California is accelerating its rollout of clean energy, even as the White House is racing to unravel climate regulations.

On Tuesday evening, the California Assembly passed a bill requiring 100 percent of the state’s electricity to come from carbon-free sources by the end of 2045, putting one of the world’s most aggressive clean-energy policies on track for the governor’s desk.

Given the size of California’s economy and the bill’s ambitions, it “would be the most important climate law in US history,” says Danny Cullenward, an energy economist and lawyer at the Carnegie Institution for Science.

The actual impact of the measure on global emissions and climate risks will be negligible, unless the rest of the world responds in similar ways. But California is effectively acting as a test bed for what’s technically achievable, providing a massive market for the rollout of clean-energy technologies and building a body of knowledge that other states and nations can leverage, says Severin Borenstein, an energy economist at the University of California, Berkeley.

“We are showing that you can operate a grid with high levels of intermittent renewables,” he says. “That’s something that can be exported to the rest of the world.”

California’s actions stand in stark contrast to the direction of federal policy under President Trump, who pulled the nation out of the landmark Paris climate agreement within six months of taking office (see “Exiting Paris, Trump cedes global leadership on climate change”). Among other measures, the administration is working to water down the Clean Power Plan, prop up the coal industry, and roll back federal vehicle emissions standards while revoking the ability of states like California to set their own.

Reaching 50 percent

The new California bill, known as SB100, will still need to go back to the Senate to approve changes made since it passed there with a comfortable margin last year, but that's not expected to be a hurdle.

The measure also moves up the state’s earlier time line to reach 50 percent renewables, from 2030 to 2026. Notably, however, California regulators have said the state’s major utilities could reach that milestone as early as 2020, underscoring the rapid pace at which the energy transformation has unfolded since the state first put its renewable standards in place in 2002. (In fact, California would already be well beyond the 50 percent threshold if the state’s legal definition included carbon-free electricity sources like nuclear power and large hydroelectric plants.)

Borenstein says the state hasn’t always picked the most cost-effective paths, noting that customers are still paying inflated rates as a result of some excessively high early wind and solar contracts. But crucially, the rapid transformation occurred without wrecking California’s flourishing economy. The state’s gross domestic product climbed by $127 billion last year, making it the world’s fifth-largest economy.

The critical question now is: Can the state’s grid get to 100 percent clean energy as affordably and reliably?

Solving the second half

Many energy researchers believe the second half of California’s clean-energy puzzle will be considerably more difficult, and expensive, to solve than the first.

Back in 2002, California got off the starting block with a fair amount of existing wind, geothermal, hydroelectric, biomass, and solar thermal power. From this point forward, every additional percentage point of clean power needs to be built from scratch. In addition, the state plans to close its last nuclear plant in the coming years, eliminating a carbon-free source that provides about 10 percent of the state’s electricity.

But perhaps the thorniest challenge is that the output of renewable sources like wind and solar vary greatly by the day and season. As renewables come to represent a larger portion of the state's total electricity generation, managing that intermittency could become increasingly costly and complex.

“The amount of effort to achieve the last 20 percent might well be as much as it took the reach the first 80,” Jane Long, a former associate director at Lawrence Livermore National Laboratory and energy researcher who has closely studied the mix of technologies that could be required to meet California’s emissions goals, said in an email.

Among other things, it could require expensive investments in energy storage, politically tricky expansions of transmission infrastructure, or greater reliance on controversial carbon-free sources like advanced nuclear, or fossil-fuel plants coupled with carbon-capture technology (see “The $2.5 trillion reason we can’t rely on batteries to clean up the grid”).

Added flexibility

A crucial nuance of the California bill is that it employs a wider definition of clean-energy sources than the state’s earlier rules, using the language “zero-carbon resources” rather than “renewables,” which means it could include those technologies or others that may emerge in the years ahead.

Long says that flexibility will be key to achieving the state's goals. It's also likely to require additional technological innovation, potentially including the development of storage technology that can work on a seasonal basis and affordable means of producing carbon-neutral fuels, she adds.

Some are more optimistic. Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley, argues that it won’t be increasingly challenging to reach California’s next set of goals. Among other things, he believes we’re sure to see the necessary energy storage advances in the years to come and says the state has yet to harvest other “low-hanging fruit,” like using electric vehicles as a form of distributed storage.

Moreover, Kammen says we’ll see a pronounced shift toward the “low-carbon lifestyles” embraced by younger citizens in the next few decades, as the state increasingly plans cities around dense housing and public transit rather than single-family homes and cars.