SAN FRANCISCO (AP) — Gov. Jerry Brown told attendees at an environmental conference Friday that climate change must be prevented or humans might one day be forced to live on another planet.

Addressing the Greenbuild Expo in San Francisco, Brown lauded California's cap-and-trade auction for greenhouse gas emissions, which began this week. It was the formal launch of the nation's most ambitious carbon-trading market, which for the first time established a market-based system to put a price on greenhouse gas emissions.

The Democratic governor said future generations will be living "indoors ... or we'll be living on some other planet." He urged other states and the nation to follow California's lead, saying the state can only do so much by itself.

"Human impact on climate is real," Brown said. "It is growing, and we need to take steps to stop it or there will be catastrophic consequences."

Brown said he has always been a steward of the environment, saying he earned the nickname "Gov. Moonbeam" decades ago during his first gubernatorial stint in part because of his interest in solar energy.

"That was not a term of endearment," the governor joked. "The people who invented that term aren't around anymore, but I am."

Brown got the nickname during his first stint as governor during the 1970s from Chicago newspaper columnist Mike Royko, who said Brown's ideas seemed far-out at the time. Royko later retracted the label, and many of Brown's early proposals once thought outlandish, such as launching communications satellites into space, have since become reality.

While Brown lauded the state's cap-and-trade system, business groups have sued to invalidate it. They call it an illegal tax that will send jobs and businesses to other states.

The governor, who seemed energized by his Nov. 6 victory when voters approved his deficit-reducing ballot initiative to raise taxes, said he realizes that climate policies seem abstract in a struggling economy and at a time when basic services are being cut. Yet he said it is all connected.

"Dealing with the environment seems more a luxury than a necessity," Brown said. "But my message is the two go hand-in-hand."

Brown said people who believe environmental action comes at the expense of economic progress "miss the point."