California’s crusade against planet-warming emissions seems at times disconnected from the reality of its gridlocked freeways. But that hasn’t stopped a push for change.

State officials want new cars to burn less gasoline for each mile they travel, and to use cleaner fuel. They are making electric cars easier to buy and adding bike lanes along major thoroughfares. Cities and counties have ripped apart streets to build new rail lines and bus corridors.

But California will have to do far more if it wants to meet a crucial 2030 climate change goal. Then, state greenhouse gases must fall roughly 40% from what they are now. It’s a steep, unprecedented drop — and whether California can transform transportation will determine whether it can meet those goals.

The challenge is huge. Even as power plants and other sectors have cleaned up, greenhouse gas emissions from transportation in California have actually grown in recent years, a sign that cleaner and more fuel-efficient cars have been unable to overcome population growth, a growing economy and suburban sprawl.

More worryingly, transportation accounts for about 40% of the state’s emissions tally, more than any other sector, with most of the pollutants coming from passenger vehicles.

“It’s the most stubborn and difficult part of the work we have ahead of us,” Gov. Gavin Newsom said recently.

Slashing emissions likely requires a dramatic shift in lifestyle and human behavior: more housing packed next to transit, motorists retiring their gas-burning vehicles to drive electric cars, and an intricate but well-functioning transportation system that encourages people to drive less.

California must make those sweeping changes while defending itself against the federal government, which is trying to roll back a series of environmental policies. Just last week, President Trump said his administration would revoke the state’s authority to set its own auto emission standards. Trump is also trying to unwind Obama-era fuel-efficiency rules and has attacked a deal four automakers reached with California to hold themselves to tougher requirements anyway.

The state is fighting the federal government in court; the latest lawsuit was filed last week.

And politicians are pressing for technological solutions. Assemblyman Phil Ting, D-San Francisco, wants to phase out sales of new gas cars — an idea that’s been championed at the national level by the likes of Sen. Jeff Merkley, D-Ore., and California Sen. Kamala Harris as part of her presidential campaign.

Yet California may not fulfill former Gov. Jerry Brown’s goal of putting 5 million electric cars on the road by 2030 unless zero-emission vehicles become more affordable and easy to charge.

“The only time we have general adoption is when you don’t have to make hard choices,” Ting said. “Think about when cell phones came out and how long it took. You knew at some point, everybody would have a cell phone, but it took a long time.”

California has already taken huge strides getting more clean cars on the roads, expanding its network of charging stations and offering rebates to make pricey electric vehicles more affordable.

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But it takes time to switch people from whatever they are driving now to something cleaner. The results of new policies might not be evident for 10 years, said Simon Mui, a senior scientist in San Francisco for the Natural Resources Defense Council.

And then there’s the even harder part: reconfiguring the landscape so many people don’t have to drive in the first place.

“It will involve not just a transition, but really a transformation of how we move people and goods throughout the state,” Mui said.

In the Bay Area, the quest for a leaner, less fuel-emitting future is complicated by continuing population growth.

Projections show nearly 10 million people living in the nine counties by 2040, and an additional 5 million in the combined outlying regions of Sacramento, San Joaquin, Stanislaus, Merced and Monterey counties. Housing prices, flight to the suburbs and long commutes have all strained the perceived boundaries of the Bay Area.

Take Justin Kreighbaum, a safety officer who spent 2½ years driving more than 70 miles from his house in rural Lake County to work at Novato Community Hospital.

He made the trek twice a week: along a tangle of backcountry roads in his Ford F-150 truck, down Highway 29, past the Petrified Forest and onto Highway 101. Just getting to work took 1½ hours — on a good day.

“The real rough drive is the drive home,” Kreighbaum said. “I can get to Novato in one hour and a half, (maybe) an hour 45. Getting home I’m lucky if it’s two hours.”

Kreighbaum represents a new normal for the Bay Area — superlong commutes that put more and more cars on the roads, as people move far from city centers in search of cheaper housing. Distances previously unheard of have become more common: Clovis to San Jose. Los Banos to Menlo Park. Roseville to San Francisco. Lake County to Novato.

“Any time you stretch the amount of driving someone has to do, you’re increasing the amount of emissions they put in the air,” said Dave Clegern, spokesman for the California Air Resources Board. “The idea that people are driving from a community in the Central Valley to a job in San Francisco, because they can’t afford a home there, is not something we’d like to see.”

Problems with transportation, land use and affordability are intertwined, said Daniel Rodriguez, associate director of the Institute of Transportation Studies at UC Berkeley. Many low-income people suffer long and costly commutes in exchange for bigger and less expensive homes. Some cope by driving older vehicles that tend to be less fuel-efficient.

“Two factors trigger all this (pollution), and they feed on each other,” said Matthew Lewis, an environmental consultant and spokesman for the pro-housing group California YIMBY. “One is sport utility vehicles,” he said, noting that state tailpipe standards allow larger SUVs, or those with four-wheel drive, to emit more than cars.

“The other is the failure of cities to build more housing and mass transit,” Lewis said. “You combine those things, and it’s a ticking climate bomb.”

The deficiencies of public transportation come from an 80-year development pattern that favored the automobile. When the Golden Gate Bridge and Caldecott Tunnel opened in 1937, residents moved to Marin County and to the hills of central Contra Costa County. Contra Costa became part of the BART District, but Marin opted out, and to this day, its transit options are sparse.

Meanwhile, residents throughout the Bay Area have pushed back against housing near transit. BART is finally planning a construction spree with help from a state law that loosens development restrictions on its property. Housing is starting to pop up on the Caltrain railroad line that snakes down the Peninsula, in cities like Belmont, San Mateo, San Carlos and Redwood City. But development isn’t keeping pace with an explosion in job growth.

“A 70-year development pattern isn’t going to be reversed overnight,” said John Goodwin, spokesman for the regional Metropolitan Transportation Commission. He and others pointed out that cities, by resisting density, have pushed the Bay Area’s affordable housing out to suburbs and wildfire zones.

Counties have begun building new transportation systems to reach that population, though it’s not cheap. The SMART train began rolling through Marin and Sonoma counties two years ago; its board is now pitching to voters a sales tax extension to keep the rail line operating. Officials in Tri Valley and San Joaquin County are pressing for a new Valley Link railroad that would run from Stockton to the Dublin/Pleasanton BART Station, and possibly help unclog Altamont Pass. It won’t open until at least 2026.

What’s left is a transportation system with many lines that don’t link up, run by agencies that don’t always work well together. It largely shuts out commuters like Kreighbaum, whose only mass transit option to the Novato hospital is to drive to Santa Rosa and catch a SMART train.

Early this month, Kreighbaum transferred to Sutter Santa Rosa Regional Hospital, where he was already working part time. The commute to Novato was too brutal.

Despite the litany of challenges, Mui, of the Natural Resources Defense Council, said California is fully capable of reducing its greenhouse gas emissions from the transportation sector in a major way.

The problem, he said, is us.

“We are our own worst enemies in this climate,” Mui said. “We have all of the technology solutions and the ability to do this, speaking as a scientist. (But) we resist what’s possible on a daily basis.”

Chronicle staff writer Alexei Koseff contributed to this report.

J.D.Morris and Rachel Swan are San Francisco Chronicle staff writers. Email: jd.morris@sfchronicle.com rswan@sfchronicle.com Twitter: @thejdmorris, @rachelswan