Over more than a decade, taxpayers have forked out millions of dollars to pay for environmentally friendly equipment to help improve the air quality around the nation’s busiest ports.

Many of the exhaust-spewing vehicles chugging into coastal cargo hubs at Los Angeles and Long Beach have disappeared, replaced with cleaner-burning alternatives. Pollutant levels have plummeted. And visits to the hospital for asthma have declined.

Can the ports reduce pollution further? Experts say it’s like a dieter trying to lose the last stubborn five pounds — now comes the hardest part.

Despite all the improvement, the port corridor remains the largest stationary source of pollution in the region.

“There was a lot of low-hanging fruit,” said Chris Cannon, director of environmental management at Port of Los Angeles. “Now the low-hanging fruit is gone. It is going to be much harder to achieve emission reductions.”

And the road to greener ports is getting more tangled:

• The effort to replace mammoth fossil fuel-powered gear with electric engines is expensive.

• A new administration is shaking things up in Washington. It’s unclear if a new president and new leadership at the Environmental Protection Agency will embrace the alternative fuel boom fostered by President Barack Obama.

• Closer to home, Gov. Jerry Brown last year ratcheted up his expectations for a zero-emissions future for the state’s industry.

• Environmentalists demand fast action and goals with shorter timetables — while organized labor is slow to embrace any technology that could cut union jobs.

Amid all the hubbub, the ports have a plan, and they’re working to update it.

The green road map

Zero-emission port operations, powered almost entirely by clean-burning gear, has long been the dream of California environmentalists who railed at the pollution-steeped past of the Los Angeles and Long Beach harbors.

That’s exactly what port leaders are attempting to move toward with their Clean Air Action Plan.

Officials are working on an update to their decade-old anti-pollution strategy, which they credit with helping to cut harmful diesel particulate matter linked to respiratory ailments by 85 percent.

The draft of the update, released in November, proposed a reduction in greenhouse gases to levels 80 percent below 1990 rates by 2050. Officials also want to slash emissions from ships and cargo-moving equipment such as cranes and forklifts.

The draft, however, rankled the very people who for more than a decade have been cleaning up the port.

Industry is weary of any new rules that will empty out its pocketbooks — such as a proposal that could place a fee on older trucks that call at the ports.

Indeed, converting to cleaner fuels won’t be cheap. The Pasha Green Omni Terminal Demonstration Project — replete with recharging solar battery stations and electric forklifts — broke ground at the Port of Los Angeles last year. The price tag: $26 million, $14.5 million of which would be paid largely through and cap-and-trade funds.

Consulting firm Moffat & Nichol estimates it would cost $23 billion for the two ports and Oakland to replace equipment with all zero- or near zero-emission technology.

Unions worry the ports’ drive to clean up will prompt officials to invest not only in greener technology but in computerized robotic equipment that could cost blue-collar union jobs.

The plan is advancing as regional air regulators look to update their own 15-year plan, which until now has relied heavily on voluntary compliance. Some on that board are pushing to slash emissions at the ports, too.

“It’s really a big challenge for us trying to find the balance, being protective of community health and reducing environmental impact, and at the same time remaining economically viable,” said Heather Tomley, director of environmental planning at the Port of Long Beach.

Uncertain days at EPA

Further complicating matters is the new administration in Washington and what could be very different priorities at the EPA.

Under the Obama administration, big incentives helped alternative-fuel businesses flourish. In that wave, the ports benefited from millions of dollars to replace trucks and cranes.

While it’s not yet certain how President Donald Trump feels about alternative fuels, it’s clear he’s bullish about domestic oil.

Trump has promised to boost drilling and clear paths for major pipelines such as the Keystone and Dakota Access projects. And his cabinet includes appointees friendly to the oil industry.

Will Trump also extend Obama’s efforts to support green power? Boosting both isn’t out of the question. Officials from such industries as solar and wind power have tried to persuade the president that their businesses are booming. And that could gain favor with a business-buoying chief executive who promised the nation jobs.

“There is so much uncertainty on what is going to happen with the current administration, so we don’t know how funding or regulations are going to impact what we do here in California,” Weston LaBar, executive director of the Harbor Trucking Association. “Industry is not against clean technology, but the tech has to be affordable and commercially viable.”

Oklahoma Attorney General Scott Pruitt, nominated to run the EPA, has a history of criticizing and suing the agency. Environmentalists cite his tight relationships with oil and gas industry executives who have donated to his political campaigns.

In the past, Pruitt has been quoted questioning the validity of global warming, a phenomenon Trump once declared a hoax.

But in response to questions from Democrats during his Senate confirmation hearing, Pruitt said he disagreed with Trump’s earlier claims that it is just Chinese propaganda intended to harm America’s economic competitiveness. “I do not believe climate change is a hoax,” Pruitt said.

Amid the debate, committee Republicans voted unanimously to send Pruitt’s nomination to a likely vote before the full Senate in the coming days. He is expected to be confirmed along largely party lines.

After that vote, the scene at the EPA — and Trump’s feelings about cleaner-burning fuels — should become clearer. And then perhaps port officials might know if they can count on rekindled incentives to help fuel their green dreams.

Truck-packed roads

The biggest riddle for ports to solve arguably isn’t the heavy equipment on the docks. It’s the endless parade of big rigs that pick up containers from port terminals and deliver them to acres of warehouses in the Inland Empire and beyond.

Those heavy-duty vehicles — carrying televisions, clothing, auto parts and a wide array of consumer goods — contribute the largest share of the smog-forming nitrogen oxide and greenhouse gasses emitted near the coastal cargo centers.

To discourage older, dirtier trucks, port leaders proposed requiring operators of big rigs 10 years or older to pay a fee beginning next year. And by 2035, the cargo haulers would have to drive only zero-emission vehicles.

But truck operators, who in the last decade have spent $1 billion to buy cleaner-burning engines, believe the goals go too far.

“The more expensive they make it to move cargo, the less likely cargo owners will import and export their cargo through the ports of Los Angeles and Long Beach,” said LaBar of the trucking association.

The Pasha projects fueled officials’ faith that electric engines could eventually replace diesel power, but LaBar remains skeptical. He believes the ports rely too much on the notion that electric power will fix the problem. Earlier efforts to adopt LNG technology failed, he said, and wound up dearly costing truckers who bought vehicles powered by that fuel.

Currently, electric trucks don’t yet have the gusto to haul containers that can weigh up to 95,000 pounds. And electric trucks can cost three times more than their petroleum-powered counterparts.

LaBar is also no fan of tougher rules. “Onerous regulations,” he said, “increase the cost of doing business here.”

Move it, move it

Amid all these competing interests, port officials say one thing is clear: they need more time.

They plan to ask commissioners to extend the public discussion period on their environmental update, which was originally set to end in mid-February.

“We need to figure out what the path is going forward,” said Tomley, the Long Beach port’s environmental planning director and a caretaker of its Green Port Policy.

Technology, while moving at a brisk pace, isn’t going to move quickly enough to solve all the ports’ puzzles this year.

And environmentalists, increasingly concerned about climate change, shun any effort that isn’t about immediacy.

“It’s stating the obvious that the ports have to do more quickly,” said Adrian Martinez, an attorney for Earthjustice.

Tough standards need to be in place, he said, and the port can’t just shoot for goals 30 years down the line.

“We have to get a lot more pollution reductions to make it safe to breathe,” Martinez said. “Until we see a plan that commits to an enforceable path to make that happen, there is going to be that tension between environmentalists, the community and the port.”

The Associated Press contributed to this report