President Trump announced on Thursday, June 1, 2017 that he will pull the U.S. out of the Paris Agreement. This decision is hugely controversial. Indeed, even Exxon Mobil and ConocoPhillips, the world’s two largest oil producers, opposed withdrawal. Moreover, nearly every country in the world (including North Korea) has signed onto the Agreement. By withdrawing, the U.S. has become one of only three countries to abstain from participation, joining ranks with Nicaragua and Syria.

President Trump Will Withdraw Under the Terms of the Agreement

An hour prior to the President’s announcement, White House Energy Advisor Michael Catanzaro told Capitol Hill Staffers that the administration will withdraw according to the terms of the Paris Agreement in lieu of other, more extreme exit mechanisms. This is significant because the Agreement’s terms will require the U.S. to wait until November 2020 to withdraw. Specifically, Article 28.1 of the Agreement provides that a party may withdraw by giving one year’s written notification to the U.N. Secretary-General, beginning three years after the Paris Agreement’s entry into force. Because the Paris Agreement came into force on November 4, 2016, the United States will only be able to give notice for withdrawal starting on November 4, 2019. Under this timeframe, the U.S. will withdraw one year later, on November 4, 2020, which is the day after the 2020 presidential election is scheduled.

The President Avoided Alternative, More Expeditious Withdrawal Mechanisms

Rather than waiting for 2020, President Trump could have opted for one of two alternative withdrawal mechanisms. First, he could have withdrawn from the Paris Agreement’s parent agreement, the UNFCCC. This would have expedited the withdrawal process, but would have been of questionable legality under U.S. law, given the UNFCCC’s status as an Article II treaty.

Similar to the terms of the Paris Agreement, Article 25.1 of the UNFCCC allows parties to withdraw by giving one year’s written notice to the U.N., beginning three years after the UNFCCC’s entry into force. Because the UNFCCC entered into force in 1994, the U.S. could give notice of withdrawal at any time. Once effective, withdrawal from the UNFCCC would automatically trigger withdrawal from the Paris Agreement itself, which provides under Article 28.3 that “any party that withdraws from the Convention will be considered as also having withdrawn from this Agreement.” Withdrawing under the UNFCCC would therefore have ended U.S. obligations under the Paris Agreement much earlier than withdrawing under the terms of the Paris Agreement itself.

However, it is not clear that the President could withdraw from the UNFCCC under U.S. law. Unlike the Paris Agreement, the UNFCCC was entered into as a treaty after receiving consent from the Senate under Article II of the Constitution. Because of this, there is, in theory, a colorable argument that withdrawal from the UNFCCC would require approval of the Senate. Note, however, that while the Constitution requires the Senate’s advice and consent to approve treaties, it is silent with respect to the Senate’s role when it comes to withdrawal.

In practice however, it would likely be very difficult to successfully challenge a unilateral withdrawal from the UNFCCC by the Trump Administration. Senators have twice tried, and failed, to challenge a unilateral withdrawal from an Article II treaty by a president. In Goldwater v. Carter, several senators, led by Barry Goldwater, challenged President Jimmy Carter’s unilateral termination of the Sino-American Mutual Defense Treaty with the Republic of China (Taiwan). The Supreme Court dismissed the complaint 6-3, but did so on jurisdictional grounds without reaching the merits. Despite providing no clarity regarding the constitutionality of unilateral executive withdrawal, the dismissal has furthered the view that presidents may unilaterally withdraw from Article II treaties without consequences. Similarly, in 2002, senators challenged President George W. Bush’s withdrawal from the Anti-Ballistic Missile Treaty with Russia in Kucinich v. Bush. There, the district court also did not reach the merits, dismissing the case on the grounds that the plaintiffs lacked standing and raised a non-judiciable political question.

President Trump’s second alternative withdrawal mechanism was to reject the Agreement’s validity by claiming that the Obama Administration did not have authority under U.S. law to enter into the Paris Agreement. This approach would have been problematic for multiple reasons. First, it is not clear under U.S. law that President Obama lacked legal authority. Pursuing this option would therefore be subject to legal challenge and could potentially set harmful legal precedent that would hamper future actions by the president. The arguments for and against legal authority have been described by others.

Second, withdrawing outside of the terms of the Paris Agreement would be a violation of international law, which recognizes that heads of state have full powers to enter into international agreements. Article 46 of the Vienna Convention on the Law of Treaties specifically addresses this issue, providing that a state may not withdraw from a treaty on the grounds that its acceptance violated domestic law, unless the violation was “manifest and concerned a rule of its internal law of fundamental importance.” While the United States is not a party to the Vienna Convention on the Law of Treaties, the United States views the Convention as “customary international law on the law of treaties.”

What Happens Next: Effects of U.S. Withdrawal

Impact on Paris Agreement

It is unlikely that the Paris Agreement will fall apart. Under international law, U.S. withdrawal would not cause the Paris Agreement to terminate. While the Agreement’s entry into force required acceptance by 55 countries representing 55% of total global greenhouse gas emissions (Article 21.1), U.S. withdrawal would not terminate the treaty even if it were to reduce the total greenhouse gas emissions below 55% of global emissions. This is so because Article 55 of the Vienna Convention on the Law of Treaties specifically provides that “a multilateral treaty does not terminate by reason only of fact that the number of parties falls below the number necessary for entry into force.”

