Pope Francis is not the first pontiff to insist that caring for the environment is a responsibility of every Christian. But he has been the most forceful, mincing no words in his often harsh and anguished calls for urgent action.

Earlier this year, he issued the first-ever papal encyclical on the environment. The document made clear he intends to use the power of the papacy, magnified by his immense popular appeal, to press the world’s political leaders to take forceful steps to reduce greenhouse gases.

(Read "Will the Pope Change the Vatican? Or Will the Vatican Change the Pope?")

He is expected to make a direct appeal on his trip this week to the United States, where he will speak