California is toughening a little-known but important climate policy aimed at cleaning up fuels used in transportation.

The Air Resources Board approved a requirement Thursday that fuel producers cut the carbon intensity of their fuels 20 percent by 2030, as part of a policy called the Low Carbon Fuel Standard. The previous target — which still applies — had been 10 percent by 2020. The air board characterized the new policy as “the most stringent requirement in the nation.”

The idea of the program is to nudge fuel producers into increasing the supply of — and demand for — hydrogen, electricity, renewable diesel and other alternatives to conventional gasoline and diesel. For the first time, companies can also get credit for greener aviation fuels.

Some Californians buying electric cars may find it easier to get credits from utilities for their vehicles, and more may be eligible, under the new rules.

The changes “will take California’s climate fight up another notch,” air board Chairwoman Mary Nichols said in a statement.

Transportation accounts for about 39 percent of California’s greenhouse gas emissions (a tally that does not include wildfires), and emissions from the sector have been creeping up even as other economic sectors get cleaner.

As of last month, the Oil Price Information Service estimated that the Low Carbon Fuel Standard added 12 to 14 cents to prices at California gas pumps.

The fuel standard’s tighter requirements are one of several ways California has recently ramped up its battle against climate change. The air board on Friday told carmakers that they must meet California’s fuel-economy standards, even if the Trump administration loosens federal requirements. And Gov. Jerry Brown recently signed a law requiring the state to get 100 percent of its power from carbon-free sources by 2045 — along with an executive order that goes beyond the electricity sector and calls for California to be carbon-neutral by 2045.

Kate Galbraith is the San Francisco Chronicle’s assistant business editor. Email: kgalbraith@sfchronicle.com Twitter: @kategalbraith