Pope Francis waves to faithful in St. Peter’s square last spring. (Vincenzo Pinto/AFP/Getty Images,)

Climate change skeptics are in Rome this week. The Heartland Institute, a Chicago-based think tank that has often questioned the science behind human-caused climate change, announced that it was sending a delegation to Rome to “inform Pope Francis of the truth about climate science: There is no global warming crisis!” Marc Morano, who runs the climate-skeptic Web site ClimateDepot, is also in attendance.

“The Pope has picked a contentious scientific issue in which — now going on almost two decades of no global warming, sea ice recovering, sea level rise rates stable to even decelerating, on almost every metric from polar bears on down – the global warming narrative has weakened,” says Morano’s Web site.

The reason behind the rhetoric is a Vatican summit on Tuesday, organized by the Pontifical Academy of Sciences and Pontifical Academy of Social Sciences, among others, on the “Moral Dimensions of Climate Change and Sustainable Development.” The event, which features U.N. Secretary General Ban Ki-moon and academic luminaries like Jeffrey Sachs, is widely seen as a lead-up to the release of a momentous and much anticipated papal encyclical on the environment.

[Pope Francis poised to weigh in on climate change with major document]

But close watchers of the Vatican and the climate debate suggest that while those skeptical of human-caused climate change — or resistant to policy steps to cut greenhouse gas emissions — have good reasons to worry about what’s coming from the pope, there’s probably very little they can do to stop it.

Let’s take the latter point first. The “skeptical delegation” (in Morano’s words) can certainly seek to influence Pope Francis, but here’s the thing. The encyclical is probably already written, observes Teresa Berger, a professor at the Yale Divinity School who focuses on Catholic theology.

“The news out of the Vatican in the past few weeks has been that the pope is editing his draft,” says Berger. “Which means he isn’t going to start this week, after having listened to all the folks assembling now for this conference, and write his encyclical. The body of the work is done.”

[2015 the ‘last effective opportunity’ to safely limit warming, says Vatican conference statement]

If the work isn’t expected to come out until summer, Berger adds, that’s mainly because the Vatican has to translate it into a large number of languages so that Catholics worldwide can read it.

As for the impact of the document when it’s released? Here, yes, climate change doubters have a great deal to worry about.

The potential for the pope’s encyclical to strongly influence U.S. Catholics is clear from polling numbers released by the Yale Project on Climate Change Communication in March. The study found that U.S. Catholics were more likely than other U.S. Christians (such as, for example, evangelical Protestants) to believe global warming is happening and caused by humans.

But even though Catholics already are more convinced about global warming than, say, evangelicals, the polling data nonetheless show ample room for movement among them.

For instance, the study found, American Catholics had high regard for Pope Francis — 76 percent had a “positive opinion”  of him — but were largely unaware that he was going to release a papal encyclical about the environment. And only 10 percent of U.S. Catholics were “very worried” about climate change.

Meanwhile, just 36 percent of U.S. Catholics thought addressing climate change would help the world’s poor — a key message that most expect Pope Francis to articulate.

Thus, the doors are wide open for Francis to come in, raise the consciousness of Catholics about climate change, connect it to their values, and thereby increase their levels of worry and activism. Many conservative Catholics will likely resist what the pope has to say, but among the bulk of U.S. Catholics, there is clearly the potential for movement.

And that’s just the beginning of the likely impact, notes Yale’s Berger. “Given the moral authority that this pope seems to have, I think the impact of the encyclical will be huge, not only on the Catholic population, both in the U.S. and beyond, but on people more generally,” she says. “There aren’t many encyclicals that before they were even published have garnered as much attention as this encyclical already has.”

Berger opened a recent Yale panel devoted tothe encyclical, and its impact, including not only religious voices but also secular, scientific ones. One of those, Yale forestry and environmental studies dean Peter Crane, says he’s not a Catholic, but from a scientific and international policy perspective, he expects the pope’s encyclical to be influential indeed.

A lot of that has to do with its timing — coming halfway between the historic U.S.-China climate accord and the Paris climate conference scheduled for December, where the world will aim to achieve new greenhouse gas emissions targets.

“I think it’s a positive development in the whole unfolding process that’s really been going on ever since Copenhagen,” says Crane. The encyclical is will help the world get “lined up for one of these conferences where there will actually be some progress” — namely, Paris.

The most powerful aspect of the encyclical will likely be the way that it moralizes the issue of environmental stewardship — taking it out of the realm of science and making it about people’s duties to others (and to the planet). This is something President Obama himself has tried to do by repeatedly framing the climate issue as being about our obligation to our children and grandchildren — but the pope’s ability to moralize is, naturally, even more powerful.

“If Pope Francis’s previous style is anything to go by, I think he will not mince words,” says Berger. “So I expect there to be something about the pope condemning sins of exploiting the Earth.”

So, for all of these reasons, it does seem that Pope Francis is winding up to make a dramatic wave in the climate debate. Climate skeptics are right about that. It’s just not clear what they can do about it.

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