Under President Donald Trump, the U.S. officially withdrew from the Paris Agreement on global warming on Nov. 4. Trump’s successor, Joe Biden, wants back in. But averting the worst effects of climate change will be an uphill battle even with the involvement of the U.S., the second-largest producer of carbon dioxide emissions, behind China. Global warming already is fueling wildfires, hurricanes and mass migrations, and in the U.S., Trump dismantled regulations meant to stifle greenhouse gas emis