California Governor Jerry Brown signed landmark legislation Monday committing his state to a 100 percent clean electricity grid by 2045.
"This bill and the executive order put California on a path to meet the goals of Paris and beyond," Brown said at a signing ceremony in state capital Sacramento.
"It will not be easy. It will not be immediate. But it must be done."
At least 20 countries and twice as many large cities have made similar pledges, but California -- the fifth largest economy in the world -- is by far the biggest jurisdiction to do so to date.
The electric sector represents 16 percent of the state's greenhouse gas emissions. More broadly, California has set ambitious goals to slash greenhouse gas emissions 40 percent by 2030, compared to 1990 levels.
On the international stage, California has emerged as a leader on climate action as US President Donald Trump has opted out of the landmark 2015 Paris climate treaty and moved aggressively to dismantle the policies of his predecessor, Barack Obama.
"It's impossible to overstate how significant it is for a state as large and influential as California to commit to 100 percent clean energy," said Sierra Club Executive Director Michael Brune.
"California is showing the world that a transition to 100 percent clean energy is within reach."
But Brown cautioned that reducing emissions enough to meet the Paris goal of capping global warming below two degrees Celsius (3.6 degrees Fahrenheit) remains a daunting task.
"Have no illusions," Brown said. "California and the rest of the world have miles to go before we achieve zero-carbon emissions."
- 'Governor Moonbeam' -
Scientists estimate that the global economy will have to become "carbon neutral" -- removing any additional CO2 put into the atmosphere -- by mid-century or shortly thereafter to avoid catastrophic climate impacts, including deadly heatwaves and superstorms engorged by rising seas.
The new bill mandates that least 60 percent of electricity will have to come from renewable energy, especially solar and wind.
This leaves the door open to power generated by nuclear reactors, geothermal sources or even natural gas, if CO2 emissions are converted into fuel or syphoned off into secure storage underground, a technology known as carbon capture and storage.
The outgoing governor, who leaves office in January, will host subnational governments, cities and businesses this week in San Francisco for the three-day Global Climate Action Summit.
Brown, 80 years old, first proposed renewable legislation 40 decades ago during his inaugural stint as California chief executive, earning him the nickname "Governor Moonbeam."
"Brown really launched the modern renewable energy industry when he was governor the first time," said Jim Williams, a professor of Energy Systems Management at the University of San Francisco and author of a seminal 2012 study that mapped the decarbonization of California's economy by mid-century.