"If you grant the president emergency powers to declare a climate emergency, it’s just like COVID.” Such a declaration could give Biden “vast and unchecked authority to shut down everything from communications to infrastructure,” and the ability to censor free speech and freeze financial assets.
-- U.S. Oil and Gas Association President Tim Stewart

The nonprofit Center for Biological Diversity issued a non exhaustive “legal guide” saying that Biden, under an emergency declaration, could:

End crude oil exports
End all offshore oil and gas leasing
Restrict U.S. fossil fuel exports and end hundreds of billions of dollars in overseas fossil fuel investments
Dramatically accelerate a transition to clean energy