Even the greenest, most eco-friendly politicians rarely utter the words Gov. Jerry Brown spoke at the Vatican’s climate change symposium last week.

To prevent the worst effects of global warming, one-third of the world’s known oil reserves must remain in the ground, Brown told the gathering of government officials from around the world. The same goes for 50 percent of natural gas reserves and 90 percent of coal.

But it’s a concept few politicians will touch. That’s because it raises a question no one wants to answer: Whose oil has to stay put?

“They’ve all got their own oil,” said environmental activist and author Bill McKibben, who first popularized the issue with a widely read 2012 article in Rolling Stone. “Recognizing that you’ve got to leave your own oil — and not somebody else’s — in the ground is the next step.”

Take California.

No state has done more to fight global warming. By 2020, under state law, one-third of California’s electricity must come from the sun, the wind and other renewable sources. Brown wants 50 percent renewable power by 2030 and has called for slashing the state’s oil use in half by the same year.

But he has shown no interest in cutting the state’s oil production. He has touted the economic potential of California’s vast Monterey Shale formation, whose oil reserves drillers are still trying to tap. And he has steadfastly refused calls from within his own party to ban fracking.

“If we reduce our oil drilling in California by a few percent, which a ban on fracking would do, we’ll import more oil by train or by boat,” Brown told “Meet the Press.” “That doesn’t make a lot of sense.”

California remains America’s third-largest oil producing state, behind Texas and North Dakota. The industry directly employs 184,100 Californians, helps support an estimated 271,840 other jobs and yields $21.2 billion in state and local taxes each year, according to the Los Angeles County Economic Development Corporation.

“Just like we have a plan for increasing renewables, we need a plan for phasing out oil drilling in California,” said Dan Jacobson, state director for Environment California.

It’s difficult for politicians to even talk about something as stark as putting limits on pumping oil, he said.

The percentages Brown cited come from a study published this year in the scientific journal Nature. The researchers calculated that in order to keep average global temperatures from rising more than 2 degrees Celsius — 3.6 degrees Fahrenheit — above preindustrial levels, the world’s economy can pump no more than 1,100 gigatons of carbon dioxide into the atmosphere between 2011 and 2050. Burning the world’s known fossil fuel reserves would produce roughly three times that amount, they wrote.

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