A trillion here, a trillion there. Senator Elizabeth Warren on Tuesday night proposed spending $1 trillion for “decarbonizing our electricity, our vehicles, and our buildings.” That, she said on Twitter, was “on top of the $2 trillion I’ve already committed to green research, manufacturing, and exporting. We’ll create millions of jobs and achieve the goals of the #GreenNewDeal.” With the top Democratic candidates about to take part in a Wednesday CNN town hall on climate change, most have put forth sweeping plans that would transform the U.S. ecomomy. Yet the proposals would be enormously expensive, and their authors have come...