Bishop of Rome, Vicar of Jesus Christ, Protector of the Planet?

Pope Francis comes to the United States to preach for action on climate change.

Pope Francis, Naomi Oreskes
Pope Francis has made climate change a central issue, penning an encyclical on the topic earlier this year, to which historian Naomi Oreskes (right) wrote a foreword.

Photo illustration by Lisa Larson-Walker. Photos by Joe Raedle/Getty Images, Sage Ross/Flickr Creative Commons.

Pope Francis arrives in the United States on Tuesday, and climate change—an issue he’s called “one of the principal challenges facing humanity in our day”—figures to be a central theme in his message to Americans.

Although his time in the United States will have all the pomp and frills of a papal visit—parades, a canonization, lots of Masses—Pope Francis arrives not only as a diplomatic envoy on behalf of the world’s Catholics but also as an advocate for billions of people across the developing world. This is a pope more comfortable riding a public bus than the popemobile, who has eschewed the papal palace in Rome for a small apartment. While in the United States, he’ll meet with homeless people and visit a prison. This pope has prioritized the poor more than any pope in recent memory, which helps explain why climate change is so high on his agenda.

A recent poll showed climate change is the most important issue for the world’s poorest countries, and for good reason: On our current course, we’re locking in several decades of rapid environmental change, and the world’s poor stand to bear the brunt of the damages.

Francis knows this. In fact, he understands the urgency of climate change perhaps better than any other world leader. In his climate encyclical earlier this year, Francis framed climate change as a key human rights issue and called for a radical transformation of global politics and wasteful high-consuming lifestyles. After decades of delay, the scale of action required to meet the internationally agreed-upon target for global warming—no more than two degrees Celsius above pre-industrial levels—has now become nearly infeasible. Francis knows this too. Simply put, we’re entering an especially important moment in world history, approaching the point of no return on one of the most consequential problems humans have ever faced.

That urgency is part of what brings Pope Francis here. World leaders have so far failed to steer the planet away from climate Armageddon, and his visit marks the start of a frantic period of last-minute diplomatic activity before key negotiations in Paris this December. Francis’ call for radical action will be directed squarely at the country that may need to hear it most: the United States.

Despite recent moves on climate change by the Obama administration, America continues to be a major hindrance: The U.S. Congress, which Francis will address on Thursday, has been a blockade to global climate negotiations for years, and our carbon emissions remain among the highest per person of any major country on the planet. America has had a unique role in creating this problem, and is uniquely positioned to help fix it. Francis’ message to America, grounded in climate science, should be a riff on the Bible verse: “From everyone to whom much is given, much shall be required.”

Still, on climate change, Francis seems hopeful. He realizes that despair has the same practical effect as denial, and so he is mobilizing for action. Helping him on this journey are activists and academics who know a lot about how to motivate mass movements.

Earlier this year, Brooklyn-based indie publisher Melville House released the first secular edition of Pope Francis’ groundbreaking encyclical with a deeply moving foreword by Naomi Oreskes, a Harvard University historian who has written extensively on the bridge between science and morality. Oreskes’ most recent book imagines a Chinese historian in the year 2393 pondering the origins of the climate-induced collapse of Western civilization. That book is a parable for the consequences of our current moment, in which “merchants of doubt” (the title of another successful book she co-authored) throw the planet into the trash bin in a fit of self-interest. Oreskes and Francis have the same basic message: The rich are trashing the Earth, and it’s got to stop.

In her foreword to the encyclical, Oreskes compares Francis’ message to Uncle Tom’s Cabin and Silent Spring, books that “called attention to facts that had long been known but upon which people had failed to act.”

Our situation is not an accident—it is the consequence of the way we think and act: we deny the moral dimensions of our decisions and conflate progress with activity. We cannot continue to think and act this way—to disregard both nature and justice—and expect to flourish. It is not only not moral, it is not even rational.

This is the message Pope Francis will bring to America: The world needs Americans to take a dramatically different path, a moral path, and the time is now. Each of us has the ability and the responsibility to put the common good ahead of our own interest every day, and the health of our planet demands it. Echoing Pope Francis, Oreskes writes in the foreword: “It is not a question of people versus the environment and choosing which is more important. It is a question of abandoning the notion of ‘versus’ altogether.”

For all the attention, it’s still an open question how much impact the pope’s visit will actually have. In an email, Oreskes joked that “having written a book about the future, I’ve about exhausted my predictive capacities. I really have no idea how his visit will be received.” American Catholics already generally agree with the pope’s message, but agreeing and working to profoundly change your society are two very different things.

What’s clear is the pope won’t settle for anything less than a complete rethink of our individual relationship with each other and the environment. Throw in the urgency of climate change, and “halfway measures simply delay the inevitable disaster,” as the pope wrote in his encyclical. That’s probably a hard message to handle for Americans, who’ve grown accustomed to a deeply individualistic ethic and a pervasive just-in-time culture. But we’ve come to the point when the stability of the world may depend on deep change in the United States—and this week will be among the first chances to see how we react when one of the world’s moral leaders calls us out.

Read more of Slate’s coverage of Pope Francis in the U.S.