Pope Francis was not talking about the apocalypse or the end of the human race. If we wreck the environment, the worst I would expect is to plunge humanity into another Dark Age. This would bring suffering like we can scarcely imagine, but humanity (and Christianity) would scrape by somehow.
One plausible environmental-disaster scenario is outlined in the book Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future, by Peter D. Ward. In brief, fossil evidence suggests that some of the mass extinctions in earth’s distant past resulted from overly warm oceans releasing massive amounts of hydrogen sulfide gas, poisoning the atmosphere and wiping out the ozone layer. The concern of course is that the current trend of carbon dioxide buildup and global warming may trigger another such catastrophe.