The mood at the conference had been dismal. Scientists were worried about losing federal funding for research and even the NASA satellites that collect basic climate data.
Brown declared that should Trump turn off the satellites, California would launch its own.
And that wasn't the only warning Brown had for the Trump administration.
Brown has spearheaded his own international climate agreement with more than 160 cities and states. California, he said, is the 6th largest economy in the world.
"We got the scientists," he said. "We got the lawyers, and we're ready to fight. We're ready to defend."
And playing defense may well be in California's future. The state has rules limiting carbon pollution from cars, but it can't have those rules without permission from the federal Environmental Protection Agency.
That came up at the nomination hearing last week for Scott Pruitt, who could run the EPA.
California Sen. Kamala Harris asked Pruitt if he could uphold the state's special permission — its waiver — for tougher car rules.
"Senator as you know, administrators in the past have not granted the waiver and in fact have granted the waiver," Pruitt said. "That's a review process ..."
He said he would not know his intention without going through the process.
This is not a new fight for California. In 2007, the Bush administration denied the state's request to have tougher pollution rules for cars, saying it would create a patchwork of regulations. California took the federal government to court, but before the case was decided, Obama was elected.
California got its waiver.
Michael Wara, a professor at Stanford Law School, said this isn't just about California.
"The first legal disputes are going to be about cars," he said. "And I'd be surprised actually if we didn't see those disputes."
Thirteen other states have adopted the same clean car standards. But the bigger question, Wara said, is a political one.
"How a Pruitt EPA responds to some of these issues with California is really going to test Scott Pruitt's and the administration's commitment to conservative values," he said.
Pruitt sued the EPA as the attorney general of Oklahoma. He argued his state had the right to set its own environmental rules.
"So that logic would seem to imply that California should have the right to set its own agenda," Wara said. "But we'll see how that trades off against the desire to roll back regulations related to greenhouse gases."
This is something California will be watching closely, with lawyers at the ready.