Paris Agreement explained: why the US keeps leaving, and why the UNFCCC matters more

The Paris Agreement is not a simple climate promise, and the United States (US) story is not just a story of one president leaving and another rejoining. It is a thirty-year pattern: the US helped build the global climate framework, struggled to make climate treaties durable at home, then used the same flexibility that made Paris possible to leave it twice.

For related background, read our climate policy and carbon markets guide, plus guides to net zero, carbon credit prices, CORSIA aviation carbon markets, UK CBAM and climate risk in investment portfolios.

Quick answer

The Paris Agreement is the 2015 United Nations climate agreement that asks countries to submit and strengthen national climate plans over time. Those plans are called nationally determined contributions. The agreement does not legally force a country to hit its emissions target, but it does create legal obligations around participation, reporting, communication and regular updating of targets.

The US keeps leaving because the Paris Agreement was joined through executive action, not ratified by the Senate as a treaty. That made it politically possible for the Obama administration to join without a two-thirds Senate vote, but it also made it possible for later presidents to withdraw by executive action. The deeper issue in 2026 is not only Paris. It is the US decision to begin withdrawing from the United Nations Framework Convention on Climate Change (UNFCCC), the parent treaty that underpins the whole process.

Key point

Leaving Paris weakens the target-setting framework. Leaving the UNFCCC would be larger because the UNFCCC is the legal foundation for the annual climate negotiations, emissions reporting architecture and future agreements built under the climate convention.

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Paris Agreement vs Paris Accords

The formal name is the Paris Agreement. It was adopted in December 2015 and entered into force in November 2016. The term "Paris Accords" is widely used in media and political speech, but it can make the agreement sound looser than it is.

Paris is a legal instrument adopted under the UNFCCC. It has rules on joining, leaving, communicating national climate plans and reporting progress. But it is not a treaty that directly fines countries for missing emissions targets. That distinction matters. Paris is serious, but it is built around national commitments, transparency and diplomatic pressure rather than a global climate police force.

Under Paris, countries submit nationally determined contributions, often shortened to NDCs. These are the national plans that set emissions targets, sector policies and timeframes. Countries are expected to update them over time, with each round representing progression beyond the previous one. The strength of Paris depends on whether countries keep making stronger plans and then implement them at home.

The US climate treaty problem started before Paris

The US did not begin as an outsider to climate diplomacy. It helped build the original framework.

The UNFCCC was adopted in 1992 and opened for signature at the Rio Earth Summit. The US Senate gave advice and consent to ratification that same year under President George H.W. Bush. The convention did not set binding national emissions cuts. It created the framework for cooperation, reporting and future negotiations.

The first major break came with the Kyoto Protocol. Kyoto was adopted in 1997 and set binding emissions targets for developed countries. The Clinton administration signed it, but the Senate had already passed the Byrd-Hagel resolution by 95 votes to zero, warning against any agreement that imposed binding limits on the US without also requiring new commitments from major developing economies. Kyoto was never submitted for ratification. President George W. Bush later rejected it. The US never became a Kyoto party.

Paris was designed partly around that failure. Instead of a top-down treaty that imposed binding targets on developed countries, it used bottom-up national plans. Instead of asking the US Senate to approve a formal treaty, the Obama administration joined through executive authority. That made Paris possible for the US. It also made it fragile.

Timeline: how the US joined, left, rejoined and left again

One correction is important. It is tempting to say the US has "never stayed" in Paris, but formal membership is more complicated. The US was a Paris party from 2016 until 4 November 2020, then outside the agreement until formal re-entry on 19 February 2021. It remained a party again until the second withdrawal took effect in 2026. The real issue is not that the US was outside Paris for most of the agreement's life. It is that partners cannot rely on US participation surviving a change of administration.

Why the UNFCCC move matters more than Paris alone

Paris sits under the UNFCCC. The UNFCCC is the parent convention that created the annual negotiating process and the wider architecture for climate reporting and cooperation. If Paris is a major legal instrument, the UNFCCC is the frame that holds it.

In January 2026, a White House presidential memorandum directed US agencies to withdraw from a wide list of international organisations, conventions and treaties, including the UNFCCC, the Intergovernmental Panel on Climate Change (IPCC) and the International Renewable Energy Agency (IRENA). The White House described the package as a broader withdrawal from international bodies that it considered contrary to US interests.

