The Paris Climate Agreement: What You Need to Know

President Biden has recommitted the United States to the Paris climate agreement, the international accord designed to avert catastrophic global warming.

The move by Mr. Biden fulfills a campaign promise to return to the pact on day one of his presidency. A letter to the United Nations signed by the president on Wednesday formally started the 30-day process of bringing the United States back into the accord.

What does the decision mean? We have extensive coverage here at The New York Times, but it is a complex issue and you may be wondering where to start.

In December 2015, nearly every country in the world, 195 in all, agreed to the first global pact aimed at reducing emissions of planet-warming greenhouse gases. It was a landmark diplomatic achievement and the pinnacle of President Barack Obama’s environmental agenda.

Scientific studies show that if the world’s carbon emissions continue unchecked, atmospheric temperatures will continue to rise. The planet won’t just be hotter, but it will also suffer from rising sea levels, more powerful storms, droughts that lead to food shortages and other severe disruptions.

The idea of the Paris climate accord was that every country, rich and poor, would set goals to curb carbon emissions in an effort to avert the worst effects of climate change.

Former President Donald J. Trump has called climate change a hoax, and he pledged during his campaign in 2016 to “cancel” the Paris deal.

Early in his presidency, Mr. Trump made clear his belief that joint global action on climate change was incompatible with his “America first” message. He also predicted that the Paris Agreement would be harmful to the economy. Environmental advocates disputed the argument that the Paris accord would cost jobs.

Mr. Trump announced on June 1, 2017, that the United States would withdraw from agreement. He cited some dubious data in his speech.

Under the terms of the agreement, the United States could not formally exit until Nov. 4, 2020. That happened to be the day after the presidential election, a time frame that guaranteed the agreement would be a 2020 campaign issue.

While Americans make up just over 4 percent of the world’s population, they are responsible for almost a third of the excess carbon dioxide in the atmosphere. China emits more carbon into the atmosphere today, but the United States had a long head start on burning coal, oil and natural gas. Some said it was a moral imperative for the United States to stay in the Paris pact.