One of the first things President Biden did after his inauguration was to sign an executive order saying the US will rejoin the Paris Agreement. Today, that move is official. United States Secretary of State Antony Blinken posted a tweet saying the US "is once again a Party to the Paris Agreement."
Blinken went on to say that "the work to reduce our emissions has already begun" and that "we will waste no time in engaging our partners around the world to build our global resilience."
The Paris Agreement is an international accord with 143 countries to work on reducing greenhouse gas emissions and was made in 2015. Barack Obama signed the US on in 2016, but Donald Trump withdrew the next year, saying he wanted to "give America a level playing field." Trump's move attracted plenty of criticism, including from Elon Musk, who threatened to (and later did) leave Trump's Strategic and Policy Forum where he advised the president. 25 companies wrote an open letter published in the New York Times begging the then-president to keep the US in the agreement.
The process of leaving the accord formally began in 2019, and a four-year rule tied to the agreement made it so that the date for a country to officially withdraw would be four years after joining. That ended up being November 4th 2020 — the day after Election Day in the US. Rejoining, however, appears to be a much speedier process.