to meet these goals, while climate monitors predict 2023 will be the hottest in human history, with the Northern Hemisphere's summer marked by heatwaves, droughts and wildfires.
In Wednesday's document, entitled "Laudate Deum" (Praise God), Francis expressed hope that the forthcoming COP28 "will allow for a decisive acceleration of energy transition, with effective commitments subject to ongoing monitoring".
He referenced concerns about the UN talks being held in oil-rich United Arab Emirates, noting that while it was a "great exporter of fossil fuels" it also made "significant investments" in renewable energy sources.
"To say that there is nothing to hope for would be suicidal, for it would mean exposing all humanity, especially the poorest, to the worst impacts of climate change," wrote Francis.
Mariam Kemple Hardy, of campaign group Oil Change International, welcomed the pope's intervention, saying he addressed "the root cause of the climate crisis: fossil fuels".
UK Catholic overseas aid agency CAFOD also praised his call for action on energy, and saying that with his text, he "holds world leaders and industry giants to account".
- 'Concern with profit' -
Climate change has been a major theme of Francis' ten years as head of the world's 1.3 billion Catholics, and while drawing on science, he has also offered a strong moral message.
In his 2015 text, the pope blamed a pursuit of economic growth for the destruction of the planet and warned rich countries must lead the way in repairing the damage.
In Wednesday's text, he said that "regrettably, the climate crisis is not exactly a matter that interests the great economic powers, whose concern is with the greatest profit".
He also repeated a call for changes in the "irresponsible lifestyle" of rich countries, saying relying on technological innovation was not enough.
He noted that "emissions per individual in the United States are about two times greater than those of individuals living in China, and about seven times greater than the average of the poorest countries".
Many doubt, however, that the new document, which Francis wrote in Spanish, can have the same impact as the first.
But Bill McKibben, the founder of 350.org, an international environmental organisation, said: "The work of spiritual leaders around the world may be our best chance of getting hold of things.
"Yes, the engineers have done their job. Yes, the scientists have done their job. But it's high time for the human heart to do its job. That's what we need this leadership for."
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