On 20 January 2021, Joseph Robinette Biden Jr. was inaugurated as the 46th president of the United States. On that day, he faced an unprecedented set of challenges, including global climate change – one of four stated policy priorities of his administration (along with the coronavirus pandemic, economic recovery and racial equity53) – in addition to the immediate issue of the then looming Senate trial of President Trump and ongoing threats of violence from Trump supporters in the wake of the 6 January insurrection at the US Capitol. Because climate change is a global commons problem (Stavins, 2011) and international cooperation is necessary to limit free-rider incentives, Biden had promised during the presidential campaign (Democratic National Committee, 2020) to initiate on the day he would take office the process of re-joining the Paris Agreement (from which President Trump withdrew the United States on 4 November 2020 – the earliest date permitted by the agreement). Thirty days after Biden filed the necessary paperwork with the United Nations on Inauguration Day, the United States again became a party to the Paris Agreement, on 29 February 2021. That was the easy part.
Stavins, Robert N. "What will happen with US climate policy under the Biden Administration?" Greening Europe's Post-COVID-19 Recovery. Ed. Simone Tagliapietra, Guntram B. Wolff, and Georg Zachmann. Bruegel, February 2022, 140-151.