San Francisco Mayor Willie Brown proposed yesterday that his city pledge to cut its greenhouse gas emissions by 20 percent below 1990 levels by 2012. Brown said the goal “is as much about protecting our national security as it is about protecting our environmental quality of life.” If the city’s Board of Supervisors passes Brown’s resolution, San Francisco will become the 116th U.S. city to promise to cuts greenhouse gas emissions in the absence of federal leadership on the issue. The Kyoto treaty on climate change, from which the Bush administration has withdrawn, would have committed the U.S. to reducing its overall emissions by 7 percent from 1990 levels by 2012.