California has been a model for the nation when it comes to cutting climate-warming emissions without sacrificing economic growth, but a new report shows that the state's car culture is presenting a challenge to continuing progress.

Even as fuel efficiency has improved and electric cars have begun catching on, Californians as a whole are driving more, pushed by lower gas prices and longer commutes, according to a report published today by Next 10, a California-based think tank, and Beacon Economics, an independent research firm. Emissions from the transportation sector rose in 2015, slowing the state's overall emissions reductions to just 0.34 percent that year, the most recent year with complete data.

The findings highlight the challenge the state faces, said F. Noel Perry, Next 10's founder, as it will need to cut emissions by about 5 percent per year for a decade beginning in 2020 to hit its climate goals.

"California is facing a very important and critical moment," he said. "We need probably a new set of policies to push us even stronger, faster and more efficiently to reduce these transportation emissions."

After California passed its signature climate change legislation in 2006 establishing a cap-and-trade carbon market, gross domestic product grew by nearly $5,000 per person through 2015, even as emissions fell by 12 percent, according to the report. That economic growth was nearly twice the national average, and job growth in California also outpaced the nation as a whole. Put another way, California produced nearly twice as much economic output per unit of energy than the rest of the country.

Decoupling Economic Growth from Emissions

Historically and in much of the world, emissions have climbed inexorably with economic growth, so this "decoupling" is critical to meeting the ambitious goals of the Paris climate agreement. California has been a beacon for the possibility, and it's not alone. A December report from the Brookings Institution found that more than 30 states have cut emissions while expanding their economies, many at even higher rates than California.

Even as President Donald Trump begins to unravel federal efforts to cut emissions, political leaders in California have sought to strengthen the state's climate policies. In July, state lawmakers passed a bill extending the state's cap-and-trade system through 2030.

So far, California has largely remained on track to meet its ambitious climate goals, which aim to cut emissions to 1990 levels by 2020 and 40 percent below that by 2030. Renewable energy sources accounted for 27 percent of electricity generation last year, ahead of a required 25 percent. And the 2020 emissions target is well within reach. But the cuts will need to accelerate dramatically after 2020. And now, thanks mostly to cars, they're moving in the opposite direction.

The transportation sector now accounts for nearly 40 percent of the state's greenhouse gas emissions, with most of that coming from passenger vehicles. Californians drove an additional 2.7 billion miles in 2015, overwhelming any gains made in fuel efficiency. Housing costs are pushing people to move farther from work, the report says, and public transportation ridership was down across the state and the nation as a whole. Nationally, the Trump administration has begun the process of reviewing federal fuel efficiency standards for cars and light trucks. California has its own fuel efficiency standards, but they require a federal waiver.

Electric Cars Hold Potential, but Charging Network Lags

State officials are hoping that electric cars may help reverse the growth in vehicle emissions. They've set a goal of getting 1.5 million zero-emissions vehicles on the road by 2025 and have implemented a $133 million rebate program. As of the end of last year, California accounted for more than half of all zero-emissions vehicles sales in the country, according to the report. And in the first three months of this year, they made up 5 percent of all auto sales in the state.

Unfortunately, the report says, the required charging network is not keeping pace: California has one of the lowest ratios of charging outlets to vehicles in the nation. Utility companies, pushed by state mandates, are planning $1 billion in spending to improve the situation.

Electrification of the transportation sector, of course, will require a huge build-out and improvement of the grid, too, but the report says the state is well on its way. Since 2010, state regulators have adopted programs to boost the energy storage capacity that's needed to handle intermittent power from wind and solar, and in 2013 required that utilities install 1.3 gigawatts of storage capacity by 2020.

"There are some very strong signs of growth in the storage sector," said Adam J. Fowler, research manager at Beacon Economics. "I think the challenges are ones we can mount."