Gregory Mankiw is a Harvard professor, one of the most influential economists in the world, and a Republican. He served as Chair of the Council of Economic Advisors under President George W. Bush, and advised both the 2008 and 2012 Mitt Romney campaigns for president.

And he thinks the US needs a carbon tax.

In an interview with National Geographic — conducted, incidentally, by actor Leonardo DiCaprio, who stepped into the role of science reporter for "Before the Flood" — Mankiw explained why he believes a carbon tax is the best option for curbing US emissions.

A "carbon tax" is actually a series of taxes that increases the cost of activities like burning coal or buying gasoline that pump carbon dioxide into the atmosphere.

"You want to tax bad activities that have negative effects on other people in society," he said. "We raised the price of cigarettes by putting a tax on cigarettes, people then consume fewer cigarettes."

Carbon dioxide is a key contributor to climate change — accounting for 80.9% of all US greenhouse gas emissions. It gets released into the atmosphere when fossil fuels, wood, and solid wastes burn, and absorbed by plants.

Scientists agree that human activity has caused atmospheric carbon dioxide to spike dramatically in the last half-century to levels not seen on Earth in 15 million years, leading to severe consequences the world over.

(Donald Trump, the chosen presidential nominee of Mankiw's party, does not seem to accept this observable reality.)

"I think trying to appeal to people's social responsibility [to reduce emissions] is really very very very hard, because people have lives and they have lots of things to worry about. They don't want to think about climate change every time they do every decision. They can't," Mankiw said.

"What a carbon tax does is it nudges them in the direction of doing the right thing," he said.

DiCaprio said he was surprised a Republican would support a new tax.

"Well one of the important things to keep in mind is that if you have a carbon tax, you can turn around and cut other taxes in response," Mankiw said. "For example, the payroll tax. So this is a tax shift, rather than a tax hike."

Given that Mankiw has served so prominently in a presidential administration, DiCaprio asked, "how come we don't have a carbon tax already?"

"Well, politicians don't always do what professors want them to do," Mankiw said.

He compared the issue to gay marriage. Obama ran in 2008 opposing gay marriage, but came around once the polls (and Vice President Joe Biden) did.

"We need to preach to the American people," Mankiw said. "Once the American people are convinced, politicians fall in line very quickly."

Watch the documentary "Before the Flood" below.