Au revoir, Paris Agreement. As of Wednesday, under United Nations rules, the United States is officially out of the global climate accord. Here’s a look at how it happened, what it means and what might happen next.

How did we get here?

You could be forgiven for thinking the United States quit the global climate change agreement a long time ago. Ever since 2017, when President Trump announced his intention to abandon the pact, he’s spoken about withdrawal as if it was a done deal. In fact, however, pulling out of the Paris Agreement has been a lengthy process.

On Nov. 4, 2019, the earliest possible day under United Nations rules that a country could begin the final withdrawal process, Secretary of State Mike Pompeo filed paperwork to do so. It automatically finalized a year later. So, as of Wednesday morning, the United States is officially no longer a part of the group of nations pledging to address climate change.

President Trump has called the Paris Agreement “job-killing” and said it would “punish the American people while enriching foreign polluters.”

Technically, though, the Paris Agreement doesn’t require the United States to do anything. In fact, it’s not even a treaty. It’s a nonbinding agreement among nations of all levels of wealth and responsibility for causing climate change to reduce domestic emissions.

The accord essentially ties together every nation’s voluntary emissions pledge in a single forum, with the understanding that countries will set even tougher targets over time over time. The United States under President Barack Obama promised to reduce its emissions about 28 percent below 2005 levels by 2025, but progress on that goal stopped under the Trump administration.

There are some reporting requirements to ensure that countries are making progress, but the Trump administration flouted those and so far has suffered no consequences.

Who’s still in, and what are they doing?

Almost every country in the world. Of the 195 countries that signed the Paris Agreement, 189 went on to formally adopt the accord. Initially Nicaragua and Syria withheld their support from the pact but both eventually joined the agreement.

As of Wednesday, in addition to the United States, the countries that originally signed but have not formally adopted the Paris Agreement are: Angola, Eritrea, Iran, Iraq, South Sudan, Turkey and Yemen.

So far, no other country has followed the United States in renouncing the Paris Agreement. At one point President Jair Bolsonaro of Brazil threatened to do so but he later reversed course.

In recent weeks there have been a spate of ambitious climate commitments from Europe and Asia. The European Parliament voted last month to cut emissions 60 percent by 2030, with the goal of reaching carbon neutrality by 2050. That measure will now be considered by the European Union’s council of ministers. China vowed to become carbon neutral by 2060. That pledge was followed by ones from South Korea and Japan, both of which vowed to zero out net emissions by 2050.

“There’s momentum continuing to build even with the U.S. pulling out,” said Alden Meyer, a director at the Union of Concerned Scientists and a 30-year veteran of international climate negotiations.

“The question is, would it continue without the U.S. fully on board?” he said.

Will U.S. greenhouse gas emissions spike?
Not necessarily. Leaving the Paris Agreement does not in itself mean the United States will stop addressing climate change.

On the other hand, it does mean the federal government has formally abandoned, for now at least, President Obama’s goal of cutting emissions about 28 percent below 2005 levels by 2025.

In reality, the United States under President Trump walked away from that target years ago. Right now, we’re about halfway to the Obama-era goal and not on track to meet it. So, while emissions probably won’t rise, they also won’t fall fast enough to avert the worst effects of climate change.

Is the U.S. withdrawal final?

No. Any future president could opt back in.

Former Vice President Joseph R. Biden Jr. has pledged that he would recommit the United States to the Paris Agreement on Day 1. In practical terms that means on Inauguration Day, Jan. 20, his administration would send a letter to the United Nations notifying it of America’s intention to rejoin. The American return would become official 30 days later.

Other countries would most likely give a Biden administration some time to get on its feet but would also want to see strong early signs that the United States has substantial plans to cut domestic emissions from cars, power plants and other sources.

By the time the United States joins other countries at the next United Nations climate conference, scheduled for Glasgow in November next year, it would be expected to have an emissions-cutting target even more ambitious than the Obama-era one.

If the United States stayed out of the agreement, it could still have a voice in United Nations climate negotiations. That’s because it would still be a member of the United Nations Framework Convention on Climate Change, the body that created the Paris Agreement. America would, however, be reduced to observer status, which means its negotiators would be allowed to attend meetings and work with other countries to shape outcomes but not be allowed to vote on decisions.

“They will still have influence, but nothing like they would as full players,” Mr. Meyer said.