If there were a headquarters for U.S.-based opposition to Pope Francis' climate change plans, it would be Chicago.

That's the home of the Heartland Institute, a libertarian public policy think tank that has organized conferences, churned out podcasts, videos, and white papers, and recently arranged a trip to Rome -- all in an effort to build opposition to the pope's environmental message.

Through its efforts, the free market-oriented group has emerged as the most active and vocal organization taking on the pontiff over an issue that some scientists and pols see as the cause of our time. Al Gore and the United Nations also have previously been in the organization's crosshairs on environmental issues.

Related: Five key questions about the Pope and climate change

Heartland, which was founded in 1984 and has courted controversy in the past, does not make its finances public, but reportedly has been bankrolled in the past by global energy giants like Exxon Mobil, as well as the Koch brothers.

Ahead of the release of the pope's encyclical Thursday, leaders of Heartland told CNNMoney that they reject the pontiff's reported claim that human activities cause global warming, believe that he should stay out of the issue, and charged Francis has been misled by leftist scientists and activists.

"We hope to see the pope's encyclical tempered or modified to reflect evidences that disclaim human activities cause climate change," said H. Sterling Burnett, managing editor of one of heartland's environmental newsletters.

Burnett recently authored an article on Heartland's blog titled, "Francis is out of his element: the pope should focus on fighting proverty, not climate change," in which he says the pope "has been badly misinformed and led astray."

Countering the pope: Last week, Heartland, which has an annual budget of $6 million and about 30 staffers, organized its Tenth International Conference on Climate Change, a two-day seminar in Washington, D.C., focused on repudiating the view that climate change is caused by man. The gathering featured speeches by Senator Jim Inhofe and Britain's Lord Christopher Monckton, both well-known climate change skeptics and critics of Francis' entry into the hotly debated topic.

During his address, Monckton explained what can happen when religious leaders wade into matters of science by citing Pope Urban VIII, who in the 17th century put Galileo under house arrest for his theory that the Earth and other planets revolve around the sun.

"Do not for the second time make the unwise mistake," Monckton warned Pope Francis.

Related: Pope, GOP collide on climate change

Heartland also took its campaign to the pope's doorstep. Members traveled to Rome in April to hold a press conference challenging the Vatican's efforts on climate change, but were unable to get an audience with the pope. The Heartland trip was timed to coincide with the Vatican's own environmental workshop.

The purpose of going to Rome was to discourage the pope from "lending his moral authority to politicized and unscientific climate agenda of the United Nations," according to a statement.

On the media front, Heartland has put together podcasts -- one is titled, "Heartland attacked by name at Vatican climate conference," along with various articles, including one headlined, "Pope's climate plan would leave billions impoverished."

Past controversies: Alleged internal Heartland financial documents were leaked in 2012 by DeSmogBlog, an online group dedicated to debunking what it calls global warming misinformation. The documents allegedly revealed Heartland had received substantial funding from organizations like the Murray Energy Corporation, Marathon Petroleum Company and the General Motors Foundation, which stopped its support in 2012.

Heartland claims the documents are fake and maintains that financial support from the energy groups amounted to less than 5 percent of its budget. (A spokesman for Exxon told The New York Times in 2009 that the company had ended funding Heartland and other groups involved with the climate change issue because of the controversy it generated.)

Heartland also came under heavy fire several years ago over its billboard campaign that featured the Unabomber and the statement that he believed in global warming in what was seen as likening climate scientists to terrorists.

What the critics say: Dr. Michael E. Mann, professor and director at the Earth System Science Center, blasted the Heartland-organized trip to Rome as "a stunt -- an act of desperation, like a little kid throwing a tantrum in public."

"There is no depth the Heartland will not stoop to in their efforts to poison the public discourse of the issue of climate change," Mann told CNNMoney.

And Dr. Jonathan T. Overpeck, director at the University of Arizona's geosciences department, dismissed Heartland's claim that the pope should not involve himself in the climate change isse.

"It's the pope's business," said Overpeck.