Demonstrators gather at Lafayette Square on June 1, 2017, after President Trump's announcement that the United States would pull out of the Paris climate agreement. (Salwan Georges/The Washington Post)

The United States became the first and only nation to withdraw from the Paris climate accord on Wednesday, even as the outcome of the presidential race remained unknown.

The nation’s formal exit from the global effort to combat climate change — a departure set in motion by President Trump more than three years ago — marked the only sliver of certainty in a sea of ambiguity about the future trajectory of U.S. climate and environmental policy.

If Trump were to eke out a victory, the U.S. government would all but vanish from international efforts to slow the Earth’s warming, in favor of promoting fossil fuels. Democratic nominee Joe Biden has called climate change “the existential threat to humanity” and vowed to immediately rejoin the Paris accord if elected. But even if he wins the White House, his plan to invest trillions of dollars toward making the United States a greener nation will face a deeply divided Congress.

Either way, Tuesday’s election could result in the one thing scientists say the nation and the planet can no longer afford: More years of U.S. stagnation on climate action.

The uncertain election outcome doesn’t change the scientific reality of rising temperatures: That the burning of fossil fuels and other human activity is warming the Earth at an alarming rate. Since the late 1800s, the world has warmed by 1 degree Celsius (1.8 Fahrenheit).

And the world remains woefully off target in its goal to cut greenhouse gas emissions enough to remain “well below” 2 degrees Celsius (3.6 Fahrenheit), a threshold beyond which scientists say the planet’s ecosystems would suffer irreparable harm.

The United States could miss its own Paris accord commitment to lower carbon emissions 26 percent to 28 percent below 2005 levels by 2025. According to an analysis by the Rhodium Group, the country is on track to cut its emissions between 20 and 27 percent, depending on how quickly the economy recovers from the pandemic. But it would need to cut emissions by 43 percent over the next decade to be on track to reach net zero emissions by 2050 — a goal that the European Union, Japan, South Korea and other nations have set in a push to slow the world’s warming.

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Trump and Biden would undoubtedly lead the country in opposite directions, with the president poised to lock in a reliance on fossil fuel for decades, while Biden aims to make deep cuts in America’s greenhouse gas emissions in the near term.

But even as the electoral map appeared to tilt in Biden’s favor Wednesday, signs pointed toward the GOP retaining control of the Senate. That outcome would dim the prospects that a Biden administration could shepherd a comprehensive climate bill through Congress.

Major pieces of Biden’s $2 trillion plan to usher in a greener future for the nation — including requirements on utilities to use clean energy, incentives for Americans to buy more electric vehicles and a massive injection of money to build out lower-emissions infrastructure — would require the cooperation of both chambers on Capitol Hill.

In particular, any legislation would need the consent of Senate Majority Leader Mitch McConnell if the GOP holds on to the Senate. The Republican from the coal-mining state of Kentucky has a long history of opposition to federal action on climate.

“Given the fact that Biden is probably not going to have a compliant Senate to work with, don’t look for him getting policy done through a reconciliation bill or even an ambitious climate bill,” said Eric Washburn, an ex-aide to two former Senate Democratic leaders, Thomas A. Daschle (S.D.) and Harry M. Reid (Nev.). “So he’s going to have to do what he’s going to do to promote his clean energy and climate agenda administratively.”

Should Biden win the White House, he could use his authority to appoint climate-friendly leaders to head federal agencies and enact a slew of policies, including more stringent limits on oil and gas drilling, carbon pollution from power plants, and federal mileage standards. And he’d probably provide significant support to expanding the nation’s charging network for electric vehicles, which currently make up less than 2 percent of cars and trucks on U.S. roads.

On Wednesday evening, Biden wrote on Twitter that rejoining the accord would be perhaps his very first action in office if he prevails. “Today, the Trump Administration officially left the Paris Climate Agreement. And in exactly 77 days, a Biden Administration will rejoin it.”

Trump, for his part, is planning on doubling down on his record of trying to bolster fossil fuel production in the United States, including expanding drilling on federal lands and waters, speeding the construction of pipelines, and scaling back the federal government’s authority to limit industrial activities that produce greenhouse gas emissions.

Late last month, for example, the Interior Department proposed starting seismic testing in December to search for oil deposits on more than half a million acres of the Arctic National Wildlife Refuge’s coastal plain, which has been off-limits to development for four decades.

“The president’s vision on the need to be energy independent remains a major, major focus,” Interior Secretary David Bernhardt said in an interview.

As uncertainty loomed Wednesday about the future of U.S. domestic and foreign policy, the rest of the world was left to react to the country’s official departure from the Paris accord — a move that Trump had been promising since he took office, saying he considered it an unfair deal for the country.

A joint statement from the leaders of Chile, France, Italy, the United Kingdom and the United Nations expressed “regret” at the U.S. withdrawal. But they insisted that the world would carry on regardless.

“We remain committed to working with all U.S. stakeholders and partners around the world to accelerate climate action, and with all signatories to ensure the full implementation of the Paris agreement,” the group wrote, adding, “There is no greater responsibility than protecting our planet and people from the threat of climate change. The science is clear that we must urgently scale up action and work together to reduce the impacts of global warming and to ensure a greener, more resilient future for us all.”

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The U.S. decision to withdraw, while long expected, also encountered stiff criticism at home.

“A shameful retreat,” Bob Perciasepe, a former top Environmental Protection Agency official and president of the Center for Climate and Energy Solutions, called the move in an email.

“Leaving the Paris agreement means handing over the mantle of international climate leadership to China, Germany, and other countries,” Sen. Edward J. Markey (D-Mass.), a chief supporter of a Green New Deal, said in a statement. “Leaving the Paris agreement means the road we must travel to stop climate catastrophe will see higher temperatures, more superstorms and wildfires, more climate displacement — and we simply do not have the time to be delayed or distracted.”

Not everyone was troubled by the U.S. withdrawal. Myron Ebell, who directs energy and environmental policy for the libertarian Competitive Enterprise Institute, and who lobbied the White House in 2017 to fulfill Trump’s promise to exit, hailed the withdrawal as “the most consequential deregulatory action taken by President Trump."

“Let the celebrations begin,” he said.

Many countries, including those most vulnerable to the effects of climate change, are still pleading for the United States to do more to cut the world’s carbon output. Last week, Maldives Foreign Minister Abdullah Shahid brought up the issue during Secretary of State Mike Pompeo’s visit to the low-lying nation.

“Without the U.S. and the other larger countries, countries like the Maldives will not be able to do much. Our carbon footprint is very small, but we are going to be the first to suffer,” Shahid said, standing beside Pompeo at a news conference. “You can see the islands are very low; the sea is threatening us. It’s an existential threat to us.”

Pompeo assured him that the United States would help but said staying in the 2015 agreement would not make a difference when the carbon emissions from countries such as China continue to rise.

“Everybody seems focused on the Paris climate accords, and yet the countries that have signed on to the Paris climate accords haven’t begun to do what ingenuity has led to from the United States of America,” he said, adding that U.S. greenhouse gas emissions are falling.

Some state and corporate leaders have continued to press ahead with Paris climate targets in the absence of federal leadership. Twenty-three states, along with the District of Columbia, have set economywide greenhouse gas emissions targets, according to the Center for Climate and Energy Solutions. So have some major corporations.

“While the Trump Administration pulled out of the Paris agreement, the American people never supported that decision,” former New York mayor Mike Bloomberg, who has funded the effort to keep the United States on track with its international pledges, said in a statement, “and cities and states and businesses across the country resolved to do their part to stay in.”