Huffington Post: Ironically, the most ambitious U.S. action in the fight against global warming is coming from big cities and their mayors. It seems preposterous on its face. Each city’s emissions are only a tiny fraction of the global pie. Cities can’t force utilities to shift to renewables, or make Detroit produce smarter cars. Nonetheless, some of the biggest U.S. mega-cities are stepping up in a big way on global warming: In San Francisco: Mayor Gavin Newsome committed his city to …