Still, the U.S.’s withdrawal undermines global cohesion on climate change and may impact participation by other countries. While the EU and China have already signaled their intentions to recommit to the Agreement, some commentators speculate that President Trump’s decision could cause more reluctant countries to rethink their pledges, or even withdraw from the Agreement.

Impact on U.S. and Global Emissions Trajectories

The direct impact of the U.S.’s exit on the trajectory of U.S. and Global emissions in the near term will be limited, with much greater potential consequences for U.S. foreign policy. This is so for multiple reasons.

The course of U.S. emissions over the next decade will be determined not by U.S. participation in the Paris Agreement, but instead by a combination of domestic federal and state policy decisions, some of which have already been taken by President Trump. Under the Obama Administration, the U.S. had pledged to reduce its emissions by 26-28% below 2005 levels by 2025. And some progress has already been made — the Energy Information Agency reports that emissions in 2015 were already 12% below 2005 levels. Since President Trump took office, however, he has taken steps through his America First Energy Plan to roll back the Obama-era regulations that were put in place to reach these goals. According to Climate Action Tracker, “[t]he Trump Administration’s policies, if fully implemented and not compensated by other actors, are projected to flatten U.S. emissions instead of continuing them on a downward trend.” Climate Action Tracker explains that, in order for the U.S. to reach its 2025 Paris Agreement commitment, the U.S. “would have had to implement both the Clean Power Plan and the Obama Administration’s full Climate Action Plan. Current U.S. policies, including the Clean Power Plan, would only reduce emissions to 10% below 2005 levels by 2025.” This means that additional policy beyond the status quo is necessary to meet the 2025 target.

Under the America First Energy Plan, it is clear that the Trump Administration will not pursue the necessary additional reductions. However, it is possible that this gap could be filled by regional and state efforts. Indeed, Michael Bloomberg — the current U.N. special envoy for cities and climate change—has said that U.S. states, cities, and businesses will continue to work to meet the U.S.’s commitment under the Paris Agreement. Similarly, California announced that it will partner with New York and Washington States to create the United States Climate Alliance, which will “convene states committed to upholding the Paris Climate Agreement and taking aggressive action on climate change.” Given these developments, it is possible that the U.S. could still make progress toward its 2025 targets. Whether it meets those targets, however, will be determined by domestic policy initiatives rather than the president’s decision to leave the Paris Agreement.

Similarly, neither President Trump’s decision to exit the Paris Agreement, nor his recent rollback of U.S. regulations is likely to have a major impact on global emissions by 2030. Climate Action Tracker reports that both China and India are projected to reduce their emissions beyond their Paris Agreement pledges. So much so that their reductions will likely outweigh any increase in U.S. emissions that could result from President Trump’s policies.

Impact on U.S. Foreign Policy

The real impact of withdrawal from the Paris Agreement will instead be felt in the realm of U.S. foreign relations. Commentators appear to agree that, by withdrawing from the Agreement, the U.S. has ceded leadership on an increasingly important global issue. Moreover, representatives from some countries have already indicated that this will impact relations with the U.S. on other issues, including trade. The impact on climate issues will be compounded because the Paris Agreement is hardly a final solution to climate change; pledges to reduce GHG emissions under the Agreement, even assuming the U.S. meets its 2025 target, are currently insufficient to prevent global temperatures from rising more than 2 degrees Celsius (the point at which scientists agree we will see “truly dangerous change”). In order for the world to keep temperatures below the 2 degree scenario, all countries would need to reach peak emissions by 2030 and then steadily make further cuts. This means that pledges to reduce emissions must become more ambitious over time, a prospect that almost certainly entails future international negotiation and diplomacy. By withdrawing from the Paris Agreement, the U.S. has signaled that it is unlikely to take the lead on these issues — at least for now.

Industry Opposition

Notably, the president’s decision was opposed by industry, including several large oil companies. Commentators assert that this trend stems in part from the energy industry’s recognition that global efforts to address climate change are on the rise and are here to stay. Under this view, pulling out of the Paris Agreement merely weakens the ability of the U.S. to take an international leadership role in shaping inevitable climate protection efforts. Indeed, in expressing its support for the agreement, ConocoPhillips stated that the Agreement “gives the U.S. the ability to participate in future climate discussions to safeguard its economic and environmental best interests.” Support for the agreement also appears to be consistent with recent shareholder activism in favor of greater efforts to address climate change. According to U.S. News and World Reports, the majority of Exxon shareholders recently voted to support a new transparency standard requiring the company to disclose how its business operations will be impacted by climate change. The energy industry’s position is perhaps also explained by the fact that, as noted above, exiting from the Paris Agreement will have no impact on the U.S.’s emissions trajectory in the longer term. In other words, withdrawal leads to no tangible change in U.S. domestic policy, and instead merely prevents the U.S. from having a seat at the table in the future.