As climate change deniers face growing scrutiny and skepticism, a new undercover investigation by the environmental group Greenpeace shines new light on academics-for-hire, who are willing to accept secret payments from fossil fuel companies to sow doubt about global warming.

The sting operation publicized Tuesday involved two Greenpeace UK employees posing as representatives of oil and coal companies, and asking U.S. academics to write papers touting the benefits of rising carbon dioxide levels and the benefits of coal use in developing countries.

Professors from Penn State and Princeton University "agreed to write the reports and said they did not need to disclose the source of the funding," according to reporting by Greenpeace Energydesk, a journalistic arm of the international environmental organization.

Energydesk reporters Lawrence Carter and Maeve McClenaghan continue:

Citing industry-funded documents—including testimony to state hearings and newspaper articles—Professor Frank Clemente of Penn State said: “In none of these cases is the sponsor identified. All my work is published as an independent scholar.”

Leading climate-sceptic academic, Professor William Happer, agreed to write a report for a Middle Eastern oil company on the benefits of CO2 and to allow the firm to keep the source of the funding secret.

Among the exposé's other findings:

• US coal giant Peabody Energy also paid tens of thousands of dollars to an academic who produced coal-friendly research and provided testimony at state and federal climate hearings, the amount of which was never revealed.
• The Donors Trust, an organization that has been described as the "dark money ATM" of the US conservative movement, confirmed in a taped conversation with an undercover reporter that it could anonymously channel money from a fictional Middle Eastern oil and gas company to U.S. climate skeptic organizations.
• Princeton professor William Happer laid out details of an unofficial peer review process run by the Global Warming Policy Foundation, a UK climate skeptic think tank, and said he could ask to put an oil-funded report through a similar review process, after admitting that it would struggle to be published in an academic journal.
• A recent report by the GWPF that had been through the same unofficial peer review process, was promoted as “thoroughly peer-reviewed” by influential columnist Matt Ridley—a senior figure in the organization.

Happer, the Princeton professor, was invited to speak on Tuesday before the U.S. Senate at a 'Data or Dogma' panel organized by GOP presidential candidate Ted Cruz. Greenpeace investigator Jesse Coleman cornered him there to ask about the revelations.

Watch the video below:

Late last month, Happer—who has said "more CO2 would benefit the world"—appeared at a climate skeptic summit in Texas, Energydesk reports. There, he defended CO2 production saying: "Our breath is not that different from a power plant." He went on to say, "If plants could vote, they would vote for coal."

As Carter and McClenaghan point out, the Greenpeace investigation follows recent reports showing fossil fuel companies burying the truth about climate change, while funding spurious research to cast doubt on the scientific consensus and make it "difficult for ordinary Americans to even know who to trust."