NATURE CLIMATE CHANGE | ADVANCE ONLINE PUBLICATION | www.nature.com/natureclimatechange 1 opinion & comment COMMENTARY: Better out than in Luke Kemp Continued US membership in the Paris Agreement on climate would be symbolic and have no efect on US emissions. Instead, it would reveal the weaknesses of the agreement, prevent new opportunities from emerging, and gift greater leverage to a recalcitrant administration. A ter the election of President Trump and a two-house Republican majority, many fear for the future of US climate policy. he new administration has indicated that they will abolish Obama’s climate legacy through executive orders 1 . he repeal of domestic measures will likely result in the US missing its irst nationally determined contribution (NDC) under the Paris Agreement, which is an inadequate target of reducing emissions by 26–28% compared to 2005 levels by 2025. If other countries adopted comparable targets, global warming would likely exceed 2 °C (ref. 2). he US would need to implement the Clean Power Plan and additional measures to reach its NDC 3 . Preliminary research suggests that the policies of the Trump administration would instead lead to emissions increasing through to 2025 3 . Now the predominant concern for much of the international community is that the US will withdraw either from the Paris Agreement, or the overarching United Nations Framework Convention on Climate Change (UNFCCC) 4 . he former would take four years and the latter only one. Both are legally possible and within the presidential mandate 5 . he conventional wisdom is that a US withdrawal would be a worst-case scenario for international climate policy. However, a sober analysis of the political, legal, and inancial impacts suggests otherwise. he modiied matrix of risks posed by a recalcitrant US administration summarized in Table 1, and explored in detail below, highlights the paradox of US participation: a rogue US can cause more damage inside rather than outside of the agreement. Domino efect here are fears that a US exit could trigger others to free-ride or withdraw. One analysis found that if the world were to follow a laggard US in delaying action by 8 years, cumulative future emissions over the next century would be doubled, and the 2 °C target would be out of reach 6 . Such concerns are reasonable, but speculative. he closest parallel is the Kyoto Protocol, which the US signed but never ratiied. he refusal to ratify empowered the international community to adopt the 2002 Marrakech Accords. he spirit of solidarity was short- lived. In 2007, amidst the second term of the Bush administration, parties launched the Bali Road Map. his marked the beginning of the process of abandoning Kyoto and creating an alternative agreement with US participation: the Paris Agreement. he example of Kyoto suggests that US withdrawal is unlikely to unravel Paris in the short-term, but a second Republican term may trigger a domino efect. Whether the experience of Kyoto repeats itself is for now uncertain; the architecture of Paris is diferent and international politics have evolved. However, a domino efect could also occur without withdrawal. he success of Paris largely relies on its pledge and review process to create political pressure 7 , and drive low-carbon investments 8 . A great power that wilfully misses its target could provide political cover for other laggards and weaken the sot power of process. his would lay bare the weaknesses and legal porousness of the Paris Agreement and undermine any public and investor conidence vested in the agreement. Paris may forfeit legitimacy due to the loss of a major emitter, but it is equally likely that its legitimacy will be grievously injured by the US blatantly violating the spirit and purpose of the agreement. A laggard at the table Continued membership would also raise the problem of the US watering down the provisions and details of the Paris Agreement (known as the ‘Paris rulebook’), which are to be conirmed by the end of 2018. In a consensus-based process Table 1 | A modiied risk matrix for US participation in the Paris Agreement on climate. Risk Likelihood Consequence Withdrawal required? US misses its domestic targets High Moderate–severe (moderate if it only impacts US emissions, and severe if it creates a domino effect) No US actions create an incentive for others to free-ride or withdraw Low–medium Severe (potential reneging on targets, free-riding and a cascade of withdrawals) No (this may be exacerbated or lessened by formal withdrawal) US obstructs and waters down the ‘Paris rulebook’ and other negotiations Medium Moderate–severe (the impacts depend on US diplomatic actions. At worst, this could result in a stalling of the Paris rulebook development and weakening of key provisions, as well as the obstruction of negotiating issues beyond both the rulebook and 2018) No (this requires continued participation) Cancellation of climate financing High Major–severe (undermining of public and investor confidence. Potential slowdown of action in developing countries. Severe if it creates a domino effect) No ©2017MacmillanPublishersLimited,partofSpringerNature.Allrightsreserved.