VATICAN CITY - At a climate summit at the Vatican, California Gov. Jerry Brown said the world may have reached a tipping point on global warming and that humanity must reverse course or face extinction.
He said ``millions are suffering already.''
Brown is at the Vatican amid stepped up efforts to bring support for policies to reduce greenhouse gas emissions. The summit was called by Pope Francis following the recent release of his encyclical on climate change.
Brown addressed dozens of mayors and other local government officials from around the world Tuesday, issuing criticism of Republican politicians and business interests skeptical of climate change.
The governor is also scheduled to address the gathering Wednesday.
Brown, a onetime Jesuit seminarian, has made climate change a central theme of his tenure in his second stint as California governor.
Francis at the summit said he has "a lot of hope'' that negotiators at Paris climate talks will reach an ambitious agreement to reduce global warming.
The Vatican also is angling to make sure the U.N.'s new Sustainable Development Goals, to be finalized in September, address the problems of human trafficking and modern-day slavery.
San Francisco Mayor Edwin Lee also attended the summit and announced the city and county of San Francisco will "completely phase out the use of petroleum diesel'' in its municipal fleet of vehicles and replace it with renewable diesel by the end of the year.
"This action will reduce the greenhouse gas emissions from our diesel fleet by over 60 percent,'' Lee said, to applause.
He added that thanks to federal and state credits, San Francisco's fuel costs will actually be reduced as a result.