President Trump is expected to announce this afternoon whether the U.S. will be withdrawing from the Paris accord — the historic global agreement, reached in 2015, to set targets for reducing greenhouse gas emissions and limiting the rise in average global temperatures.

Trump is revealing his decision in the Rose Garden at the White House.

During his campaign, Trump vowed to "cancel" U.S. participation in the deal. World leaders and business figures have urged him to reconsider.

Barack Obama used his authority as president to join the Paris accord without a vote in Congress. That means Trump can also remove the U.S. from the accord without a vote. But it will take a while: Under the terms of the agreement, he wouldn't actually be able to withdraw until November 2020.

Leaving the underlying treaty — the United Nations Framework Convention on Climate Change — would be faster and could be completed within a year. But that treaty was U.S. Senate-ratified. Presidents have unilaterally exited Senate-ratified treaties before, but it's rare and controversial.

Urged to stay, urged to leave

A wide chorus of voices are calling for Trump to recommit to the Paris agreement: Other world leaders and hundreds of scientists, of course, but also CEOs of major energy companies and other big U.S. corporations. Even many of Trump's own advisers support the deal, according to The New York Times.

But the accord has a number of detractors. More than 20 Republican senators have called for Trump to leave the deal. Influential Trump advisers, reportedly Steve Bannon and Environmental Protection Agency Administrator Scott Pruitt, also urged him to withdraw.

And then, of course, there's the argument advanced by candidate Trump. On the campaign trail, he criticized the agreement that the U.S. formally signed onto last year. He has said the deal is "unfair" to the U.S., objecting in particular to the requirement that wealthy nations help developing countries build renewable energy sources.

Trump has also signaled, more broadly, that fighting climate change is not a priority for his administration. He has denied the existence of climate change in the past and appointed as the head of the EPA a man who doesn't accept the overwhelming scientific consensus that humans are causing global warming by releasing carbon dioxide into the atmosphere. And he's already overturned several Obama-era efforts to reduce emissions.

A hard-fought diplomatic agreement

The Paris accord was reached in 2015 after lengthy negotiations. The deal relies on voluntary cuts in emissions by all the member nations — nearly 200 of them.

The agreement also, significantly, sets a global target: to keep the rise in the average temperature no higher than 2 degrees Celsius above pre-industrial levels. And it calls for some $100 billion a year in funding, from developed countries toward developing countries, to support green energy sources.

But the accord fell short of what some parties had hoped for. Island nations — which face an existential threat from rising sea waters — had pushed hard for a target of 1.5 degrees Celsius. (Why not even lower? Well, as of 2015, the global average temperature had already risen by 1 degree Celsius, and even with robust efforts to cut emissions, some further increase is essentially inevitable.)

And the agreement relies on voluntary cuts in emissions, which is seen by some critics as a major weakness.

Still, the fact that the world managed to agree on a target was celebrated as a diplomatic achievement, one multiple world leaders have emphasized as crucial to support. After the recent G-7 meetings, Indian Prime Minister Narendra Modi met with German Chancellor Angela Merkel and vocally supported her commitment to action on climate change.

Modi said failing to act on climate change would be "a morally criminal act." Merkel had previously vowed to "convince the doubters" among world leaders that "protecting the climate matters to all of us."

The doubters are in the minority. Only two countries — Syria and Nicaragua — have completely rejected the deal.

Several dozen countries have signed but not fully approved — including Iran, Turkey and, most significantly, Russia, which is a major emitter of greenhouse gases. But three-quarters of the countries on earth have fully committed to the accord.

A pact in name, or in deed?

It's important to note that the Paris accord is only as strong as each nation's actual reduction in emissions. That means leaving the agreement isn't the only way to weaken it: Trump could keep the U.S. as a signatory, but continue to slash the programs that would actually make it possible to reach the target for emissions.

The opposite is also true: U.S. greenhouse gas emissions could continue to go down, at least in the short term, even if Trump withdraws from the accord.

As NPR's Christopher Joyce recently reported, emissions in the U.S. have declined by about 12 percent since 2005.

"The U.S. has successfully bent its greenhouse gas emissions curve," Kate Larsen, of the economics research team Rhodium Group, told Christopher. "And we are going to continue to reduce emissions over the next 10 years, likely regardless of Trump policy."

But while the Paris accord isn't synonymous with U.S. emissions cuts, that doesn't mean Trump's decision on the accord is meaningless.

Economist Marc Hafstead, who's with Resources for the Future, told Christopher that exiting the deal "could potentially have political ramifications — to the extent that our pulling out of the agreement is going to cause other countries to do less."

It would also threaten the $100 billion a year pledged to help developing countries achieve emissions cuts, as Bloomberg has reported.

Meanwhile, many analysts see a U.S. departure from the deal as paving the way for China to take the lead on climate change.

It's not just a question of intangible moral leadership, or even of the potential profits from green energy that would be on the table. The Atlantic reported last year that a U.S. departure would likely result in less-transparent mechanisms for actually enforcing the Paris accord — because Chinese "faulty and unreliable energy statistics" could play a prominent role.

By staying in the deal, the U.S. would keep a spot at the negotiating table — and potential influence over how the agreement is enforced. A vote to remain also would mark a beginning — not an end — of questions about how the Trump administration will affect global climate change efforts.