Mayors from around the world met this weekend to cut emissions. SAN FRANCISCO – Sunday, when mayors from around the world gathered in this most environmentally aware of American cities to mark World Environment Day, they hoped to make a clear statement: Local communities - even more than nations - can be the pioneers of environmental reform. The choice of place and time could hardly have been more auspicious. In recent months, it has become increasingly obvious that a critical mass is developing around perhaps the most nettlesome issue of modern American environmentalism - climate change - and that states,...