Originally published 2021-02-26.
The United States officially became a party to the Paris climate agreement again on 19 February 2021. President Joe Biden had signed the paperwork on his first day in office, 20 January 2021, and under the treaty's rules the return took effect 30 days later. It reversed Donald Trump's withdrawal, which had come into force on 4 November 2020, the day after that year's election.
The story did not end there. On 20 January 2025, at the start of his second term, Trump ordered a second exit, which took effect on 27 January 2026. Below is a clear guide to what the Paris Agreement is, why the 2021 return mattered, what targets the US set, and how the country ended up leaving twice.
What the Paris Agreement actually is
The Paris Agreement was adopted on 12 December 2015 at the COP21 climate conference at Le Bourget, just outside Paris. All 195 countries taking part in the UN climate talks, plus the European Union, agreed it by consensus, and it entered into force on 4 November 2016.
Its main goal is to hold the rise in global average temperature to well below 2 °C above pre-industrial levels, while pursuing efforts to keep it to 1.5 °C. Rather than imposing one set of cuts on everyone, it asks each country to set its own plan, called a nationally determined contribution or NDC, and to submit a new, more ambitious one every five years.
Two details are often misunderstood. First, the national targets themselves are not legally binding; what is binding is the duty to set them, report on them and update them. Second, the deal is not signed by "200 countries", a figure that circulated widely. As of January 2026 it had 194 parties, and Iran was the only major emitter that had not ratified it.
How the US left the first time
Trump announced in a Rose Garden speech on 1 June 2017 that the United States would pull out, arguing the deal was unfair to American workers and industry. The treaty did not allow an instant exit, though. The earliest the US could formally give notice was 4 November 2019, and the withdrawal took effect a year later, on 4 November 2020. So, despite some reports at the time, the US did not actually leave in 2019; it left in late 2020.
Biden had criticised the decision throughout his campaign. In his inaugural address he spoke of the climate crisis in stark terms:
"A cry for survival comes from the planet itself."
The return in February 2021
Rejoining was quick by comparison. A single executive order and a letter to the United Nations were enough, and after the 30-day waiting period the US was back. Biden marked the day on social media with a short message: "Today, America is officially back in the Paris Climate Agreement."
Environmental groups and many foreign governments welcomed the move, but some warned that the years outside had cost the world precious time. Christiana Figueres, who led the UN climate secretariat when the Paris deal was reached, told the Guardian that the real damage of the exit was the signal it had sent:
"It's the political symbolism that the largest economy refuses to see the opportunity of addressing climate change."
Todd Stern, the lead US negotiator in Paris in 2015, described rejoining as a big first step but only a first step, according to Global Citizen.
The targets the US set
Under Barack Obama, the original US pledge was to cut greenhouse gas emissions by 26 to 28% below 2005 levels by 2025. Biden's team promised to go much further.
- 2030 target: at a virtual Leaders Summit on Climate on 22 April 2021, attended by 40 world leaders, Biden announced a new goal of cutting emissions by 50 to 52% below 2005 levels by 2030.
- Clean power by 2035: his climate plan aimed for a carbon-free electricity sector by 2035.
- Net zero by 2050: the long-term aim was net-zero emissions across the whole economy by mid-century.
- Investment: during the campaign Biden proposed spending around $2 trillion on clean energy and infrastructure. Much of the climate spending that eventually passed came through the 2022 Inflation Reduction Act rather than a single plan of that size.
Whether those targets could be met was always uncertain, because each depended on decisions by future Congresses and administrations.
Leaving again in 2025 and 2026
That uncertainty became reality. On 20 January 2025, the day of his second inauguration, Trump signed an order taking the US out of the agreement again. Under the treaty's one-year rule, the second withdrawal took effect on 27 January 2026. In February 2026 the US went further and notified the UN that it would also leave the underlying UN Framework Convention on Climate Change, effective a year later, the first country ever to do so.
For other countries, the back-and-forth has been a reminder that a big emitter's commitments can change with each election. For the agreement itself, the rest of the parties stayed in.
Is the Paris Agreement working?
The honest answer is: partly. The first global stocktake, published in September 2023, found that expected warming by 2100 had fallen from roughly 3.7 to 4.8 °C, as estimated around 2010, to around 2.4 to 2.6 °C under current policies and pledges. If countries also meet their long-term net-zero promises, the figure could be closer to 1.7 to 2.1 °C.
That is real progress, but it is still well above the 1.5 °C aim, and 2024 became the first calendar year in which the global average temperature exceeded 1.5 °C above pre-industrial levels, according to the Copernicus climate service. A single year above the line does not mean the long-term goal has been formally missed, since the target refers to a multi-decade average, but it shows how little room is left.
Common questions
Can a president leave the treaty alone? In practice, yes. The US joined by executive action rather than a Senate-ratified treaty, which is why presidents have been able to join and leave by order.
Does leaving stop US climate action? Not entirely. States, cities and companies continue their own programmes, though federal policy has a major effect on the pace of change.
Why does one country matter so much? The US is the world's largest economy and its biggest historical emitter, so its choices shape the ambitions of others.
Sources: Wikipedia: United States and the Paris Agreement; Wikipedia: Paris Agreement; Global Citizen: The United States officially rejoins the Paris climate agreement; Scientific American: Biden promises to slash greenhouse gas emissions 50 percent by 2030; The Guardian, 19 February 2021.
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