Former President Trump’s return to the White House now jeopardizes a slate of California clean air rules that were intended to advance the state’s climate goals and bring it closer to complying with federal air quality standards.

In the last four years, California has adopted some of the nation’s most innovative air regulations, including a ban on new gasoline-powered car sales by 2035 and a prohibition against diesel-fueled trucks visiting state ports and railyards in 2036.

However, many of these rules, which were approved by the California Air Resources Board, have not been approved by the Biden administration and now face outright rejection by the incoming Trump administration. In the aftermath of election day, climate leaders in California and abroad said they were bracing for broad opposition to many such initiatives.

“There is no denying that another Trump presidency will stall national efforts to tackle the climate crisis and protect the environment, but most U.S. state, local and private sector leaders are committed to charging ahead,” said Dan Lashof, U.S. director of the World Resources Institute. “And you can count on a chorus of world leaders confirming that they won’t turn their back on climate and nature goals.”

Although California is the only state in the nation that has been granted authority to set its own vehicle emission standards, the U.S. Environmental Protection Agency must still approve waivers to make these rules effective. Currently, eight clean air rules await EPA waivers — and some have been waiting for as long as four years.

The choice to defer action on these rules has not only delayed the implementation in California, but in other states that have chosen to adhere to California’s more stringent climate rules instead of the federal government’s.

The Biden administration has until Jan. 19 to sign the rules. Otherwise, it will be up to the incoming administration to decide, and Trump has voiced disdain for California’s unique ability to set emissions standards.

Even if Biden’s EPA acts before the Jan. 19 deadline, a potential Republican sweep of the U.S. House of Representatives and Senate could allow for any newly passed rules to be overturned.

The Congressional Review Act allows Congress to scrutinize any major federal rules adopted within the final 60 legislative session days. If the EPA had decided on the rules in early 2024, the Biden administration could have avoided such a review.

The California rules that are now at risk are:

• The Small Off-Road Engines rule, adopted in 2021, would ban the sale of gas-powered yard equipment, including leaf blowers, lawn mowers and other equipment. It was scheduled to go into effect this year. It is expected to prevent 887 premature deaths and provide $9 billion in public health benefits.
• The Commercial Harbor Craft rule, adopted in 2022, would require new ferry boats and excursion vessels to be zero-emission where feasible. It also calls for more watercraft, including commercial sportfishing boats, to replace their older engines with newer, cleaner models to reduce pollution. It was scheduled to go into effect this year. It is expected to prevent 531 premature deaths and provide $5 billion in health benefits.
• The In-Use Locomotive rule, adopted in 2023, would establish age limits for trains operating in California and gradually phase out diesel engines. The rule would guarantee all train fleets would be zero-emission no later than 2058. It was slated to go into effect this year. It is expected to prevent 3,233 premature deaths and provide $32 billion in public health benefits.
• The Advanced Clean Cars II rule, adopted in 2022, would require an increasing percentage of new cars sold at California auto dealerships to be zero-emission or plug-in hybrids. The regulation would eventually culminate in a ban on selling new, gasoline-powered cars by 2035. It is slated to go into effect in 2026. It is expected to prevent 1,287 premature deaths and provide $13 billion in public health benefits.
• The Advanced Clean Fleets rule, adopted in 2023, would ban fossil-fuel-powered cargo trucks from registering to serve California ports and rail yards. It would ultimately require all cargo trucks serving the ports to be zero-emission in 2035. It also established zero-emission requirements for governmental and large commercial fleets. It was scheduled to go into effect this year. It is expected to prevent 2,526 premature deaths and provide $26 billion in public health benefits.
• The In-Use Off-Road Diesel-Fueled Fleet rule, adopted in 2022, would phase out some of the dirtiest engines from agricultural and construction equipment. It was scheduled to go into effect this year. It is expected to prevent 571 premature deaths and provide $6 billion in public health benefits.
• The Transport Refrigeration Units rule, adopted in 2022, would phase out diesel-powered refrigeration units for cargo trucks. It was slated to go into effect last year. It is expected to prevent 177 premature deaths and provide $2 billion in public health benefits.
• The Heavy-Duty Omnibus rule, adopted in 2020, would establish cleaner engine standards and require warranties for new heavy-duty vehicles. It was scheduled to go into effect this year. It is expected to prevent 2,480 premature deaths and provide $23 billion in public health benefits.

Asked about the Biden administration’s decision to hold off on approving California’s clean air rules, EPA spokesperson Shayla Powell said: “EPA is reviewing the requests closely to make sure its decisions are durable and grounded in the law.”

Collectively, California’s eight pending clean air rules were expected to prevent 11,000 premature deaths and provide $116 billion in health benefits over the next three decades, according to the American Lung Assn. But federal inaction has delayed the enforcement of seven of the eight regulations.

“The benefits that are delivered when these programs are enacted in full are going to benefit everybody,” said Will Barrett, national senior director for clean air advocacy for the American Lung Assn. “We know air pollution is not — and cannot be — a partisan issue. That’s not how we look at it. Everyone deserves clean air and everyone deserves the benefits that these programs will deliver.

“Now, if they’re delayed, we need to make up those lost tons of pollution reductions.”

In two historic federal endowments, the EPA awarded about $910 million in grant money to Southern California to reduce smog-forming emissions from trains, trucks and cargo-moving equipment at the Port of Los Angeles.

However, incentives alone will not enable California to reach its zero-emission goals.

“If any of those are delayed, revoked or anything, it leaves a gap,” Barrett said. “And that gap is paid for by the lungs of the breathers in California. That’s what’s subsidizing polluting industries — people’s health.”

Given the uncertainty, many businesses have been left in limbo, according to Erik Neandross, president of clean transportation solutions for TRC Companies, which provides environmental consulting for trucking fleets. He said many of his clients are working to electrify their fleet, but they are in the minority. Most other companies are deferring action until there’s more regulatory clarity.

“We can’t really get a strong read out of EPA if and when they’re going to give the waiver,” Neandross said.

The Supreme Court recently overturned federal agencies’ ability to interpret vague statutes, delivering a serious blow to environmental regulators such as the EPA. And if California’s rules face legal challenges, Trump-appointed judges still sit on a number of federal courts that could decide California’s environmental issues.

“There are so many question marks, and it’s very easy for the fleets to just say, ‘You know what? We’re just going to wait.’”