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The voluntary climate pact was originally written to try to reduce warming and withstand the changing political winds in the United States.

In his first hours in office, Trump started the yearlong process to withdraw from the pact for a second time; in 2021, then-President Joe Biden had the U.S. rejoin on his second day in office.

Once the withdrawal takes effect next year, the U.S. will join Iran, Libya and Yemen as the only United Nations countries that are not part of the agreement.

The U.S. withdrawal, while expected, triggered heavy reactions from around the world. That's because the U.S. is historically responsible for the largest share of heat-trapping gases in the atmosphere, has been a leader in international climate negotiations and is the world's largest producer of the fossil fuels that cause the problem.

When the agreement was signed Dec. 12, 2014, then-President Barack Obama called it "the best chance to save the one planet we have."

What is the agreement?

The main goal is to keep long-term global temperatures from warming 1.5 degrees Celsius — or 2.7 degrees Fahrenheit — above preindustrial times and if not that, below 2 degrees Celsius by slashing planet-warming emissions from coal, oil and gas.

"The Paris Agreement is a framework, not a stand alone solution," said Mohamed Adow, founder of PowerShift Africa and a veteran climate negotiations observer. "Tackling climate change is not a pass-or-fail scenario. The Paris Agreement was never a solution itself, just a structure for countries to take action. And in large part that is what countries are doing."

The pact is part of the United Nations Framework Convention on Climate Change, which started in 1992 with the Rio Earth Summit. Technically, it is not a treaty, so its adoption did not require U.S. Senate approval.

Is it mandatory?

It works as a binding but voluntary program. Every five years each participating country is required to submit a goal or plan for what it will do about heat-trapping emissions of carbon dioxide, methane and other gases. Those goals — called National Determined Contributions, or NDCs — are supposed to be more ambitious every five years, Cambridge University climate negotiations historian Joanna Depledge said.

The latest five-year pledges are due next month. Biden submitted a plan for the U.S. last month to reduce emissions by as much as two-thirds by 2035 compared to 2005 levels.

"The countries themselves" decide what it's in those goals with no punishment for countries missing goals, Depledge said.

Every two years, countries have to report how much greenhouse gases they emit.

The pact also says rich countries, such as the U.S., need to help poor countries decarbonize their economies, adapt to the impacts of climate change and be responsible in some ways for damage done by climate change.

Last year international negotiations set a goal of rich nations contributing $300 billion a year to help poor nations with climate change. The U.S. disputes that the $300 billion goal is legally binding, Depledge said.

How much does it cost?

No industrialized country is assigned a portion of the $300 billion.

Historically, the U.S. was criticized for providing less than its share of the global financial climate aid, given its history as a major climate polluter and it being the world's largest economic power.

"Formally, there is no agreement on how much the U.S. should provide. However, our work on Fair Shares — based on U.S. historical emissions and ability to pay — finds that the U.S. contribution should be $44.6 billion per year," Mercy Corps climate lead Debbie Hillier said.

Last year, Biden announced that the U.S. climate aid to poor nations was up to $11 billion a year.

How did it come to be?

The 1998 Kyoto Protocol — which Al Gore and the Clinton administration helped forge — called for mandatory emission cuts but was rejected by nonbinding votes in the U.S. Senate. President George W. Bush withdrew the U.S. from the deal.

That eventually led to an agreement being fashioned in Paris in a way that didn't need U.S. Senate approval and was not mandatory. A 2014 bilateral deal between the U.S. and China paved the way for the Paris agreement.

"One of the main reasons that countries are not legally required to actually meet the emissions reduction pledges they put forward under the Paris Agreement is because the Obama administration indicated that with the increased political polarization around climate change over the two decades following the Rio Earth Summit, obtaining 67 votes in support of the agreement in the U.S. Senate would have been challenging," said veteran climate analyst Alden Meyer of the European think-tank E3G.

Has it worked?

Last year Earth temporarily passed the primary Paris 1.5 degree threshold, several of the global monitoring groups said. While the 1.5 degree goal is about a 20-year average, the overwhelming majority of scientists say the world is likely to eventually breach the 1.5 mark for good.

The long-term warming is now 1.3 degrees — 2.3 degrees Fahrenheit — above preindustrial times. Climate Action Tracker, a group of scientists, has the world on path for 2.7 degrees Celsius, or 4.9 degrees Fahrenheit, of warming.

Experts call the Paris agreement a partial success, saying negotiators never figured the deal alone would be sufficient.

Mercy Corps' Hillier said that while reduced warming projections are "far from sufficient, it is shows that the collective commitments under the Paris Agreement have made a difference."

What does withdrawal mean?

Once withdrawn, the U.S. can attend negotiations but not be part of decision-making.

There's little direct impact on domestic U.S. climate policy, but "the decision may undermine U.S. credibility in climate diplomacy, likely reducing its influence in global environmental policy," said Scott Segal, a Washington lawyer who represents energy interests, including fossil fuel companies.

Several experts say the U.S. will lose out on a trillion-dollar-plus renewable energy boom, leaving other countries like China to rule the green economy.

"The world is more likely to warm slightly more," Climate Analytics and scientist CEO Bill Hare said. "The more the world warms the faster we will experience more extreme weather events such as flooding, extreme hurricanes, fire, weather, drought, and heat. The U.S. will not be exempt from such events."