"This isn't just a matter of nobody else in the church thinking of doing anything like this and then here comes the pope with some wild statements about the environment," said William Dinges, a professor of religion and culture at the Catholic University of America.
More than two decades ago, Pope John Paul II declared that the world's ecological crisis had been partly caused by the burning of fossil fuels in a speech to mark the World Day of Peace. The pope blamed "political obstacles" for a lack of significant action to fix the problem.
"The effects of ecological problems transcend the borders of individual states, hence their solution cannot be found solely on the national level," John Paul said, adding: "political obstacles, forms of exaggerated nationalism, and economic interests—to mention only a few factors—impede international cooperation and long-term effective action."
The Pontifical Academy of Sciences, the scientific arm of the Vatican, called on "all people and nations to recognize the serious and potentially reversible impacts of global warming caused by the anthropogenic emissions of greenhouse gases and other pollutants" in a 2011 report.
Francis's encyclical has the potential to shake up the debate over global warming, but there are no guarantees.
Francis enjoys a high degree of popularity among American Catholics and the general public. The pope wants the encyclical to set the stage for a strong global climate deal when diplomats descend on Paris for United Nations climate talks this year. Francis also will deliver a speech to Congress in September and speak with President Obama, a meeting where they are expected to discuss the threat of climate change.
But skeptics aren't waiting for the pope to speak.
Rick Santorum, a Republican presidential contender and devout Catholic, has urged Pope Francis to leave "science to the scientists." The Heartland Institute, a conservative think tank that refutes the reality of global warming, even sent a delegation of scientists to the Vatican in an attempt to persuade Francis to stand down.
Still, skeptics won't stop the Vatican.
The Vatican has long pointed out that the world's poor will suffer the most as a result of global warming, a key reason for the Catholic Church's call for action to halt rising temperatures.
"The fact that some States, power groups, and companies hoard non-renewable energy resources represents a grave obstacle to development in poor countries," Benedict wrote in an encyclical, adding: "Those countries lack the economic means either to gain access to existing sources of non-renewable energy or to finance research into new alternatives."
Francis is expected to amplify the Vatican's moral call for climate action.
"At the end of the day, he's commenting on this not because he's a scientist or a politician. He's commenting on this because climate change affects all human beings and that makes it a moral issue," Thomas Wenski, the archbishop of Miami, said in an interview.