The Paris climate pact agreed at the end of 2015 was a historic achievement after more than two decades of failed efforts to reach a global consensus on climate change. It became legally binding about a year later, after countries responsible for 55% of global greenhouse gas emissions had ratified it.

Only two countries have not joined: Syria, which was crippled by war at the time of negotiations, and Nicaragua, which refused to sign up because it considered the deal too weak. Therefore, as the US pulls out it becomes the only country in the world to argue that the Paris accord demands too much of signatory nations.

What does it commit member states to doing?

The Paris deal bridged yawning divides on how to limit emissions, partly by focusing more on ambitions than obligations. It aims to keep the overall increase in global temperatures below 2C, with an aspiration of limiting them to 1.5C. Scientists say these are the largest increases possible without risking catastrophic and irreversible change in the world’s climate.

Developed and developing countries, which squabbled for years about who should foot the bill for cleaner growth, must all limit their emissions and try to achieve a zero-emissions economy by the end of this century.

Finance will be provided to poor nations to help them cut emissions and cope with the effects of extreme weather.

How effective will it be, if implemented fully?

There are no legally binding requirements, and even the voluntary caps on emissions agreed so far are actually far too loose to hit the treaty’s own targets. Instead they are likely to lead to warming of 2.7 to 3C. But governments are committed to meet every five years to review targets and progress, starting in 2023, potentially ramping up their goals.

Environmentalists and governments backed the compromise deal in the hope that it would spur investment in clean technology, while both peer pressure and example would spur officials to tackle their emissions.

There is some indication that just the existence of the deal is having a positive effect, and the market for green energy is growing so fast that US firms including Exxon Mobil have pushed Donald Trump to stay in.

Does it matter if the US leaves?

The US is not bound to any targets by the Paris accord, and is already moving away from Barack Obama’s commitment to cut emissions. It will continue to do so regardless of whether it remains part of the global deal.



There are fears that formal withdrawal could dent the soft-power impact of the agreement. But other major emitters including the European Union and China point out that the US would be as free to rejoin as it is to leave.

They have vowed that their commitment to tackling climate change will not be affected, perhaps bolstered by polls showing that the deal has the backing of a firm majority of US voters.