During his State of the State address last week, California Governor Jerry Brown defiantly declared, “We cannot fall back and give in to the climate deniers.” Just hours after President Trump announced his intention to resume construction of the Keystone XL and Dakota Access pipelines, Brown declared, “The science is clear,” and said there is much California can and will do on its own to combat the climate crisis.

A coalition of climate-justice advocates and labor groups in the Bay Area have a proposal that they say is a prime example of how California can do this. In spite of its reputation as a haven for environmentalism, California is home to the third-largest oil-refining sector in the United States, which exports a considerable amount of gasoline, jet fuel, propane, and other fossil-fuel products to surrounding states. Oil processing is already California’s largest industrial emitter of greenhouse gases, but things could get even worse in the coming years: The state’s refineries have developed a greater technical capacity to convert lower-quality, denser oil into engine fuels than those in other parts of North America, meaning they’re at the leading edge of the oil industry’s long-term pivot towards refining dirtier-burning sources, including the tar sands—something California’s existing climate policies may do little to prevent.

In response, a coalition of groups, including Communities for a Better Environment (CBE), the Sierra Club, 350 Bay Area, the Asian Pacific Environmental Network, the California Nurses Association, and numerous others, are pushing to make the San Francisco Bay Area the first place in the world to place limits on oil refineries’ overall greenhouse-gas (GHG) and particulate-matter emissions. The proposal would prevent oil corporations from making the Bay Area a center of tar-sands refining by enforcing a cap based on historic emissions levels.

But not everyone agrees on this approach. Staff members with the Bay Area Quality Management District (BAAQMD), whose board of directors will vote on the proposal in May, oppose the emissions cap. They say it’ll simply push GHG emissions into other areas of the country and might even interfere with California’s effort to combat climate change through its cap-and-trade program, a stance shared by the oil industry. For his part, Governor Jerry Brown has yet to weigh in on the Bay Area caps. With less than four months until the vote, emissions cap proponents are doing everything they can do ensure the proposal’s passage.

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On a weekday morning this past June, CBE Senior Scientist Greg Karras was among dozens of people who signed up to speak about the emissions cap at a BAAQMD meeting inside a plush office tower near San Francisco’s Embarcadero waterfront. The idea for a cap on oil refinery emissions was born from an incident some 15 miles and a world away from San Francisco’s financial district. In 2012, an explosion and fire at Chevron’s massive refinery complex in Richmond—an industrial East Bay city predominantly composed of low-income and working-class people of color—endangered 19 workers and sent 15,000 neighbors to local hospitals with respiratory ailments. Within months, a coalition of environmental-justice, environmental, and labor groups had organized to oppose the oil companies’ push to refine cheaper, dirtier crudes.

After years of lobbying the BAAQMD to implement strong refinery-wide emissions regulations, the coalition submitted a proposal for “enforceable numeric limits” on emissions in September 2015. The proposed cap is based on historic emission levels, so it would not force refineries to change their business, but it would prevent them from processing dirtier oil.

At the meeting in June, BAAQMD’s board of directors was finally receiving a presentation from their executive staff on the emissions-caps proposal’s feasibility. Members of the coalition proposing the cap watched as the BAAQMD staff explained their opposition to the plan, saying it would undermine the state’s cap-and-trade system, which allows companies that lower emissions to sell pollution “allowances,” thus permitting other companies to increase emissions, or purchase a certain number of “offsets” from carbon-saving projects elsewhere in the United States or in Quebec. BAAQMD Advisory Council Chairman Stan Hayes said he agreed with the goal of reducing greenhouse-gas emissions, but that regional refinery caps would lead to “a game of climate change Whack-a-Mole,” where one area swats down emissions and then they pop up somewhere else.

By the time Greg Karras’s turn came to speak, his frustration was palpable. Karras has been instrumental in conceptualizing the cap, and he has served as one of its chief defenders. “Staff and its advisory council ignore the urgency and effectiveness of curing the failure to put any overall emissions limit on refineries because they ignore four undisputed facts,” he began. He then recited a string of devastating information he has developed through years of intensive study: Bay Area refineries’ collective carbon-dioxide emissions have increased; refining tar sands and other more polluting crudes would drive these emissions drastically higher; and long-lasting infrastructure to produce lower-quality and tar-sands oil is already being constructed.

At the following meeting, the BAAQMD’s directors unanimously voted to set a date for a vote on the caps: May 17, 2017. In the meantime, the dissenting staff members are crafting an environmental-impact review of both the caps idea and a separate pollution-control regime that they view as a preferable alternative. Advocates of the caps are working to secure resolutions of support from Bay Area city councils and continuing to turn out for the BAAQMD’s meetings.

The emissions caps would have both local and global implications, proponents say. The Los Angeles Basin and the San Francisco Bay Area are, respectively, the first- and second-largest oil-refining centers west of Houston, and the Bay Area is an especially large exporter of gasoline, jet fuel, diesel, propane, and other petroleum products to surrounding states.

