Pope Francis said the devastation caused by the recent hurricanes should prompt world leaders to act now on climate change.
The Pope said: “If we don’t turn back, we will go down.”
Pope Francis strongly backed the 2015 Paris Agreement on reducing global warming, which Mr Trump walked away from this year.
The Pope spoke out as Hurricane Irma has brought strong winds, torrential rains and left millions without power in Florida.
He said: “You can see the effects of climate change and scientists have clearly said what path we have to follow.
“All of us have a responsibility, all of us, small or large, a moral responsibility.
“We have to take it seriously. We can’t joke about it,” he said.
“Each person has their own. Even politicians have their own.”
The Pope said: “If someone is doubtful that this is true, they should ask scientists. They are very clear. These are not opinions made on the fly. They are very clear. Then each person can decide and history will judge the decisions.”
Pope Francis recently urged political leaders to “support the consensus of the world” that climate change is harming the world’s poorest.
He has called on world leaders to “listen to the cry of the Earth and the cry of the poor, who suffer most because of the unbalanced ecology”.
President Trump has declared a major disaster and emergency aid for Florida, describing the hurricane as a “big monster”.
Dozens of people have been killed after Irma raged west across the Caribbean.
After criticism that the UK has been slow to help nationals trapped in the storm’s aftermath, Foreign Secretary Boris Johnson said there had been an “unprecedented” relief effort from the Government.