The US' withdrawal from the Paris Agreement caused many to worry for the future success of the agreement due to its large historical and continued contribution to climate change (representing more than a quarter of all historical CO2), and its important role on the global political stage. Its participation is therefore seen as key to the success of any global effort to combat global warming. There were also concerns that its departure would "demoralise" other nations in their efforts to reduce CO2 emissions.
However, President Biden's election win allayed those fears when he signed an executive order on his first day as President to re-join the Paris Agreement. The US hosted a two-day virtual climate summit (the "Climate Summit") on 22-23 April 2021, where it set out its new NDC target to cut carbon emissions by 50-52% below 2005 levels by the year 2030. The Climate Summit was a success and has arguably put the US back on the climate world stage. Read more about the Climate Summit.