What Is the Paris Climate Agreement That Joe Biden Will Rejoin, Why Did Donald Trump Leave?

President-elect Joe Biden plans to rejoin the historic Paris climate agreement on the day he is inaugurated, according to his chief of staff Ron Klain.

So what is it, and when and why did president Donald Trump leave?

What is the Paris Agreement?

Also known as the Paris climate accord and the Paris climate agreement, it is a legally binding treaty aimed at tackling climate change.

Every nation, other than Nicaragua and Syria, signed up to it in Paris on December 12, 2015, pledging to work to keep post-industrial levels of global warming "well below" 2 C, and hopefully at 1.5 C. This would "significantly reduce the risk and impacts of climate change," according to the document. There were 194 signatories as of January 2021, and 189 countries had adopted the accord. Nicaragua and Syria have also since joined.

The signatories, which originally included countries such as the U.S. and China, agreed to work towards reaching a global peak of greenhouse gas emissions as soon as possible, while recognizing that this will take longer for developing countries. Carbon neutrality by the second half of the century was the goal.

Every five years, the targets are revisited and made more ambitious. When then-President Barack Obama signed up, the U.S. agreed to cut emissions by between 26 to 28 percent below levels released in 2005 by 2025.

The treaty came into effect on November 4, 2016. Obama described the accord as a "turning point" and "the best possible shot to save the one planet we got."

When and why did the U.S. leave the Paris Agreement?

In June 2017, the U.S. became the first country to formally withdraw from the agreement, but did not officially leave until November 4, 2020, due to rules in place to prevent signatories from leaving. By chance, this was the day after the 2020 presidential election.

Having previously falsely claimed climate change is a hoax, leaving the agreement was one of the key promises that Trump made on the 2016 campaign trail.

In a statement in 2017, Trump argued that the agreement would negatively affect the U.S. economy and jobs market, would not mitigate climate change, and unfairly favored other countries such as China and India. He said he would attempt to negotiate another deal. "If we can, that's great. And if we can't, that's fine," he said.

In 2019, Trump described it as "terrible, one-sided" and "a total disaster for our country."

In response, over 200 city mayors promised to continue working towards the aims of the agreement, by signing up to the American Climate Alliance.

As well as leaving the Paris Agreement, the Trump administration has rolled back dozens of environmental rules and regulations in the U.S.

The day that the U.S. left, Biden tweeted that the country would rejoin should he be elected president, and it seems he will hold to that pledge. It would take 30 days for the rejoining to come into effect. In addition to signing up to the accord, Biden has announced his administration will spend $2 trillion on a climate change plan.

Some experts believe it will take time to undo the diplomatic damage done by Trump's decision. Michael Oppenheimer, a climate-policy researcher at Princeton University, told Nature: "The United States can't simply jump back in and pretend it's all back to 2015... It will need to work to regain trust."

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President-elect Joe Biden pictured at the Queen theater on December 19, 2020 in Wilmington, DE. Biden plans to re-join the Paris Agreement. Joshua Roberts/Getty Images