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Jeff Chiu, Associated Press
California Gov. Jerry Brown, right, speaks as Virgin Group founder Sir Richard Branson, left, and Dr. Rajendra Pachauri, chair of the United Nations' Intergovernmental Panel on Climate Change, listen during a panel at the Governor's Conference on Extreme Climate Risks and California's Future at the California Academy of Sciences in San Francisco, Thursday, Dec. 15, 2011.

SAN FRANCISCO — The United Nations' top climate official on Thursday lauded California's efforts to help mitigate global warming by reducing greenhouse gases but said the state needs to more quickly adapt to the risks that extreme weather and a rising sea pose to agriculture and the coastline.

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 said.

"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 lumped together global-warming skeptics, including GOP lawmakers and the Cato Institute, calling them 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.

"The Cato Institute has speakers that say environmentalism is a greater threat to capitalism than Marxism itself," he said, evoking laugher from the audience.

Patrick J. Michaels, a senior fellow in environmental studies at Cato, said the institute has never denied climate change, but disputes temperature projections by the UN, saying the sensitivity of temperature to changes in carbon dioxide levels have been overestimated.

"Governor Brown clearly has not read anything that the Cato Institute has published on global warming. Rather than deny it, we believe that indeed the surface temperature of the planet is about one degree Celsius warmer than it was 120 years ago and that increasing atmospheric carbon dioxide has contributed to this," Michaels said in a statement to The Associated Press.

"On the other hand, it is also clear that the rate of observed warming is falling beneath the midrange projections from the United Nations' Intergovernmental Panel on Climate Change."

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's, 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.