Getting the United States out of the Paris climate accord.
His own words from his speech to Congress: : "The United States accounts, as all of you know, for less than 15 percent of carbon emissions. The rest of the world accounts for 85 percent. That’s why I kept my commitment to rejoin the Paris Accord,
because if we do everything perfectly, it’s not going to matter."
https://www.nytimes.com/2021/04/29/us/politics/joe-biden-speech-transcript.html