Speaking to a crowd of cheering supporters in the White House Rose Garden yesterday, the president of the United States, Donald Trump, announced he would take the country out of the United Nations Paris Agreement on climate change.

The political optics were clear. "Our withdrawal from the agreement represents a reassertion of America's sovereignty," the president said.

He was followed by Scott Pruitt, an avid opponent of international climate action, appointed by Trump to the lead the US Environmental Protection Agency. "By exiting, you're fulfilling yet one more campaign promise to the American people," he told the president.

The domestic political motivation is obvious: Trump has not been able to deliver on any of his signature campaign pledges so far, and this was low-hanging fruit.

What is less clear is how the withdrawal will actually work. On this point, there are number of unresolved questions.

What can Trump renegotiate?

In his speech, Trump stated he would "work with Democratic leaders to either negotiate our way back into Paris, under the terms that are fair to the United States and its workers, or to negotiate a new deal that protects our country and its taxpayers."

That was only one small part of the speech, and the president did not elaborate on what exactly he would want renegotiated.

The suggestion was immediately rejected by world leaders and the UN. The leaders of France, Germany and Italy issued a joint statement: "We deem the momentum generated in Paris in December 2015 irreversible, and we firmly believe that the Paris agreement cannot be renegotiated." The European Union and China released a similar declaration today.

Even if a renegotiation were possible, it is unclear what Trump wants changed - given that the emissions reduction commitments in the treaty are already nonbinding.

"It's merely hand-waving - this line was probably thrown in at the last minute," Tom Burke, chairman at climate think-tank E3G, told DW. "How can you renegotiate a deal that you've said you want to leave?" he added.

"I suspect this was more about managing the headlines, so he could keep his campaign promise but still try to look reasonable."

How will the US leave?

Curiously, Trump appears to have opted for a formal procedure for withdrawing from the agreement, even though he had several other options that could have been more immediate.

Barack Obama did not deem the agreement to be a "treaty," and therefore did not submit it to the Republican-controlled US Congress for approval.

Trump could have changed this designation, and put it to a vote - where the Republican-controlled US Senate would have rejected it, annulling Obama's signature. This option would have given more of a sense of a mandate.

Trump also could have pulled the US out of the UNFCCC, the framework administering the agreement. But he chose to keep the US as a UNFCCC member.

Yet another option would have been to stay in the treaty, and just not do anything about it. But this could have opened up the administration to legal challenges.

The more cumbersome route Trump chose in the end will mean a withdrawal process of four years from after treaty ratification, which happened on November 4, 2016. That would mean the US will have withdrawn by just before the presidential vote in 2020.

How difficult would it be for the US to get back in?

Because of the long withdrawal process, the US may not have actually left before the next US election in 2020. In fact, it looks like the deadline for withdrawal will fall one day after the 2020 election.

"The next presidential election may be in part a plebiscite on this issue - should the US stay in or out?" Sam Adams, director of the World Resources Institute in the US and a former mayor of Portland, Oregon, told DW.

Even though the deadline will likely fall in the period between the election and the inauguration of a new president, Adams believes the UN could find a way of refusing to formalize the withdrawal until after the new president takes office.

"If the next president is elected clearly supporting staying in the agreement, even though they will not have been sworn in, that seems like a clear signal to the rest of the world."

If the withdrawal is completed, Adams says it would be "difficult, but not impossible" for the US to rejoin in the future, because it would need the assent of all countries involved. "The situation of a country who signed, left, and wants to rejoin is different than a country like Nicaragua or Syria, which didn't join in the first place," he says.

"This is a scenario being discussed fervently in the halls of the UN. But as with all processes, the UN has a level of discretion" to bend the rules, he says.

Can the US act as a spoiler?

Many of the rules in the agreement still need to be set over the coming years, and during this period the United States will technically still be a signatory while it is withdrawing. This has led to concern that the US delegation will try to sabotage the process, in order to make the agreement collapse and work on a new deal, one which Trump might view as more favorable to the US.

But Burke believes the Americans' ability to do this is limited, because they can't veto decisions. "Yes, during the withdrawal process the US will be there - but nobody's going to be paying attention to them," he said. "Nobody will feel like they need to make concessions to the US," he added.

Although decisions in the framework are taken by consensus, that doesn't mean that any single country can scuttle the whole thing. In Paris, "Nicaragua protested, but the chairman was able to read the mood in the room and gavel it through," Burke explained. 

Nicaragua and Syria are the only two countries in the world that did not sign the agreement - the former because it does not believe the deal goes far enough to stop climate change, and the latter because of its civil war.

"It would be very difficult for America to really obstruct things - but they could slow things down by being very bureaucratic," he added.

Will other countries need to fill the US gap(s)?

The US departure will leave two shortfalls for the agreement: in emissions reduction pledges and in climate finance.

Barack Obama pledged the United States would contribute $3 billion (2.7 billion euros) to the Green Climate Fund by 2020, out of a total amount of $10 billion from developed countries.

Developed countries, the reasoning goes, industrialized earlier and therefore have a larger historical responsibility for climate change. The funding will be used to help developing countries develop renewable energy economies and adapt to the impacts of climate change.

The US has already put $1 billion into the fund. Now Trump has said America will not give a penny more. This leaves a $2 billion hole, and the delegates will have to decide in the coming year whether to increase everyone else's contributions to fill it, or to reduce the total fund amount.

On emissions reduction pledges, the situation is less clear. Since pledges are voluntary, there is no official total number that needs to be reached.

But without the United States, the framework will be further from its goal of preventing a global temperatures rise of 2 degrees Celsius by 2050 - the level scientists say could cause catastrophic climate change.

Burke says countries are expected to ratchet up their pledges in the coming years anyway, and will probably still do so. "Our expectation was that most countries will outperform their [pledges], and I think there's still a reasonable chance that the US will hit their 2025 target anyway."

But make no mistake: this is a setback, he says. This will make ratcheting up the commitments more of a challenge. "Everyone understands that Paris didn't go far enough, but it put us on the right road to go further faster in the future."