The UN climate conference in Paris is most likely humanity's last chance to thwart global environmental disaster, Pope Francis says, warning the world was "at the limits of suicide".
The Pope, who wrote a major document on the environment last June, made the comment in an hour-long news conference aboard the plane returning him to Rome on Monday at the end of a six-day trip to Africa.
The freewheeling conversations have become a trademark of his papacy and the few times he takes direct questions from journalists.
Francis, who visited Kenya, Uganda and the Central African Republic, also said the continent was "a martyr of exploitation" by wealthy countries who lust after its natural resources and try to impose Western values instead of concentrating on development.
The Pope was asked if the UN climate summit in Paris would mark a turnaround in the fight against global warming.
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"I am not sure, but I can say to you 'now or never'," he said.
"Every year the problems are getting worse. We are at the limits. If I may use a strong word I would say that we are at the limits of suicide."
He spoke of retreating glaciers in Greenland and low-lying countries at risk from rising sea levels.
"I am sure that the (Paris delegates) have goodwill to do something. I hope it turns out this way and I am praying that it will," he said.
World leaders launched an ambitious attempt on Monday to hold back the earth's rising temperatures, with the United States and China - the world's biggest carbon emitters - urging the UN climate summit in Paris to mark a decisive turn in the fight against global warming.
