Pope Francis and Climate Change.

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The leader of the world’s 1.2 billion Roman Catholics not only says the world must try to protect against an environmental catastrophe regardless of its cause, but he is preparing the first ever papal letter to the church’s bishops focused on the environment.

He hopes it will encourage U.N. negotiators meeting in Paris in December to make courageous decisions about protecting the world from global warming, he told reporters traveling with him to the Philippines.“I don't know if (human activity) is the only cause, but mostly, in great part, it is man who has slapped nature in the face," he said in January before visiting with survivors of 2013’s Typhoon Haiyan, one of the strongest tropical cyclones ever recorded, according to federal weather experts. “We have in a sense taken over nature."

Pope Francis is exceptionally well regarded at the end of his first two years as pontiff, a tenure during which he has rejected ostentatious trappings of the Vatican, embraced the poor and criticized a church that he says has become obsessed with abortion, gay marriage and contraception. An NBC/Wall Street Journal poll released in March found that only 6 percent of Americans viewed him negatively, while 55 percent had positive feelings about him.

Now with his papal letter, or encyclical, he is expected to tie together the environment, development and concern for the poor. It will be published over the summer, in time for the U.N. General Assembly’s discussion on sustainable development in September and the Climate Change Conference meeting on reducing global warming in Paris in December.

Some scholars at Catholic universities hope that given Francis' popularity, Americans will pay close attention to what he says in the encyclical and during his visit to the United States in September, when he will address Congress and the United Nations.Adorable Boy Crashes Pope's Speech

that I’ve heard of in the last 10 years,” said Daniel P. Scheid, an assistant professor of theology at Duquesne University, a Catholic school in Pittsburgh.

Catholics might be more open than others to Francis’ message. They are more convinced than other Christians that global warming is happening, are more worried about it, and are more supportive of taking action, Yale University found ina survey. Seventy percent of Catholics, for example, believe the Earth is warming compared to 57 percent of other Christians, according to the survey.

A Vatican official who helped to write the first draft of the encyclical, Cardinal Peter Turkson, said in a recent speech that Francis was “compelled by the scientific evidence for climate change” while acknowledging the debate over its causes.

“What is not contested is that our planet is getting warmer,” Turkson said in a lecture last month at Saint Patrick’s Pontifical University in Maynoor Ireland.

Rather than provide a narrow agenda for greening the church or a comparison of the merits of capitalism versus communism, the encyclical will emphasize caring for people and the environment, Turkson said.

"The threats that arise from global inequality and the destruction of the environment are interrelated; and they are the greatest threats we face as a human family today," Turkson said in what was regarded as a preview of the encyclical.

Some encyclicals have covered purely religious topics, others issues of broader interest such as birth control.

Francis’ predecessors, Popes John Paul II and Benedict XVI, also spoke out about the environment: John Paul II in 1990 called for protecting creation and the poor and Benedict XVI in 2011 urged delegates to climate change negotiations to reach a strong agreement. But by devoting an encyclical to ecology Francis makes the topic a core one for the church, Scheid said.

“Benedict tended to write like an academic, making arguments, which is good, he’s a professor,” he said. “But I think Francis will speak more as a pastor, as someone trying to speak to the heart, not just the mind.”

Jessica Hellman, an associate professor of biological sciences at the University of Notre Dame in Indiana, said she expected the pope to frame the problem as a moral one. Developed countries will likely be called on do what they can to lessen global warming without regard to what others are doing, she said.

“I think that’s a pretty powerful argument and something different from what you hear in the regular political U.N. discourse so I think it could make a difference,” she said.

She and others at the Catholic university have compiled the Notre Dame Global Adaptation Index or ND-GAIN, which measures the vulnerability of each country to climate change and ways they can adjust.

“It’s time to think of climate change not as a scientific issue but as a humanitarian crisis,” she said.

The goal of the conference in Paris is to limit warming to 3.6 degrees Fahrenheit over preindustrial levels. The United States and China, the world’s largest greenhouse gas polluters, have announced that in Paris they would commit to targets for cuts in their countries’ carbon emissions. At the end of last month, the White House described how by relying on President Obama’s executive authority it would decrease greenhouse gas emissions by nearly a third over the next decade.

The plan is opposed by Republican lawmakers in Congress, who often dispute human contribution to climate change and oppose any accord in Paris.

Francis’ popularity has not made him immune to criticism on the issue. In an editorial in December, the publication Investor’s Business Daily, argued that initiatives such as limiting the use of fossil fuels were actually regressive taxes that would hurt the poor.

“The radical climate change agenda he has made peace with would make the poor poorer and income inequality worse,” it wrote.

Patrick J. Michaels, the director of the Center for the Study of Science at the Cato Institute, makes a similar argument, saying the more affluent a society, the more it protects its environment.

“So I would think that the pope should come down on the side of economic development as a way of promoting environmental justice," he said.

The Cato Institute argues that global warming is real but that current technologies cannot accomplish legislative goals for reducing emissions.

Michael points to a recent Gallup poll showing that among noneconomic problems, the environment and pollution rank well below dissatisfaction with government, terrorism and other issues for Americans. Despite respect for Francis, Americans are more likely to challenge statements with policy or political implications with which they disagree, he said.

"What I’m thinking is that he’s going to have to be very careful in the words that he chooses when he is the United States," he said.

Environmentalists counter that action is needed now.

Dan Misleh, the director of the Catholic Climate Covenant — launched in 2006 and intended to help Catholics address climate change — agrees that Americans pay too little attention to environmental problems. But he hopes the pope will spur people to act.

"I think there will be some challenges in the encyclical that will make people stand up, pay attention and through that reflect on their own lives and lifestyles," he said.

Duquesne University will inaugurate an annual conference on the "Integrity of Creation" in September by exploring the topic of climate change. Notre Dame's Mendoza College of Business, meanwhile, will hold a conference on climate investing, also in September, with the goal of developing a set of principles to guide investments.

Christiana Peppard, an assistant professor of theology, science, and ethics in Fordham University's Department of Theology, said she doubted that Francis would convince doubters in the U.S. Congress to change their minds.

"But I think it is good to have people who are respected as moral leaders and powerful ones at that remind powerful politicians that in a globalizing world a lot more is at stake than might be discerned in election cycles or fiscal quarters," she said.(CNBC)