Gov. Jerry Brown and the governors of Oregon, Washington and British Columbia gained international attention Monday for signing a pact in San Francisco aimed at reducing the pollution that causes global warming.

But a day later environmentalists lashed out at Brown for his full-throated support during the event of fracking, the controversial practice in which oil and gas companies inject water, sand and chemicals into the ground to fracture underground rock formations and release huge amounts of fossil fuels. Those are the very substances that scientists say are causing more global warming.

Oil pumps on one of the four artificial THUMS islands off the coast of Long Beach, Calif.,  Oct. 3, 2013 . Oil companies have fracked from man-made islands
Oil pumps on one of the four artificial THUMS islands off the coast of Long Beach, Calif., Oct. 3, 2013 . Oil companies have fracked from man-made islands and platforms off the Southern California coast for years, and state regulators are only now realizing that the technique is more widespread than originally thought. (AP Photo/Chris Carlson) ( Chris Carlson )

"To see the governor support fracking at a climate change event is terribly ironic," said attorney Kassie Siegel with the Center for Biological Diversity, an environmental group. "Gov. Brown is a climate leader, but supporting fracking can undermine all that he has done."

As Brown waxed poetic about the dangers of climate change Monday, about 50 people carried signs outside the event protesting his fracking policy. As part of an increasingly energetic campaign, environmentalists -- who are also worried about air and water pollution from fracking -- have begun picketing and protesting the governor whenever he appears at environmental events to point out what they see as hypocrisy.

"I feel my blood pressure rising," said Kathryn Phillips, director of Sierra Club California. "Jerry Brown is a very interesting guy. He's very smart. And I think in this case, he's wrong."

At Monday's event, in response to questions from this newspaper, Brown offered his most extensive remarks yet defending his administration's fracking policy.

Brown said he saw no contradiction in calling climate change "the world's greatest existential challenge" Monday while refusing to impose a moratorium on fracking -- as New York has done, and as many of the environmental groups who supported his campaign for governor in 2010 now want for California.

"As you know, I signed legislation that will create the most comprehensive environmental analysis of fracking to date," Brown said. "It will take a year, year and a half, maybe a little longer. And I hope that all the people, critics and supporters alike, will participate and offer their best thoughts."

Brown was referring to SB4, a bill by state Sen. Fran Pavley, D-Agoura Hills, that requires companies that conduct fracking operations in California to notify all nearby property owners, obtain a permit from the state, conduct groundwater testing and disclose the chemicals that they are using. The law takes effect in 2015.

Brown also said that fracking has helped the environment in some ways, by increasing the supply of natural gas, which burns cleaner than coal.

"In terms of the larger fracking question -- natural gas -- because of that, and the lowered price, the carbon footprint of America has been reduced because of the substitution of natural gas for coal," Brown said. "So this is a complicated equation."

Brown noted that SB4 requires the state to conduct an independent, peer-reviewed scientific study of fracking's impact on air, groundwater, wildlife and climate by Jan. 1, 2015. That document will help policymakers, he said.

The governor also referred to the Monterey Shale, a huge underground formation running from Bakersfield to San Benito County and Modesto that the U.S. Department of Energy has estimated could hold 15 billion barrels of oil.

"Nobody is talking about doing anything there for an extended period of time -- and certainly not before the environmental document," Brown said. "I think we ought to give science a chance."

Asked whether fracking should be banned, as Monday's protesters were demanding, Brown said: "What would be the reason for that?"

Earlier this year, a study released by the University of Southern California concluded that developing the Monterey Shale could create up to 2.8 million jobs and provide California from $4.5 billion to $24.6 billion in new tax revenue.

Chris Field, a Stanford biology professor and internationally renowned climate expert, said that Brown is correct that increasing natural gas production can reduce America's greenhouse gas emissions by displacing coal. But he noted that when oil and gas companies have leaks in their piping and drilling systems, the release of just 3 percent of the methane they are trying to capture can negate the climate benefits, because methane traps heat at 21 times the rate of carbon dioxide in the atmosphere.

Still, he added, drilling for oil in California, assuming it's strictly regulated, is cleaner than burning coal or getting it from Canadian tar sands.

"Dealing responsibly with the challenge of climate change," he said, "involves careful balancing of providing the energy resources that we need in the near term while recognizing that in the long term we need to move away from fossil energy sources."

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/paulrogerssjmn.