LOS ANGELES — Foreign governments concerned about climate change may soon be spending more time dealing with Sacramento than Washington.

President-elect Donald J. Trump has packed his cabinet with nominees who dispute the science of global warming. He has signaled he will withdraw the United States from the Paris climate agreement. He has belittled the notion of global warming and attacked policies intended to combat it.

But California — a state that has for 50 years been a leader in environmental advocacy — is about to step unto the breach. In a show of defiance, Gov. Jerry Brown, a Democrat, and legislative leaders said they would work directly with other nations and states to defend and strengthen what were already far and away the most aggressive policies to fight climate change in the nation. That includes a legislatively mandated target of reducing carbon emissions in California to 40 percent below 1990 levels by 2030.

“California can make a significant contribution to advancing the cause of dealing with climate change, irrespective of what goes on in Washington,” Mr. Brown said in an interview. “I wouldn’t underestimate California’s resolve if everything moves in this extreme climate denial direction. Yes, we will take action.”