When was the last presidential race in which the two leading candidates for a major party’s nomination aggressively competed over who has the best plan to address climate change? Oh, right, never. But 2016 is a new era. This week, the Bernie Sanders and Hillary Clinton campaigns talked smack to each other on social media, fighting for the hearts of climate hawks. It started on Wednesday, when Sanders — who last month released a very ambitious, but legislatively focused, climate plan — challenged Clinton to detail her own plans. Over the last few months, Clinton has reacted to the candidacies of Sanders and fellow climate hawk Martin O’Malley, as well as grassroots activist pressure, by moving left on a couple of key climate issues. After years of avoiding…