Joe Biden says he will immediately rejoin the Paris climate pact if he is elected US president, but Chinese observers say that even if he does and Washington and Beijing work together on the issue, they will still have plenty more challenges to overcome.Biden made the pledge on Twitter on Wednesday, the day the US formally left the accord reached in the French capital in 2015.“Today, the Trump Administration officially left the Paris Climate Agreement. And in exactly 77 days, a Biden…