That is different from leaving Paris. A country outside Paris can still be a party to the UNFCCC. A country outside the UNFCCC is stepping outside the legal foundation that created Paris and the annual negotiation process. Paris also states that a party that withdraws from the UNFCCC is considered to have withdrawn from Paris.

Assuming the UNFCCC withdrawal process reaches completion in 2027, the practical effects could include fewer formal reporting obligations inside the UN climate process, no normal negotiating seat when future climate rules are set, reduced financial contribution to the UNFCCC budget and less formal influence over standards that other economies continue to use. Carbon Brief's analysis of the withdrawal makes the same broad point: the UNFCCC decision is a larger institutional rupture than Paris withdrawal alone.

Plain-English version

Leaving Paris is like leaving the main climate targets agreement. Leaving the UNFCCC is closer to leaving the forum where the climate rulebook is written.

What leaving Paris does not do

Leaving Paris does not, by itself, switch off climate policy inside the US. Domestic emissions are shaped by federal regulation, state policy, tax credits, infrastructure decisions, power markets, vehicle standards, oil and gas rules, clean technology costs and court decisions. Treaty membership is part of the picture, not the only driver.

Leaving Paris also does not make climate risk disappear for companies or investors. Banks, insurers, infrastructure owners, utilities and manufacturers still face physical risk, transition risk, supply-chain risk and policy risk. A US company selling into the European Union (EU), for example, may still face European reporting rules or carbon border rules even if the US federal government is outside Paris.

Nor does withdrawal erase legal debate. In July 2025, the International Court of Justice (ICJ) issued an advisory opinion on states' climate obligations under international law. Advisory opinions are not the same as domestic legislation or a binding judgment in a case between two named parties, but the opinion is likely to influence future climate litigation and diplomatic arguments. Withdrawal from a treaty framework does not automatically remove every source of potential climate-related legal exposure.

What COP30 showed after the US stepped back

COP30 in Belém, Brazil, was the first major UN climate summit after the second US Paris withdrawal process began. It also marked the tenth anniversary of the Paris Agreement. The summit showed that the process can continue without US leadership, but also that the finance and fossil fuel questions become harder when the world's largest historical emitter is not playing a full role.

The strongest area of progress was climate finance. The Baku-to-Belém Roadmap focused attention on the goal of mobilising $1.3 trillion a year for climate action in developing countries by 2035. Adaptation finance, loss and damage, and implementation support were central parts of the discussion. Those are not side issues. For many vulnerable countries, the credibility of the entire climate process depends on whether money actually arrives, not whether communiques sound ambitious.

The weakest area remained fossil fuels. COP28 had produced the first agreed global language on transitioning away from fossil fuels. At COP30, efforts to turn that language into a clearer roadmap ran into resistance. Brazil indicated that it would develop a voluntary fossil fuel phase-out roadmap for discussion before COP31, but voluntary roadmaps are weaker than binding agreement text.

The lesson is not that the UNFCCC process collapsed. It did not. The lesson is that climate diplomacy is now trying to operate with a major credibility gap: the country that helped create the framework is no longer a dependable participant.

Who fills the gap?

No single country replaces the US in the climate process. Different actors fill different parts of the vacuum.

China

China is the world's largest annual emitter and one of the dominant manufacturers of solar panels, batteries, wind components and electric vehicles. That gives it influence even when formal diplomacy is difficult. Clean technology scale can become a form of geopolitical power because standards, supply chains and prices often follow the largest producer.

The complication is obvious: China can be a clean technology superpower while still using large amounts of coal domestically. That means China's role is not simple climate leadership. It is a blend of industrial strategy, energy security, export power and climate policy.

The European Union

The EU has become the most important rule-maker for climate-linked trade. Its Carbon Border Adjustment Mechanism (CBAM) puts a carbon cost on some imports where the exporting country does not apply an equivalent carbon price. That creates a direct trade incentive for other countries to measure and price industrial emissions. Our UK CBAM guide explains why the United Kingdom (UK) is also moving in this direction.

The EU cannot replace US climate finance alone, and it faces its own political pressures. But it can export climate rules through market access. That may matter more for businesses than summit speeches.

Climate-vulnerable countries

Small island states, least-developed countries and climate-vulnerable economies have often been the moral centre of the UN climate process. They have the least responsibility for historic emissions and the most exposure to physical climate damage. US withdrawal makes their position harder because finance, loss and damage, and adaptation support become more difficult without full US participation.

