On 31 May 2017, rumors began to swirl that United States President Donald Trump was considering pulling out of the Paris Agreement, which the U.S. entered in 2015 and was put into effect in 2016. The agreement between 195 countries (144 of which are ratified as of May 2017) aims to mitigate and limit the effects of climate change:

Governments agreed to come together every 5 years to set more ambitious targets as required by science;

as required by science; report to each other and the public on how well they are doing to implement their targets;

to each other and the public on how well they are doing to implement their targets; track progress towards the long-term goal through a robust transparency and accountability system.

Perhaps the most pointed reaction to news that President Trump was considering pulling out of the agreement came from Jean-Claude Juncker, the president of the European Union, who on 31 May 2017 said:

That’s not how it works. The Americans can’t just leave the climate protection agreement. Mr. Trump believes that because he doesn’t get close enough to the dossiers to fully understand them. It would take three to four years after the agreement came into force in November 2016 to leave the agreement. […]. The law is the law and it must be obeyed.

Juncker’s words bring to light the lack of legal clarity surrounding what leaving the Paris Agreement entails and what, from a legal standpoint, President Trump is actually able to do both internationally and without the approval of Congress.

To understand these issues, it is first necessary to understand what makes the Paris Agreement unique. As discussed in a New York Times article from December 2015, the agreement itself did not require the authorization of the Senate, and was instead passed through an Obama executive order:

Countries will be required to reconvene every five years, starting in 2020, with updated plans that would tighten their emissions cuts. Countries will also be legally required to reconvene every five years starting in 2023 to publicly report on how they are doing in cutting emissions compared to their plans. They will be legally required to monitor and report on their emissions levels and reductions, using a universal accounting system. That hybrid legal structure was explicitly designed in response to the political reality in the United States. A deal that would have assigned legal requirements for countries to cut emissions at specific levels would need to go before the United States Senate for ratification. That language would have been dead on arrival in the Republican-controlled Senate […]. So the individual countries’ plans are voluntary, but the legal requirements that they publicly monitor, verify and report what they are doing, as well as publicly put forth updated plans, are designed to create a “name-and-shame” system of global peer pressure, in hopes that countries will not want to be seen as international laggards.

The authority for the legally binding aspects of the agreement actually comes from a treaty that the United States Senate ratified in the 1990s: The United Nations Framework Convention on Climate Change (UNFCCC), as explained by the Center for Climate and Energy Solutions:

The UNFCCC, adopted in 1992, is a treaty among governments that provides a foundation for the global climate effort. Enjoying near-universal membership, the convention was ratified by the United States with the advice and consent of the Senate. The convention set a long-term objective (avoiding “dangerous human interference with the climate system”), established principles to guide the global effort, and committed all countries to “mitigate” climate change by reducing or avoiding greenhouse gas emissions. The Paris Agreement defines how countries will implement their UNFCCC commitments after 2020.

Holly Doremus, Co-Director of the Center for Law, Energy and the Environment at the Berkeley School of Law, told us via email that the earliest the U.S. could formally withdraw is November 2020, if they decided to remain in the UNFCCC:

It’s not as easy to formally withdraw from the Paris Agreement as some of the coverage makes it sound. Article 28 of the Paris Agreement allows any party to notice its withdrawal three years from the date on which the Agreement entered into force for that party [November 4, 2016]. That means the earliest date on which the U.S. can formally notice its withdrawal would be November 4, 2019. Withdrawal becomes effective no sooner than one year later. So the US can’t withdraw from Paris alone until November 2020.

According to Axios, who on Wednesday first reported that the Trump administration was considering pulling out of the agreement, a “small team” is working out the details of how exactly to initiate leaving the deal and whether they plan on leaving only the Paris Agreement or the entire UNFCCC treaty:

Details on how the withdrawal will be executed are being worked out by a small team including EPA Administrator Scott Pruitt. They’re deciding on whether to initiate a full, formal withdrawal — which could take 3 years — or exit the underlying United Nations climate change treaty, which would be faster but more extreme.

The latter, more extreme option could theoretically bring the United States out of the Agreement (and the entire United Nations Climate Program in general) in only a year’s time, but where the ultimate authority to pull out of that treaty lies is a legally complicated issue. An increasingly relevant October 2016 note from the Center for Climate and Energy Solutions (titled “Could a Future President Reverse U.S. Approval of the Paris Agreement?”) touches on this issue:

There is some question whether the president may withdraw from the UNFCCC without Senate approval as a matter of U.S. constitutional law. Unlike the Paris Agreement, which President Obama accepted under his executive authority, the UNFCCC was ratified by President George H.W. Bush after receiving the Senate’s consent […]. Consequently, there is a plausible argument that the president may not withdraw from the UNFCCC without the Senate’s consent, for the same reason that the president may not unilaterally rescind a law enacted by Congress: Namely, termination of a law requires action by the same institutional actors that adopted the law—in this case, the president acting in conjunction with two-thirds of the Senate. In practice, however, it is very unlikely that the legality of a decision by the president to withdraw from the UNFCCC could be successfully challenged.

Doremus concurs with this assessment, but both she and the above note reference a failed attempt from senators to block Jimmy Carter’s withdrawal of the United States from a mutual defense treaty with Taiwan in 1979. In that case, Goldwater v. Carter, the Supreme Court refused to hear arguments over the legality for Carter to make that move — necessitated by the decision to open diplomatic relations with the People’s Republic of China — primarily due to the political nature of the case.

In response to this decision, a 1980 piece in the Yale Journal of International Law argued that “both the Executive and the Congress have been left with no legal precedent to guide and inform them as to the most intelligent way to terminate future treaties.” This lack of legal clarity remains essentially unchanged to this day.

Ann Carlson, Professor of Environmental Law at UCLA, told us via e-mail that if the United States stays in the UNFCCC treaty while pulling out of the Paris Agreement, they would still be bound by the obligations under the treaty:

If Trump doesn’t withdraw the U.S. from the 1992 UNFCCC, we remain a party to it and would still have to meet our obligations under the treaty, including providing data about our greenhouse gas emissions.

A simpler option, Carlson told us, would involve the Trump administration simply ignoring the obligations of the treaty without making any formal notice. She told us:

There are no real ramifications for doing so.

In response to news reports that the U.S. was considering pulling out of the agreement, Trump tweeted: