Donald Trump has confirmed the US will not honour its commitment to the landmark Paris Agreement, aimed at tackling global climate change.
Here is a look at the agreement and the impact of the US withdrawal.
What is the Paris Agreement?
The historic global deal reached in the French capital in December 2015, saw nearly 200 countries agree to curb rising temperatures.
Why do we need it?
There are fears that if greenhouse gases continue soar into the atmosphere at current trajectories, the world could face temperatures of more than 4C above pre-industrial levels by 2100.
This would be hotter globally than at any time in human history.
It could lead to sea level rises, more intense storms and flooding, further extreme droughts, water shortages and heatwaves.
Massive loss of wildlife and reduction in crop yields is also feared.
What key things does the agreement involve?
- It includes a pledge for countries to take action to limit temperature rises to "well below" 2C above pre-industrial levels
- To also pursue efforts to limit any increase 1.5C
- A five-year review of climate action plans
- Finance for poorer countries to develop along a low-carbon path and to cope with the impacts of climate change
- A goal for greenhouse gases to peak as soon as possible and be rapidly reduced
Why has Donald Trump withdrawn from the deal?
President Trump's predecessor Barack Obama signed up to the deal without ratification by the US Senate.
Mr Trump, who has described climate change as a hoax by the Chinese to hurt US manufacturing, pledged to abandon the deal during the US presidential race.
He has also promised to boost fossil fuel production at home.
What is the mechanism for withdrawing?
A country can serve notice it is quitting the deal three years after it came into force for them, and it then takes another year to leave.
It would also be possible for the US not to formally leave but simply refuse to meet its commitments, which are voluntary for countries.
What does the US quitting the agreement mean?
America has agreed to reduce its emissions by 2025 to 26-28% of 2005 levels, about 1.6 billion tonnes.
There are concerns the failure of the world's second biggest polluter to honour its commitments will make it harder for the world to curb temperature rises.
It also sets a precedent and raises fears other countries may renege on their commitments.
Nicholas Stern, a climate economist said Mr Trump's decision was "a mistake for the United States", as it is a "world leader" in the technologies needed to produce green energy which would create jobs.
Lord Stern branded low-carbon energy the "growth story" of the future, and said that other countries would now "take the lead" in developing renewable energy.
He continued that many companies, cities and states in the US would "just get on with it" in becoming greener, regardless of the decisions made by President Trump.
How have global leaders reacted to the US withdrawal?
The European Union and China are set to announce deeper commitment to climate action after a summit on Friday.
On Thursday, China's Premier Li Keqiang reiterated the importance of the deal and said fighting climate change and said "international responsibility".
China, which is the world's biggest polluter, also said it will work with the EU to uphold the agreement even if the US withdraws.
German Chancellor Angela Merkel said she was pleased China is committed to sticking to its climate treaty obligations.
Russia also said it supports the climate deal and stressed it would be less effective without the major participants.