Why UK readers should care

The UK remains inside Paris and the UNFCCC. US withdrawal does not change the UK's climate targets directly. It does, however, change the policy environment around UK companies, investors and supply chains.

1. Trade rules keep moving

Carbon border rules are becoming more important. The EU CBAM is already shaping trade expectations, and the UK has its own CBAM planned. If the US federal government steps away from global climate frameworks while Europe keeps building carbon-linked trade rules, UK companies may face a split world: US policy uncertainty on one side, European carbon reporting and pricing pressure on the other.

2. Investors still need to price climate risk

For investors, Paris membership is not a return forecast. A country can be inside Paris and still miss targets. A company can be outside a climate-friendly jurisdiction and still have strong transition plans. The practical question is whether policy volatility changes the risk profile of sectors, supply chains and assets. Our guide to climate risk in investment portfolios covers the investor side in more detail.

3. Carbon markets depend on trust

Carbon markets are not separate from climate diplomacy. Article 6 of the Paris Agreement sets rules for international cooperation and carbon market transfers between countries. Aviation markets such as CORSIA also depend on eligible credits, reporting rules and confidence that emissions reductions are not double counted. US withdrawal does not stop those markets, but it can affect confidence, standard-setting and demand signals.

4. UK companies with US exposure need scenario thinking

UK businesses with US suppliers, customers, investors or operations should not assume one stable climate policy pathway. Federal policy can change quickly, while state-level policy, European regulation, insurance markets and investor requirements may move differently. That is why climate transition planning should include policy scenarios rather than one linear assumption.

What to watch before COP31

COP31 is scheduled for Antalya, Turkey, in November 2026 under a Turkey-Australia arrangement. Several issues will matter more than the speeches.

• UNFCCC withdrawal status: whether the US withdrawal process continues toward completion in 2027 or is legally challenged, delayed or reversed.
• Finance delivery: whether the $1.3 trillion roadmap becomes more than an aspiration.
• Fossil fuel language: whether the voluntary roadmap promised after COP30 turns into a credible process.
• Article 6 implementation: whether carbon market rules become clearer for countries, airlines and corporate buyers.
• EU and UK carbon border policy: whether trade rules become the main enforcement mechanism for climate policy outside the US.
• US domestic policy: whether federal withdrawal is offset, partly or fully, by state policy, corporate procurement and clean technology economics.

FAQ

Is the Paris Agreement legally binding?

Parts of it are. Countries have legal obligations around participation, communication, transparency and updating their national climate plans. The specific emissions targets are nationally determined and are not enforced through a direct global penalty system.

Why did the US not make Paris a normal treaty?

Senate approval was unlikely. The Kyoto experience showed that a formal climate treaty with binding targets would probably fail in the Senate. The executive-agreement route made US participation possible but also made withdrawal easier.

Can the US rejoin Paris again?

Yes, a future administration could seek to rejoin Paris again, as President Biden did in 2021. In that case, the order was signed on 20 January and formal re-entry followed on 19 February after the agreement's 30-day process. Rejoining the UNFCCC after a completed withdrawal would be more complex because there is no normal modern precedent for a party leaving and then returning.

Does US withdrawal mean the Paris Agreement has failed?

No. The agreement continues without the US, and most countries remain inside the process. But US withdrawal weakens credibility, finance expectations and the sense that major economies are moving together.

Does this matter for ordinary UK investors?

Yes, indirectly. It affects climate policy credibility, carbon market confidence, clean technology supply chains, trade rules and long-term transition risk. It does not mean investors should make decisions based on one political event alone.

Useful source links

• UNFCCC: The Paris Agreement
• UNFCCC: What is the United Nations Framework Convention on Climate Change?
• UNFCCC: What is the Kyoto Protocol?
• US Congress: Byrd-Hagel Resolution
• Harvard Environmental and Energy Law Program: Paris Agreement tracker
• White House: 2025 executive order on international environmental agreements
• White House: 2026 memorandum on international organisations, conventions and treaties
• Carbon Brief: US exit from UNFCCC and IPCC explained
• World Resources Institute: COP30 and the $1.3 trillion roadmap
• UNFCCC: The road to COP31 in Antalya
• International Court of Justice: climate change advisory opinion case

Bottom line

The US climate treaty story is not only about one withdrawal from Paris. It is about a deeper durability problem. The US helped design the global climate framework, but its domestic politics have made long-term participation uncertain. For UK readers, the practical issue is not symbolism. It is how policy volatility affects trade, carbon markets, finance, supply chains and transition risk.