President Donald Trump on Thursday announced the United States was withdrawing from the 2015 Paris Agreement, saying the US would not implement financial commitments or national emissions-curbing measures pledged under the pact.
A total of 196 nations have endorsed the agreement and 147 have officially ratified it, aiming to prevent the worst-case scenarios of global warming.
It is the first pact to commit all nations to limit global warming caused by emissions from burning coal, oil and gas.
The US ratified the pact under Barack Obama in September 2016 and pledged $3 billion towards the Green Climate Fund of which it delivered $1 billion under the Obama administration.
China, the world's number one greenhouse gas polluter and Europe, number three after the US in second place, recommitted to the Paris Agreement on Thursday.
The biggest polluter not to have ratified the deal is Russia (number five after India).
What is the goal?
Nations agreed to hold global warming to "well below" two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels, and to strive for a lower limit of 1.5 degrees.
Experts say even the two-degree ceiling is a tall order, requiring an immediate and deep reduction in planet-warming emissions from fossil fuels, an industry with major influence in Washington.
Signatories to the deal will aim for emissions to peak "as soon as possible." The UN's climate science panel recommends a 40 to 70 percent cut by 2050 from 2010 levels.
Based on voluntary pledges by countries so far, the planet is on track for warming of about three degrees, many scientists say – a recipe for catastrophic floods, storms, drought and sea-level rise.
By the second half of this century, according to the pact, there must be a balance between emissions from human activities such as energy production and farming, and the amount that can be absorbed by carbon-absorbing "sinks" such as forests or storage technology.
Without the US administration on board, the goal may move even further out of reach.
Developed countries, which have polluted for longer, must take the lead with absolute emissions cuts.
Developing nations, which still burn coal and oil to power growing populations and economies, are encouraged to "continue enhancing" their efforts and "move over time" towards absolute cuts.
How to track progress?
In 2018, and every five years thereafter, countries will take stock of the overall impact of their efforts to rein in global warming, according to the text.
It "urges" and "requests" countries to update their pledges by 2020.
Some nations, including the US, set emissions-curbing targets for 2025, others for 2030. Both categories are meant to be updated every five years.
Rich countries are expected to provide funding to help developing countries make the costly shift to cleaner energy sources and to shore up defences against the impacts of climate change.
Donor nations must report every two years on their financing levels, current and intended.
In a nonbinding "decision" that accompanies the agreement but is not included in it, the $100 billion per year that rich countries have pledged to muster by 2020 is referred to as a "floor", meaning it can only go up.
The amount must be updated by 2025.
According to an Organisation for Economic Co-operation and Development (OECD) report, pledges made in 2015 alone would boost public climate financing (excluding private money) to $67 billion in 2020.
What is the level of bindingness?
The agreement makes provision for parties to withdraw, but notice can be given only three years after its entry into force in 2016. The actual withdrawal would take effect a year later.
A country can also withdraw from the 1992 UN Framework Convention on Climate Change, under whose auspices the deal was negotiated. Withdrawal would take effect a year after notification.
Trump, however, said the US would "cease all implementation" of the accord "as of today."