As President Trump moves to reverse the Obama administration’s policies on climate change, California is emerging as America’s de facto negotiator with the world on the environment. The state is pushing back on everything from White House efforts to roll back pollution rules on tailpipes and smokestacks, to plans to withdraw or weaken the United States’ commitments under the Paris climate change accord. In the process, California is not only fighting to protect its legacy of sweeping environmental protection but also holding itself out as a model to other states — and to nations — on how to fight climate change.