Some of California’s main petroleum sources—including Southern California and Alaskan north-slope oil fields—are in decline. Similar to the tar sands, a sticky mixture of sand, clay, and bitumen that involves flaying Canada’s boreal forest, many of these existing crudes are uniquely dense. As a result, West Coast refineries have developed the capacity to convert these denser oils into fuels. They’re now better equipped to do that than refineries anywhere else in the world, and it might be up to the West Coast to provide the infrastructure needed to handle the scale of tar sands–refining oil corporations are seeking in the near-future.

”The tar sands are potentially very cheap, and a lot of refineries in California and Washington are already optimized to process it,” explained Joshua Axelrod, a policy analyst at the Natural Resource Defense Council who co-authored a 2015 report called “West Coast Tar Sands Invasion.”

In addition to being California’s largest industrial emitter of greenhouse gases, oil processing is the state’s top industrial emitter of toxic chemicals and lung-penetrating particulate matter, making it a major cause of asthma, cancers, and other maladies. The tar sands contain high concentrations of toxic metals, chemicals, and sulfur, which corrode pipes and could lead to more refinery explosions such as that in Richmond in 2012, local residents fear.

Tar-sands bitumen also causes more climate pollution at all stages, from extraction to refining, than virtually any other fuel source. A 2015 report in the journal Nature found that trillions of dollars’ worth of known and extractable coal, oil, and gas reserves—including nearly all remaining tar sands and all Arctic oil and gas—must remain in the ground if the global temperature increase is to be kept under the safety threshold of 2 degrees centigrade, agreed to by the world’s nations in Paris.

Recent tar sands-oriented proposals by Bay Area refineries include 100-car oil trains and new processing units.

Karras notes that emissions caps “do not require any refinery to do anything differently.” Nevertheless, he notes, they would accomplish something unprecedented: They would effectively bar the five major oil refineries in the Bay Area from processing that portion of the world’s oil reserves that pose the greatest threat to the global climate.

It’s an idea that could spread to other refinery regions, including those in Los Angeles and Washington State, proponents say, thereby cutting off the tar-sands invasion at the knees. “We see the emissions caps as a model that other air districts can and should adopt,” says CBE Community Organizer Andrés Soto, who grew up downwind from Chevron’s Richmond refinery. Soto suffers from adult-onset psoriasis, an autoimmune disorder linked to air pollution that attacks the skin’s surface.

The oil industry has rallied against the emissions caps, including by lobbying at several BAAQMD meetings. Refinery managers have claimed that the emissions caps would kill local jobs and echoed the argument that it would push pollution to other locations. By a strange turn, BAAQMD’s own executive-staff members have been their most effective ally, prompting critics to label these staffers too cozy with the industry they regulate. “Staff with the Air District authorized permits for four tar-sands–related infrastructure projects since 2013,” CBE Staff Attorney Roger Lin says. “Yet it took them four years to bring the emissions-caps proposal before the board of directors for full consideration.”

The powerful California Air Resources Board also dealt the caps proposal a setback in September 2015, when its executive officer, Richard Corey, wrote an anti-caps letter to BAAQMD Chief Executive Officer Jack Broadbent. Corey said the caps would undermine the “efficiency” of cap-and-trade, the state’s free market–style pollution-reduction program.

Since Corey’s letter, however, the California Legislature has adopted multiple pieces of legislation that go beyond cap-and-trade, most notably last year’s Assembly Bill 197 and its companion Senate Bill 32, which require the California Air Resources Board to prioritize direct-emissions reductions by industrial emitters such as refineries.

Ultimately, the 24 county supervisors and City Council members who comprise the BAAQMD board of directors will decide the caps’ fate at their meeting a little over four months from now. Their districts include nine counties, ranging from Santa Clara County (home of Silicon Valley) in the south to Napa and Sonoma counties in the north.

Oakland City Council member Rebecca Kaplan, one of the BAAQMD board’s most vocal emissions-caps advocates, says the proposal gives local elected officials a rare opportunity to make a significant contribution to counteracting the climate crisis, while also fulfilling its mandate to protect local air quality.

“The emissions caps are a prime example of how a Bay Area regional government body can advance a progressive agenda amid the Trump administration’s efforts to take us backward,” says Kaplan, who also helped lead the Oakland City Council’s opposition to a proposed Oakland coal terminal.

As one of the few oil-refinery scientists who serves as an expert on refinery engineering issues for environmental nonprofits and doesn’t take a paycheck from the oil industry, Greg Karras has a long history to draw on of going toe-to-toe with industry scientists and recalcitrant government officials. In the past 30 years, he and his CBE colleague Julia May—the organization’s Los Angeles–based senior scientist—have helped create several national precedents in air, water, and toxic-pollution control. May refers to this approach as “trickle-up regulation.”

Karras notes that, although California refineries are already uniquely well-equipped to process the world’s dirtiest crudes, they are nevertheless in the process of committing enormous capital to the construction of new tar sands–refining equipment. The equipment’s estimated lifespans range from 30–50 years, meaning they are attempting to commit to tar-sands production for at least that many years to come.

So far, opposition by community groups has fended off most of these proposals even without the emissions caps, but it’s uncertain how much longer these efforts can hold out, Karras says.

“This is the crossroads,” Karras argues. “From here, it’s either the stoplight or the cliff. If word gets out that Bay Area elected officials are the ones with the power to create that stoplight, I think we’ll succeed.”