Pope Francis has spoken out forcefully against climate change denial. Speaking to reporters Sunday as he flew over Caribbean islands decimated by Hurricane Irma, Pope Francis said humans must make progress in tackling climate change, and called those who continue to deny its existence “stupid.”

Pope Francis: “We will not go backwards, we will go down. That is true. Climate change, you feel the effects, and the scientists tell us clearly the road to take. All of us have a responsibility—all of us—some small, some big, a moral responsibility, to not accept it, to give one’s opinion or to make decisions, and we have to take it seriously. I think it is something we cannot joke about. Whoever denies this must go to the scientists and ask them. They speak very clearly. The scientists are precise. Man is stupid, a head that does not see.”

Pope Francis was speaking with a bandage over his eye because he bumped his head on the popemobile while traveling in Colombia.