Pope Francis said Monday it was "now or never" for the world community to hammer out a deal on climate change, warning the situation was "borderline suicide".
"It's now or never," the Argentine said as the heads of more than 150 nations kicked off 12 days of talks in the French capital to negotiate a deal to roll back global warming.
Since the Kyoto Protocol on climate change was adopted in 1997, "little has been accomplished" and "every year the problems get more serious," he told journalists at a press conference on the papal plane during his return from a trip to Africa.
Sometimes it appears, "to use a strong word, that the situation is borderline suicide", he said.
"Almost everyone at the Paris talks wants a deal to be done. I am confident they will do it," he said.
The pope is a fierce defender of the environment, publishing a hard-hitting thesis on the topic this year and laying the blame for global warming squarely on man's shoulders.
Climate change fuels poverty, migration, sickness and war, he says, and fossil fuels should be abandoned in an energy revolution paid for largely by developed countries.
The Vatican is being represented at the opening of the COP21 talks by its secretary of state Pietro Parolin, the Holy See's number two.
The negotiations, taking place under the banner of the UN Framework Convention on Climate Change, are due to wrap up on December 11.
A deal is far from guaranteed: potential stumbling blocks range from providing finance for climate-vulnerable and poor countries to scrutiny of commitments to curb greenhouse gases and even the legal status of the accord.