Updated at 9:15 p.m. ET

California has established an ambitious goal of relying entirely on zero-emission energy sources for its electricity by the year 2045.

Gov. Jerry Brown signed a bill mandating the electricity target on Monday. He also issued an executive order calling for statewide carbon neutrality — meaning California "removes as much carbon dioxide from the atmosphere as it emits" — by the same year.

"This bill and the executive order put California on a path to meet the goals of Paris and beyond," Brown said in a statement. "It will not be easy. It will not be immediate. But it must be done."

As the Trump administration rolls back federal efforts to combat climate change, California has actively pursued a leading role in the international fight against global warming.

The latest announcement comes shortly before Brown heads to San Francisco for the Global Climate Action Summit.

The bill specifically requires that 50 percent of California's electricity to be powered by renewable resources by 2025 and 60 percent by 2030, while calling for a "bold path" toward 100 percent zero-carbon electricity by 2045. ("Zero-carbon" sources include nuclear power, which is not renewable.)

Previously, California had mandated 50 percent renewable electricity by 2030.

California is not the first state with such ambitions — in 2015, Hawaii established a goal of 100 percent renewable electricity sources by 2045.

But, as KQED's Lauren Sommer reported last year, "California uses about 30 times more electricity than Hawaii and is the fifth largest economy in the world."

California already gets a substantial portion of its electricity from renewable resources.

The California Energy Commission estimates that 32 percent of retail energy sales were powered by renewable sources last year.

But the supply of renewable energy varies from day to day — even moment to moment.

NPR's Planet Money reported that on a sunny day this June, nearly 50 percent of the state's electricity came from solar energy alone.

But as Sommer reported last year, that variability means it's tricky to get renewable energy supply to match up with electricity demand:

"The sun and wind aren't always producing power when Californians need it most, namely, in the evening.

"The state's other power plants, like natural gas and nuclear, aren't as flexible as they need to be to handle those ups and downs. Hydropower offers the most flexibility but is scarce during drought years."

Large-scale energy storage systems can help address that problem, Sommer said, as could a "better-connected transmission grid system."

California has dramatically stepped up its climate-change policies four times in the last four years, as Capital Public Radio's Ben Bradford reported last month.

Before the new 100 percent zero-emission goal, lawmakers approved "higher renewable energy use, tighter greenhouse gas targets, and extension of the cap-and-trade program," he wrote.

The new bill was supported by Democrats who emphasized the damaging consequences of climate change, while opposed by state Republicans who highlighted the policy's financial costs, Bradford reported.

California's utilities had been on track to meet the previous goal, of 50 percent clean power by 2030, "but scientists debate whether cost-efficient 100 percent clean energy is feasible or if it would require new technological advances," Bradford wrote.

Some cities across the U.S. have attained 100 percent renewable electricity or energy supplies — including Aspen, Colo., Burlington, Vt., and Georgetown, Texas.

And earlier this year, for one entire month, Portugal produced enough renewable energy to meet its entire electrical demand — although the country did rely on fossil fuels to balance out the periodic disconnect between supply and demand.

As NPR reported at the time:

"For most countries in the world, a fully renewable energy supply still seems like a challenging target. Some small island nations have managed it — and a few larger countries, too.

"Iceland and Norway meet essentially all of their electrical needs through hydro and geothermal power, and have for years — but those countries take advantage of extraordinary geology, making the accomplishment hard to replicate.

"Several small islands are all-green, but larger countries are rare. On particularly windy days in 2015 and 2017, Denmark exceeded its electrical needs through wind power alone.

"And several times in the past few years, Costa Rica has kept on the lights through on all-renewable power for several months, fueled by heavy rains that fed into hydroelectric facilities."

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