US President Donald Trump signs new executive orders on January 21, 2025/SCREENGRAB
In Paris in 2015, nearly 200 countries agreed to a series of measures to tackle climate change that were designed to avoid some of the worst consequences of rising temperatures.
But on his first day in office, incoming President Donald Trump announced that the US would withdraw from the deal.
What is the Paris climate agreement?
World leaders pledged to try to prevent global temperatures rising by more than 1.5C above those of the late 19th century, known as "pre-industrial" levels.
It saw almost all the world's nations agree to cut greenhouse gas emissions, which cause global warming.
Adopted by nearly 200 countries in the French capital in December 2015, the Paris Agreement came into force on 4 November 2016.
What did the Paris Agreement say?
The agreement lists a series of commitments:
To "pursue efforts" to limit global temperature rises to 1.5C, and to keep them "well below" 2.0C above those recorded in pre-industrial times
To achieve a balance—known as net zero"—between the greenhouse gases that humans put into the atmosphere and the gases that they actively remove, in the second half of this century
Each country to set its own emission-reduction targets, reviewed every five years to raise ambitions
Richer countries to help poorer nations by providing funding, known as climate finance, to adapt to climate change and switch to renewable energy
The 1.5C target is generally accepted to refer to a 20-year average rather than a single year.
So, while the year 2024 was more than 1.5C warmer than pre-industrial times, that does not mean the Paris Agreement threshold has yet been breached.
Why has Trump withdrawn from the Paris agreement?
On the day of his inauguration, incoming US President Donald Trump vowed to pull out of the Paris deal.
He signed an executive order that argued that such agreements "do not reflect our country's values or our contributions to the pursuit of economic and environmental objectives" and "unfairly burden the United States," citing costs to American taxpayers.
The US is the world's second-largest annual emitter of planet-warming gases, and its total emissions since the late 19th century are the highest of any country.
Trump made a similar announcement in 2017 at the start of his first presidency, although it was three years before the US was able to leave the Paris agreement.
His successor, Joe Biden, rejoined in 2021 at the start of his term as president.
This time, under UN rules, the US has to wait a year until it is officially out.
The US is the only nation to have withdrawn from the Paris deal, and joins Iran, Libya and Yemen as the countries outside the agreement.
Why does keeping global warming to 1.5C matter?
Scientists say that every 0.1°C temperature increase brings with it greater risks for the planet, such as longer heatwaves, more intense storms and wildfires.
The 1.5C target was agreed because there is very strong evidence that the impacts would become much more extreme as the world gets closer to 2C. Some changes could become irreversible.
The science is not completely certain, but according to the UN, the consequences of 2C global warming versus 1.5C, external could include:
Extreme hot days would be on average 4C warmer at mid-latitudes (regions outside the poles and tropics), versus 3C at 1.5C
Sea-level rise would be 0.1m higher than at 1.5C, exposing up to 10 million more people to more frequent flooding
More than 99% of coral reefs would be lost, compared with 70-90% at 1.5C
Several hundred million more people may be exposed to climate-related risks and susceptible to poverty by 2050 than at 1.5C.
What have countries done since signing the Paris Agreement?
World leaders meet every year to discuss their climate commitments, at international summits known as COPs (Conference of the Parties).
All COPs since 2015 have tracked how countries are building on what they promised in Paris.
When the agreement was signed, governments admitted the Paris targets would not limit global warming to 1.5C.
Current climate plans still put the world on track for around 2.6C to 2.8C of warming by 2100, according to the UN. This could drop to 1.9C if all net zero pledges were achieved, but this would require countries to take more action.
At COP28 in December 2023, countries agreed for the first time to "contribute" to "transitioning away from fossil fuels," although they were not forced to take any specific action.
However, there was no significant progress on this goal at COP29 in November 2024.
What did the Paris agreement promise for poorer countries?
The Paris deal restated a commitment first made in 2009 that the world's richer countries should provide $100bn (around £82bn) annually by 2020 to help developing nations deal with the effects of climate change, and build greener economies.
In 2020 only $83.3bn was raised, but the goal was eventually achieved in 2022, according to data from the OECD, external.
In 2023, countries agreed for the first time that a fund should be established exclusively for loss and damage. This is money to help countries recover from the impacts of climate change.
At COP29 countries agreed to update the 2020 goal.
Richer nations committed to providing $300bn (around £245bn) a year to developing nations by 2035, with a broader ambition for $1.3tn to be raised from private and public sources by the same date.
However, developing countries - which had hoped for more - criticised the $300bn figure as a "paltry sum"