Rajendra Pachauri, chairman of the United Nations Intergovernmental Panel on Climate Change, joined scientists, California Gov. Jerry Brown and billionaire Sir Richard Branson at a conference at the California Academy of Sciences in Golden Gate Park.
Brown organized the conference, he said, to urge people to "wake up" to the risks posed by extreme weather caused by manmade global warming, and to start thinking about what California ought to do to prepare.
He said the state needs to gird itself against floods caused by the faster snowmelts that are already happening, putting pressure on aging levees and threatening the state's agriculture industry. Warming climate also means longer and more intense wildfire seasons that will threaten homes and infrastructure such as power lines, and affect air quality.
"The greatest obstacle we face is a deep sense of complacency, a sense that things were this way yesterday and were OK and will continue," Brown told the packed auditorium.
"It's difficult to see what's not completely obvious ... the buildup of greenhouse gases and climate change, we see it, it's pretty clear." he said.
Brown called skeptics of global warming, including GOP lawmakers and the Cato Institute, a well-funded "cult" that disagrees with the vast majority of published, peer-reviewed climate science.
"The main thing we have to deal with in climate change is the skepticism, the denial and the cult-like behavior of the political lemmings that would take us over the cliff," Brown said.
"There is a wide consensus among thoughtful people, even though deniers and skeptics have created quite a PR campaign," he said.
A request for comment from the Cato Institute was not immediately answered.
Pachauri said UN studies show that 95 percent of human deaths associated with extreme weather events happen in developing countries.
Yet he said the world's large economies, such as California, can make great strides toward helping reduce global greenhouse gas emissions, especially through the simple task of retrofitting existing buildings.
"If one could retrofit buildings to make them more efficient, and if new buildings could be built to current standards, it's really a win-win situation," Pachauri said. "Overall the building sector has the largest potential for the reduction of emissions."
Former Gov. Arnold Schwarzenegger also is expected to attend the conference Thursday afternoon.