One-third of the world’s oil must stay in the ground if humanity hopes to avoid the worst effects of global warming, Gov. Jerry Brown told a climate conference at the Vatican Tuesday.

“We are going to have to set a clear goal,” Brown told a crowd of mayors and public officials from around the world. “And that goal is almost unimaginable. One-third of the oil that we know exists as reserves can never be taken out of the ground. Fifty percent of the gas can never be used and over 90 percent of the coal. Now, that is a revolution.”

For an American politician of Brown’s stature, it was a rare statement. Even those who acknowledge the threat of climate change prefer not to address the idea that tapping all of the world’s known fossil fuel reserves would trigger catastrophic levels of warming, a notion widely embraced in the environmental movement.

But Brown’s comment was particularly noteworthy for another reason.

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California, for all its efforts to fight climate change, remains America’s third-largest oil producing state, out-pumped only by Texas and North Dakota. And while Brown wants to cut California’s use of oil by 50 percent in the next 15 years, he has generally supported oil production within the state’s borders.

Brown has for years refused to ban hydraulic fracturing, preferring to regulate it instead. He has argued that finding a way to tap the oil trapped within California’s Monterey Shale formation could produce an economic boom for the state. His stance has infuriated many environmentalists, even as they laud his efforts to boost renewable power and reduce greenhouse gas emissions.

So Brown’s comments, at the Vatican global symposium on climate change and modern slavery, raised a few eyebrows back home.

“We agree, fossil fuels need to stay in the ground,” said Kassie Siegel of the Center for Biological Diversity, one of the environmental groups pushing for a fracking ban. “That’s why Gov. Brown can’t be a climate leader and expand fossil fuel production in his own state. Climate leaders do not frack.”

Brown urged the gathered mayors to push for climate action within their own countries, saying they needed to “light a fire” under their national leaders. And he took aim at opponents of such action, saying they were “bamboozling” the public with a well-financed disinformation campaign.

“We have very powerful opposition that, in at least my country, spends billions on trying to keep from office people such as yourselves and elect troglodytes and other deniers of the obvious science,” Brown said.

David R. Baker is a San Francisco Chronicle staff writer. E-mail: dbaker@sfchronicle.com Twitter: @DavidBakerSF