This story is part of "America in Transition," a weekly series of in-depth reports on key parts of Donald Trump's legacy, Joe Biden's plans for change and what's at stake for all Americans. Airing Tuesday nights on ABC News Live Prime 7 p.m. ET.
As President Donald Trump prepares to leave office, U.S. greenhouse gas emissions are plunging to levels not seen in decades.
"Our regulations are meaningful. We're seeing reductions in greenhouse gases. And, you know, should we go further? We're going to the extent that the law allows us to go with the agency," said outgoing Environmental Protection Agency administrator Andrew Wheeler in an interview with ABC News Live on his agency's four-year track record.
The U.S. is projected to generate 5.9 billion metric tons of emissions in 2020 -- down more than 9% from 2019 and about the same level as in 1983, according to a BloombergNEF study.
Climatologists and environmental advocates say the rosy headline masks a sobering reality: the decline in emissions isn't happening nearly fast enough to stave off catastrophic impacts of a rapidly warming planet.
"In 2020, U.S. greenhouse gas emissions will be substantially lower than they were in 2019, but that's not because of progress that the Trump administration has made in reducing emissions," said Trevor Houser, an energy and climate analyst with the Rhodium Group, a nonpartisan research organization. "That's because we had the largest economic recession in a generation. So that's not exactly cause for celebration."
Last year, U.S. carbon pollution slid to the lowest level since 1992, according to EPA.
"We are certainly doing a lot here in the United States, but you know a lot of the problem with the greenhouse gases is it's a worldwide issue," said Wheeler. "We have problems with China in particular and emissions coming from China and other countries."
China is the world's biggest polluter, but the U.S. ranks second. And experts say the rate of decline in U.S. carbon emissions has slowed dramatically.
"Let's be clear. We need to bring carbon emissions down to net zero within a couple of decades, and we need to bring them halfway there within the next decade," said Michael Mann, Penn State University climatologist and geophysicist and author of "The New Climate War." "That requires massive action on our part and on the part of every other country in the world."
In the United States, the impacts of a warming global climate are becoming more apparent. 2020 was the worst fire season the West has ever seen, with more than 5 million acres burned. The Atlantic and Gulf coasts saw a record-breaking 30 named storms.
Scientists say the severe weather events are taking an increasingly destructive toll, in part due to rising temperatures. This year is one of the hottest years on record.
"COVID-19 is a big problem for the country; the economic situation. But the climate really is right up there beyond any lip service given," said Aji Piper, 20, the Washington state climate activist who is one of nearly two dozen young Americans who have been suing the federal government since 2015 for contributing to climate change.
After four years of Trump's hands-off approach to climate, Piper said the harms to his family are only getting worse.
"My mom has some pretty serious lung issues. And so wildfire smoke and the increased wildfire seasons just put her health at risk in jeopardy, which is personally distressing for me," Piper said. "You don't want to see your parents suffering like that."
Mann said that while declining U.S. carbon emissions are unquestionably good for the environment, the recent trend is eclipsed by "four years of tremendous damage" to climate policy under Trump.
The Trump administration has rolled back major carbon controls, including rules on power plant emissions and methane leaks from oil and gas wells.
The five biggest Trump rule changes combined could add 1.8 billion metric tons of greenhouse gases by 2035 if they're not reversed, according to an analysis by Rhodium Group. That's more than the combined emissions of Germany, Britain and Canada in one year.
"Those rollbacks combined have already and will continue to reflect in place significantly increase the amount of greenhouse gases in the atmosphere further increasing global temperatures and further raising global sea levels," said Houser.
"I've always known that as an issue, and it's a problem," said Wheeler of climate change. "You know, taking a look at, for example, some of the modeling, I still think there's some real serious concerns on the modeling on what the impacts should be and when they might hurt."
President-elect Joe Biden insists the science is clear and that absent "urgent action," the impacts of climate change will become more catastrophic.
Former Secretary of State John Kerry will serve as special climate envoy; former EPA administrator Gina McCarthy will become domestic climate czar; and if confirmed by the Senate, North Carolina environmental regulator Michael Regan will succeed Wheeler in leading the EPA.
"It's clear with the people that he's appointing is sort of a planned agency approach to climate action. It's no longer sort of bottled up in EPA and Department of Energy, but really across the administration," Mann said.
Biden plans to push major new investments in renewable energy; ask Congress to set targets to reach net-zero emissions by 2050; and restore more stringent pollution caps, including tighter fuel efficiency standards for new cars and trucks.
"The good news is that since the election, I'm hearing from lots of the professionals at EPA and they're eager to get going. They're eager to right the wrongs and think they can do it probably much faster than I would have originally thought," said Carol Browner, who led the EPA during all eight years of the Clinton administration.
Browner credited more than two dozen states with pursuing clean energy standards despite a lack of initiative on the federal level over the past four years.
"What you're seeing is elected officials being responsive to their citizens," Browner said.
California has led the way, requiring all vehicles sold in the state to get more than 51 miles per gallon by 2026. The rule defies Trump, who set a national standard of just 40 miles per gallon.
The big U.S. automakers are preparing for a big change. "They know that we're moving to this lower-carbon future," said Kristin Dziczek, the vice president of the Center for Automotive Research. "It will certainly mean greater availability of electric cars."
"Biden is seen as somebody who's very pragmatic about these future goals around the environment and climate change and the need to protect American jobs and grow the economy here," Dziczek said.
Biden has said his plan would create 10 million new jobs in clean energy. The pandemic has hit the industry hard -- half a million jobs have been eliminated this year so far.
Wheeler said he believes Biden's crackdown on fossil fuels will hurt employment.
"If we, through government actions to address climate change, force those jobs to close, those jobs are still going to occur, they're going to happen, it's going to happen in China or India or other countries, instead of here," he said.
Browner disputed that assessment, saying her record during the Clinton administration is proof that a government-backed transition to renewable energy is a "job maker."
"It's been proven so many times over," Browner said. "We have been successful in making investments in technologies to reduce dangerous pollution. It is not a job killer."
One open question is how aggressive Biden will be in pursuing his climate agenda, and whether a narrowly divided Congress will go along.
"I'm not for the green new deal. I am the Democratic Party," Biden declared during the 2020 campaign.
The debate is brewing among Democrats, as young activists like Aji Piper promise to keep the pressure on.
"We haven't seen any real strong action beyond the incremental steps that we have taken. And while those incremental steps are good, they're still just not enough," Piper said. "It feels like going for the long haul, like doing the small things that eventually will affect the large change that we need."
"It's not too late now," said climatologist Mann, "to take the actions necessary to keep warming below a truly catastrophic level."