Rex Tillerson oversaw a major shift on climate change as chairman and CEO of Exxon Mobil Corp. as the nation’s largest oil company accepted the scientific consensus that humans are contributing to a warming planet.

Tillerson, whom President-elect Donald Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE this week tapped to be his secretary of State, took charge as Exxon Mobil's top executive in 2006.

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Just a year later, Exxon shifted from its public position of doubting climate change to declaring that there is “no question” that human activity was the source of carbon dioxide emissions contributing to the phenomenon.

“Before and after Rex Tillerson, Exxon had a very different profile, as a company, in the issues related to climate change, and that’s worth noting,” said Sam Adams, the United States director for the World Resources Institute, a nonpartisan group that advocates for international climate action.

Under Tillerson’s predecessor, CEO Lee Raymond, Exxon fought the Kyoto Protocol on global warming and other climate policies, frequently framing climate science as shaky at best.

After Tillerson took over, the company backed a tax on carbon dioxide emissions, implemented an internal accounting measure to put a fee on carbon emissions and stopped funding many groups that outright reject the scientific consensus behind climate change, all major shifts away from its previous positions.

Exxon endorsed last year’s agreement in Paris on global warming.

“The appropriate debate isn’t on whether climate is changing, but rather should be on what we should be doing about it,” then-spokesman Kenneth Cohen said in February 2007, according to Greenwire, in a statement that underlined the company’s shift.

Exxon spokesman Alan Jeffers said the company's evolution aligned with a landmark report from the Intergovernmental Panel on Climate Change, which concluded that “most” of the 20th century's global warming is “very likely” due to the increase in greenhouse gases from human activity.

“Our views have really followed the science,” he said.

Tillerson nonetheless disagrees with environmentalists’ views that fighting climate change means using less fossil fuels. Instead, Exxon looks at solutions like replacing coal with natural gas — which the company produces — and adapting to the effects of a changing world.

“Our plan B has always been grounded in our beliefs around the continued evolution of technology and engineered solutions to address and react to whatever the climate system and its outcomes present to us, whether that be in the form of rises in sea level, which we think you can address through different engineering accommodations along coastal areas, to changing agricultural production due to changes in weather patterns that may or may not be induced by climate change,” he told investors last year.

Exxon Mobil’s stated positions on climate change set it apart from other major United States-based fossil fuel companies and closer to the opinions of big international oil companies like BP, Royal Dutch Shell and Statoil.

Many of the countries in which Tillerson and Exxon Mobil did business have populations that were keen to tackle climate change. A more pro-green outlook was good business for the company.

It’s also unclear whether the positions taken by Exxon Mobil match Tillerson’s personal views.

They could be at odds with the views of Trump, who has repeatedly said climate change is a hoax. He wants to pull the U.S. out of the Paris agreement as quickly as possible and has forcefully said that a carbon tax is completely out of the question.

Environmentalists don’t buy Exxon Mobil’s shift, and as such don’t appear to believe Tillerson will have a moderating effect on his boss.

Timmons Roberts, an environmental studies professor at Brown University, said that those favoring actions on climate change should not get their hopes up about Tillerson.

“A lot of the hopes that Tillerson would be a moderating force are probably wishful thinking,” he said.

“Tillerson has said good things recently, that it’s real, that something needs to be done about it, that a carbon tax is a good way to address it. But then you look at the actions of Exxon over these years, and they’re extremely worrisome.”

He and other environmentalists say that Exxon Mobil, like its competitors, continued to expand production of fossil fuels under Tillerson. The company still donates millions to politicians fighting climate change policies.

Exxon also has been embroiled over the last year in controversy over its past climate positions.

InsideClimate News and the Los Angeles Times published reports last year concluding that as early as the 1970s, Exxon Mobil’s scientists knew that greenhouse gases from oil and natural gas caused climate change.

The reports said that despite that knowledge, Exxon Mobil for years tried to sow doubt about global warming, fearing that policies to fight it would hurt its bottom line.

The accusations have spurred condemnation from environmentalists and Democrats and investigations by the Democratic attorneys general of New York and Massachusetts. Exxon Mobil has denied the allegations, saying its positions have always aligned with prevailing science.

“Rex Tillerson hid climate science so it could cash in on disaster, instead of transitioning his company to a position of true leadership,” said Annie Leonard, executive director of Greenpeace.

She criticized Trump’s decision to make the nomination.

“This appointment is a desperate grab for power by a failing industry that is perfectly fine bringing the American people down with it.”

As secretary of State, Tillerson would be responsible for the country’s climate change diplomacy, something that is worrying green groups.

“Handing over U.S. global policy to Big Oil is an epic mistake,” Rhea Suh, president of the Natural Resources Defense Council, said in a statement. “This industry has been near the center of more conflict than any other in modern time; tapping its chief oilman as the nation’s top diplomat sends the wrong message at home and abroad.”

Tillerson's supporters, including the oil industry, usually cite his Exxon Mobil leadership as a reason that he would be an effective diplomat.

“Rex Tillerson is world class. He has decades of experience working with global leaders and overseeing the creation of thousands of jobs,” said Jack Gerard, president of the American Petroleum Institute. “He understands that American voters want to strengthen our national security, grow jobs, and protect American interests globally.”