On Tuesday, the New York representative-elect Alexandria Ocasio-Cortez joined a protest at the office of Nancy Pelosi, the House Minority Leader, in Washington, demanding that Democratic leaders take steps to draft a “Green New Deal” that would fully transition the United States away from fossil-fuel energy within a few decades. That project, though daunting, is one that climate experts say governments must take on to avoid the most destabilizing and existentially threatening impacts of climate change.

One of those experts is Dan Lashof, the director of the World Resources Institute, a global environmental-research organization. Like others, Lashof warns that federal policymakers are running out of time to craft a national strategy for dismantling the fossil-fuel economy. But he notes that certain states have set ambitious targets for reducing emissions on their own. “The time frame, I think, is captured well in the law that’s on the books in Hawaii and an executive order in California, which calls for zero net emissions by 2045,” he told me.

Transportation accounts for the largest share of carbon emissions, and in recent years several countries have set timelines for banning cars that use fossil fuels. Sales of gasoline-burning vehicles will be banned in France and most of the U.K. by 2040. The U.S. will eventually have to set its own target date.

“I think it is technically realistic—politically, it’s a different question—to say that by 2030, all new vehicles would have to be zero-emission vehicles,” Lashof said. “We know how to do that for passenger vehicles. Trucks are a bit harder, although I think we’ll see a lot of advances there with more rapid charging batteries. There may be a case for fuel cells or other solutions—renewable diesel produced from renewable electricity.”

Transportation is one area where the federal government and state governments have leaned on mandates—such as the E.P.A.’s renewable-fuel standard—to push along the adoption of cleaner technologies. As the window for large-scale action closes, direct government intervention in other parts of the carbon economy will become more urgent. “Some economists might say, just put a carbon tax in place and everything will take care of itself,” Lashof said. “I think there’s a lot of reasons why that’s not true. At the same time, I think allowing people to continue to emit carbon pollution into the atmosphere for free is a terrible idea.”

(In this week’s issue of the magazine, Bill McKibben, a founder of the grassroots climate campaign 350.org, notes that, after Exxon announced its support for a carbon tax of forty dollars a ton, representatives of the U.N.’s Intergovernmental Panel on Climate Change “laughed out loud at the idea that such a tax would, this late in the game, have sufficient impact.”)

There is growing consensus that some form of large-scale carbon removal will be necessary. (Experiments in carbon removal are, Elizabeth Kolbert wrote last year, “either the ultimate insurance policy or the ultimate moral hazard.”) “The most well-developed and technically simple but challenging way to accomplish this as a policy matter is trees,” Lashof said. “Trees take carbon dioxide out of the atmosphere. The more trees, the more carbon stored in their trunks. And there’s also things you could do to improve sequestration carbon in soils, which can, if you do it right, also improve agricultural productivity.

“The other part that is right now very expensive, but is very early stage in terms of development, is so-called direct air capture,” he continued. “This is equipment that would have to be run by a zero-emitting electricity source to achieve its goals, but that’s doable.”

Of course, in Washington, politicians, not climatologists, will assess the viability of the available approaches to the climate problem. Still, Lashof sees hope in both a new generation of climate activists and the results of the midterm elections.

“Look at some of the new members of Congress who ran for office specifically to address climate change,” he said. “People like Mike Levin, in California, who replaced Darrell Issa—that’s a huge upgrade. And there are dozens of examples like that.” The midterms saw the ouster of several climate-change skeptics, including Dana Rohrabacher, John Culberson, and Rod Blum. “I think you’re seeing people, particularly young people, really forcing the issue by saying, ‘Not only is this a good thing to do, this is something that we’ve got to do and it’s urgent.’ ”