BERKELEY — In the early 1970s, California faced a one-two punch of doubled oil prices and soaring electricity demand, with new coal and nuclear power plants projected for every eight miles of coast from San Diego to San Francisco.

The plants were never built because a motley band of physicists and state lawmakers cut energy demand down in ways as ambitious as reinventing the fluorescent light bulb and as simple as slapping energy-usage labels on refrigerators.

They also retooled the electricity market so power companies spent money helping people use less energy. Utilities were investing in selling less electricity, but they did not have to build 16 power plants andwere guaranteed payback on what they did build or buy. The state's economy kept growing even though Californians on average consume 40 percent less energy than other U.S. citizens.

Scientists said Friday that exporting California's energy-saving recipe to the rest of the nation, as well as to China and India, is the first, lowest-cost and sacrifice-free answer to the global problems of rising fossil energy prices and greenhouse warming.

"This was and remains the lowest-hanging fruit," Nobel Prize-winning physicist and Lawrence Berkeley Lab director Steve Chu said Friday. "But it cannot solve it all."

Ultimately, scientists say, the solution lies in revolutionizing the global energy supply to replace the 80 percent filled by oil, natural gas and coal with carbon-free energy sources. But the speed and size of that transformation depend on energy demand.

If nations wanted to hold the global rise in temperatures to less than 4 degrees Fahrenheit while keeping their economies growing, a half-percent increase in energy efficiency by 2050 will make the difference between replacing the equivalent of all fossil energy supplied today with carbon-free energy and having to come up with twice that amount without the extra efficiency, according to an analysis by John Holdren, a Harvard energy expert and president of the nation's largest scientific society.

"The potential of what we could get is a whole lot bigger on the demand side than what we could get on the supply side," Holdren said Friday at a Berkeley symposium honoring Abe Rosenfeld, a Berkeley physicist considered one of the founders of energy efficiency concepts.

Climate scientists say rising concentrations of greenhouse gases from human fossil fuel burning have pushed average global temperatures up by more than a degree, mostly since the 1970s. Because greenhouse gases persist in the atmosphere for decades, scientists say, another degree of warming is inevitable due to current emissions.

Many scientists now believe a doubling of pre-industrial levels of greenhouse gases in the atmosphere is unavoidable, with at least another degree or two of warming that many scientists believe could melt the Greenland ice sheets and raise global sea levels as much as 20 feet.

Dozens of coal-fired plants are planned in the United States, but hundreds are planned in India and China, which is projected to surpass the United States as the largest emitter of greenhouse gases in 2020.

The rest of the United States already is inching in a California direction on energy efficiency. Last year's federal energy act borrowed 16 of the state's 2004 appliance and building standards and made them law for the rest of the country, ending five years in which the federal government failed to upgrade any of its standards.

Energy scientists said there's a lot more to be done nationally and in California. More than 70 percent of greenhouse emissions from electricity generation are tied to powering buildings, especially in wealthy, developed countries, said Robert Socolow, a Princeton energy analyst.

"It suggests we're moving more and more to a world where it's the electricity in buildings that's the problem," he said. "We've got to learn, and California can teach us. It suggests we have to reinvent the building."

Rosenfeld, now serving on the California Energy Commission, has plenty of ideas.

His latest passion — cool roofs — just became the rule in California. If a building has a flat roof and no architectural reasons against it, the roof has to be white to reflect incoming solar energy and save up to 20 percent on air-conditioning costs. For people who don't like light-colored roofs, Berkeley scientists invented dark roofing tiles that reflect 30 to 40 percent of the infrared light that warms homes and drives up summer electricity bills.

So far, scientists calculate that applying the same idea to cars will save 2 to 3 percent on fuel economy.

"People keep asking me, 'Have we used up all the low-hanging fruit?'" Rosenfeld said. "We seem to be a long way from physical limits on refrigerators, air conditioners, cars, what have you. We've got a long way to go."