With the United States no longer a party to the Paris Agreement as of tomorrow—and with the prospect, depending on the outcome of today’s election, of a quick U.S. reentry—we’d like to offer a quick review of the legal mechanics, the challenges, and the stakes.

What will be the impact of U.S. withdrawal from the Paris Agreement?

It depends on how the long the United States stays out.

Since President Trump announced his intentions to withdraw early in the administration, no other country has given any indication of following suit. However, a more important indicator of the impact of U.S. withdrawal is the level of climate ambition countries are demonstrating. Countries were to re-up their “nationally determined contributions” (NDCs) this year. With the pandemic delaying COP 26 until next year, countries’ announcements are coming slowly. But both China and the European Union recently affirmed they’ll be strengthening their targets.

If the president is reelected—and if the U.S. consequently stays out of Paris and doesn’t significantly ramp up its climate effort—global ambition will likely suffer. With the world’s second largest (and largest historical) emitter retreating once again from the global climate effort, some countries may find it more difficult politically to strengthen their emission reduction pledges and others might use U.S. withdrawal as a convenient excuse not to do so. The longer the United States stays out, the more difficult it will be to move forward.

If Joe Biden is elected, on the other hand, the U.S. withdrawal from Paris will likely be a very brief chapter in the United States’ long in-and-out history of international climate engagement.

How easy would it be for the Biden Administration to rejoin the Paris Agreement?

Procedurally, it would be very easy. If Joe Biden becomes president, he can rejoin the Paris Agreement on Day 1 simply by depositing an instrument of acceptance with the UN Secretary-General. Thirty days later, the United States will again be a party. This is the same way President Obama joined the Paris Agreement in 2016.

As explained in this C2ES issue brief, U.S. participation in the Paris Agreement can be decided solely by the president, without seeking Senate advice and consent, among other reasons, because it elaborates an existing treaty, the UN Framework Convention on Climate Change. If Biden is president, he would have ample authority to join it as an “executive agreement.”

While formally rejoining the agreement is simple, the greater challenge for a Biden Administration would be putting forward a new U.S. NDC that’s widely viewed as ambitious and credible (see below).

What obligations would the United States assume by rejoining the Paris Agreement?

Although the agreement is considered binding under international law, a party’s obligations are largely procedural rather than substantive (another reason the agreement does not require Senate approval).

The United States’ principal obligations would be to maintain an NDC and update it every five years; to report regularly on its emissions and on its progress in implementing and achieving its NDC; and to take part in various review processes. The agreement’s transparency provisions can strengthen confidence that countries are sticking to their promises, while the five-year updating of NDCs keeps pressure on them to progressively strengthen their efforts.

Notably, the Paris Agreement gives parties discretion to determine the nature and stringency of their NDCs and does not mandate that they achieve their NDCs. Countries’ targets, in other words, are not legally binding (another reason Senate approval is not required). 

What will be the challenges of developing a new NDC?

If the U.S. were to rejoin the agreement, it would technically be required to have an NDC in place within 30 days.

Under the Obama Administration, the United States pledged to reduce its greenhouse gas emissions by 26-28 percent from 2005 levels in 2025. In the current round of NDC updating, parties are expected to put forward targets for 2030.

As C2ES senior adviser Sue Biniaz outlines in this excellent blog, the core challenge in fashioning a U.S. NDC will be balancing the need and desire for greater ambition with the need to put forward an NDC that is credible and sustainable over time. The Biden campaign’s climate strategy aims for net zero emissions by 2050, and achieving that goal will require stronger action in the near term. After years of inaction under the Trump presidency, other countries will expect the United States to make up for lost time.

But it would be counter-productive internationally to submit an NDC that the United States cannot realistically achieve. It is important, therefore, that the U.S. NDC be firmly grounded in domestic climate policy. It will take some time, however, for a new Biden administration to undertake consultations (with Congress, domestic stakeholders and the international community) and to develop and enact policies that could support an ambitious, sustainable NDC.

The administration could send a strong signal at the moment of reentry by declaring its commitment to achieving carbon neutrality by 2050, and could promise to formally submit a new NDC as soon as it’s able. (To comply with the agreement’s technical requirement for an NDC, it could in the meantime provide a placeholder or provisional NDC, for instance reinstating the Obama administration’s 2025 target.) Ideally, it would then be in a position to deliver an ambitious, credible NDC in time for the delayed COP 26 in December 2021 in Glasgow.