The U.S. has officially left the Paris Agreement. What happens next?

The withdrawal has been expected for years, but marks “a bitter moment,” experts say.

On Wednesday, November 4, the United States officially withdraws from the Paris Agreement, the landmark international accord to keep climate change in check and limit future greenhouse gas emissions. The exit occurs regardless of the outcome of Tuesday’s vote, but the winner of the presidential election will decide whether to stay out or to rejoin the landmark climate deal.

The U.S. is now the only major country in the world not committed to the accord, which aims to keep global temperatures from rising more than 3.6 degrees Fahrenheit (2 degrees Celsius) above pre-industrial temperatures. It is no surprise—President Trump announced his intention to leave the agreement in 2017 and began proceedings to do so in November 2019. But the move is in keeping with Trump’s agenda to reduce efforts to address climate change in recent years, says Kate Larsen, an analyst at the climate-focused Rhodium Group.

A future president can decide to rejoin the pact at any time, but will have to revise the country’s plans and submit new, more ambitious goals—which may be more difficult to reach after several years of delays, says Andrew Light, a climate expert at the World Resources Institute and an architect of the agreement reached under President Obama. But even without federal support, the U.S. has made progress toward decarbonization, and that forward motion is likely to continue with or without membership in the accord.

“The U.S. being absent from the Paris Agreement hasn’t stopped long-term momentum, but it has slowed it—and put the U.S. at a disadvantage,” Larsen says.

What was the Paris Agreement?

The U.S. is currently the second-highest greenhouse gas emitter in the world. It’s responsible for spewing more than 5 billion metric tons of carbon dioxide a year since 1990, to say nothing of other potent planet-warming gases, such as methane or hydrofluorocarbons.

The country is number one in overall “historical” emissions, however: the source of 25 percent of all human-produced greenhouse gases that have collected in the atmosphere since the Industrial Revolution.

After years of negotiations, signatories to the Paris Agreement decided to try to limit the amount of global warming at the end of the century to less than 2 degrees Celsius and to aim for an even more ambitious target of 2.7 degrees Fahrenheit (1.5 degrees Celsius). Passing these thresholds, mountains of science suggested, would result in catastrophic changes to the climate system that would have dangerous, costly outcomes for billions of people across the planet. Since the Agreement, a steady stream of research has shown that even the lower temperature goal is likely to spur unwelcome effects. Many of those are already apparent, from supercharged tropical cyclones to record-breaking wildfires.

Global temperatures rise fairly predictably in response to rising greenhouse gas concentrations. That means that there is an ultimate limit to how much more carbon we can put into the atmosphere, if the temperature goals are to be met: in orther words, a carbon “budget” to stick to. The agreement didn’t hammer out the exact details of the budget, leaving each country to develop plans to reduce their greenhouse gas emissions over time.

All of them pledged to review the goals every five years, keeping track of where they’d succeeded or failed and hopefully ratcheting up their ambitions. In that first round, the U.S. aimed to get its emissions down 26 to 28 percent below its 2005 levels by 2030. Other major emitters enacted similar goals: China, the current top emitter, said it would reach “peak” emissions by 2030, and the E.U. aimed to be 40 percent below its 1990 emissions by the same time.

The goals weren’t legally binding, but there was great international political pressure to cooperate, says Marina Andrijevic, an analyst at Climate Analytics. Much of that pressure was generated by the commitments from the United States: As the primary driver of emissions, the country’s willingness to participate held great weight.

“The role of the U.S. in this process cannot be overstated,” she said. “In the math of things, the agreement doesn’t work if the U.S. doesn’t do its part.”

So in 2017, when President Trump announced his intention to remove the U.S. from the deal, concerns piled up. Some analysts worried that the withdrawal would incite others to also drop out or dial back their ambitions unless the U.S. rejoined. Others were concerned that global goals simply couldn’t be met without U.S. cooperation.

The looming exit was largely seen as an abdication of the country’s responsibility for its role in the warming that has already occurred. The U.S. had pulled out of the Kyoto climate agreement years earlier.

“The world community has now had this happen two times. We’ve pulled out of a climate agreement, largely designed around us, twice. That will make them wary of negotiating with us again,” Larsen says.

Now, the U.S. has officially become the first and only signatory to leave the pact, joining Iran and Turkey as the only major emitters to eschew the international deal.

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How much ground has been lost?

At the time they were established, the emissions reductions targets the U.S set for itself were considered steep, but not unattainable. Hitting them would have required careful, coordinated efforts at the federal, state, and local levels. But the Trump administration actively disassembled many of the efforts that would have fed into success, Light says.

Most notably, the administration rolled back the Clean Power Plan, a carbon emissions standard for power plants introduced in 2015 that would have decreased greenhouse gas waste from electricity production by more than 30 percent by 2030. That would have accounted for a good part of the country’s Paris goals, since electricity production accounts for about one-third of the country’s emissions.

Similarly, Trump’s administration weakened rules controlling car and truck fuel efficiency. Transportation accounts for the largest fraction of American emissions, slightly under a third of the annual total. Under an Obama-era rule, car and truck efficiency was scheduled to increase to an average of 49.5 miles per gallon by 2030. Under the new rules, that number is 42.3 miles per gallon. The difference adds up: By 2030, an extra 300 million metric tons of CO₂ will be added to the atmosphere, compared to the Obama era rules.

The Obama Administration also developed more stringent rules about controlling methane emissions, which the Trump Administration weakened. Methane is a hyper-potent greenhouse gas, roughly 86 times more effective at trapping heat than carbon dioxide within 20 years after its appearance in the atmosphere. So any goals within that time frame—like the Paris targets—need to take its presence seriously. The Environmental Defense Fund estimated the effect of the rollbacks on emissions as the equivalent of an extra 100 coal-fired power plants.

And other progress on powerful planet-warming gases—the hydrofluorocarbons that had first been effectively internationally regulated by the Montreal Protocol—also stalled in recent years, potentially adding extra heat to the near future and putting the U.S.’s Paris goals further out of reach.

Last year, the U.S. reached a 10 percent decrease in emissions compared to 2005, according to the EPA—far from the Paris goals. This year, the broad effects of the pandemic have already lowered U.S. emissions dramatically, likely making it seem like the country is close to its target. But the costs have been enormous.

“An economic crisis, especially one that is causing such harm to working-class people, is no way to cut emissions,” says Rachel Cleetus, a climate policy expert at the Union of Concerned Scientists. “This is not a viable emissions reduction strategy.”

And, she emphasizes, the overall message of the withdrawal is dark, given how starkly climate change is already demonstrating its destructive power. “This is coming at a time when we have just seen Typhoon Goni slam ashore in the Philippines; we’ve seen Hurricane Eta, the 28th named storm in the Atlantic hurricane season in 2020; we’ve seen Colorado and California have the worst wildfire seasons on record. At a moment like this, to be stepping away from the Paris Agreement is really a bitter moment. It’s a shameful exit,” says Cleetus.

Can lost ground be regained?

The drafters of the Paris Agreement made it hard to leave and relatively easy to join, meaning a U.S. president could announce his or her intention to recommit to it and rejoin within 30 days. The leader would have to pledge stringent new commitments, however, and clearly outline the ways the country can meet them, a taller task now that several more years have passed and many more gigatons of greenhouse gases have collected in the atmosphere.

But “the good news that has emerged over time is that the Paris Agreement is much bigger than the U.S.,” Andrijevic says.

No other countries followed the U.S. out of the agreement. In fact, many others stepped into the leadership void, she says; the E.U., China, Japan, and South Korea have all recently announced ambitious new goals for how quickly they’ll reach net-zero emissions and are well on their way to meeting them. At the same time, the cost of renewable energy sources like solar and wind has plummeted, making them not only competitive but often cheaper than fossil fuel energy sources.

And even within the U.S., cities, states, and companies have quite effectively chipped away at meeting the emissions reductions goals even as the federal government stepped back. Nearly half the U.S. states and many cities, representing over 65 percent of the country’s population, have now set significant reduction targets, and over 4,000 cities, tribes, businesses, and other organizations have committed to maintaining Paris-level goals.

Those commitments add up. A recent analysis suggests that the actions of those in the pro-Paris coalitions could drive a 25 percent reduction in U.S. emissions by 2030—just under the target goal.

“The fact is that the U.S., along with every major emitter, has to get to net zero by 2050, and we do not have an administration that is taking that seriously right now. We have a bunch of non-federal leaders who are,” and that’s what’s saving us right now, Light says.

Even a tiny fraction of the money being pumped into economies in COVID-19 recovery stimulus packages could pay for huge progress toward Paris-level emissions reductions, Andrijevic's recent research has shown.

"In many ways, this is illustrative of the idea that solving climate change isn’t that expensive," she says, with effects that would be tangible within coming decades.