President Trump has decided to pull the U.S. out of the Paris Agreement. Here's what you need to know. (Daron Taylor/The Washington Post)

The Paris Agreement is the product of more than two decades of work by the United Nations and agreed to by more than 190 countries — but it was built specifically with the United States in mind.

It is the first climate deal to include all countries, regardless of wealth or development.

Despite the inclusivity of the deal, its final form was negotiated with the United States’ particular legislative system in mind.

This is the reason that the deal is called an “agreement” rather than a “treaty,” for instance. The U.S. interprets a treaty as requiring Senate approval — something that would have been impossible for the Paris Agreement in the Republican-dominated chamber. But an agreement can be approved through an executive action by the president.

The legal shape of the Paris Agreement was also crafted with the U.S. in mind. The various elements of the deal carry diverse levels of legal weight. One particular quirk is that the deal legally obliges countries to prepare climate targets, but not to implement them — another requirement for avoiding Senate ratification.

How did the climate deal all get started?

The process of negotiating the deal began in 1992, when 197 parties signed the United Nations Framework Convention on Climate Change. The treaty contained no targets to reduce emissions, but it outlined the core principles of the international fight against climate change.

Finally, 23 years later, the Paris Agreement was adopted in December 2015, and came into force on Nov. 4, 2016.

Despite the years of tense negotiations leading up to the deal, the Paris Agreement now has the commitment of almost every country in the world. A total of 195 parties have signed the agreement, while 147 have ratified the deal, meaning they formally agree to abide by its rules.

What do countries have to do under the deal?

The deal is a wide-ranging package of actions to reduce emissions and deal with the impacts of climate change, including an overarching goal to limit global temperature rise to 2 degrees Celsius (3.6 degrees Fahrenheit) rise above preindustrial levels, or a more stringent  1.5 degree Celsius (2.7 degrees Fahrenheit) temperature rise, if possible.

To this end, countries have had to submit “nationally determined contributions.” These are nationally designed pledges containing climate targets. The U.S., for example, agreed to cut greenhouse gases by 26 percent to 28 percent below 2005 levels by 2025.

How does the U.S. exit the agreement?

Leaving the Paris Agreement itself is an easy, but lengthy, task.

The deal was specifically designed so that the U.S. could join without the need for congressional approval. On Aug. 29, 2016, President Barack Obama wrote a short letter, which was deposited at the United Nations, signaling that the United States would join the agreement.

A short letter from President Trump will suffice to reverse this action. According to the Paris Agreement, nations that want to withdraw only have to leave a written notice in a U.N. depository.

But Trump won’t be able to write this letter until three years after the Paris Agreement came into force — that is to say, not until Nov. 4, 2019 — thanks to a clause in the deal itself.

It then takes another year before the U.S. can leave the agreement — bringing it to Nov. 4, 2020 — which is also the day after the next presidential election.

With these rules in place, Trump will only see the fulfillment of his campaign promise to leave the Paris agreement either after he’s won a second term as president or been voted out of office.

How could Trump try to speed up the process, if he decides to exit?

Trump could always signal — for instance, by tweet — that the U.S. has no plans to meet its international commitments in the meantime, although such a move would be unprecedented. Trump is also within his rights to weaken the U.S. emissions reductions goals without having to abandon the deal — and the administration has already taken steps to do that by moving to dismantle Obama-era regulations.

Another option would be to declare that Obama overstepped his authority in unilaterally approving the Paris agreement and that it should have been put to a vote in the Senate. Sending it to the Republican-dominated chamber would be a death sentence for U.S. participation in the deal, which would require a two-thirds majority to pass.

But it would also mean opening the legal floodgates on the other agreements enacted in this way. A 2009 study by the University of Michigan found 94 percent of international agreements by modern U.S. presidents are enacted by executive action, rather than Senate approval.

As well as weakening his own executive powers, Trump could also open these other agreements up to legal challenge if he decides that the Paris Agreement requires a Senate majority.

Finally, Trump could pull out of the original Framework Convention treaty altogether. This was approved by the Senate in 1992 under George H.W. Bush, so Trump could find himself in legally murky waters if he decides to withdraw unilaterally. It would, however, be a shorter, one-year process.

David Waskow, director of the World Resources Institute international climate initiative, calls this “the nuclear option.”

The U.S. wouldn’t be the first country to withdraw from an international climate change accord — Canada withdrew from the Kyoto Protocol in 2011.

What leverage do other countries have to try to persuade the U.S. to stay in?

Apart from their efforts to persuade Trump that pulling out of the Paris agreement would be a diplomatic disaster for the U.S., other nations have little power over the U.S. decision.

As a piece of international law, the agreement has few teeth. This is despite the fact that it will hinder other nation’s efforts to achieve the Paris goals. The more ambitious 1.5 Celsius temperature goal, already difficult, begins to look impossible without the input of the U.S., which is the world’s second largest emitter after China.

“There’s no Supreme Court of the world, so there’s a limit to what you can do, and so the enforcement provisions of the Paris agreement are based upon reporting on progress toward their targets, scrutiny by an international body of experts, and peer pressure to play nice,” says Jake Schmidt, director of the international program at the Natural Resources Defense Council.

There has been talk in some nations of a climate-related trade tariff on products coming from the U.S., but this would be outside the scope of the official agreement.

Nonetheless, this idea gained some traction in Mexico as well as France after Trump’s election.

Meanwhile, the chairman of steel giant ArcelorMittal has called for a border tax on carbon in Europe to prevent industry from leaking to less regulated countries.

A group of former Republican statesman argued earlier this year for such a tax as a method to prevent “free-riding.”

How much does this really matter?

While withdrawing from the Paris agreement would be a significant symbolic gesture on the world stage, in practical terms, it is just one more step along the road of unraveling the U.S. climate commitments.

What really matters for the planet is not whether the U.S. remains in the agreement, but whether its emissions rise or fall — and the U.S. was firmly on the path of breaking its commitments regardless.

Trump has already made it clear that he has little intention of hitting the 26 to 28 percent emissions reduction target set under Obama, having already ordered a review of the Clean Power Plan and of any regulations that “burden” domestic energy production, as set out in his Energy Independence Executive Order.

However, it is also worth bearing in mind that the majority of U.S. emissions reductions to date have not been as a result of climate regulations, but because of the switch from coal to natural gas in power plants, or because of the global financial crisis, as some academics have argued.

The U.S. energy mix is not determined by the Paris Agreement, nor solely by Trump’s energy regulations, but by economics and market forces outside the administration’s control.

On the other hand, the Paris agreement is not just about emissions reductions. The deal consists of many elements designed to keep climate change in check and ensure that poor countries are dealt a fair hand.

By shutting itself out of the Paris agreement, the U.S. does not just withdraw from its own emissions reduction commitments, but from global cooperation on a range of important climate issues.

This includes provisions on financial contributions of $100 billion a year by 2020 to developing nations, adaptation to the impacts of climate change, how to deal with losses and damage caused by such impacts, technology and carbon trading.