Outlining the Arduous Path to a Planet Powered by Clean Energy

would drive down the costs.

Solar power offers a stunning example, with costs of the panels plunging 80 percent in the last decade.

Wind turbines have been a big winner, too. They supply almost 5 percent of the electric power in the United States, and in a handful of American states and some smaller countries, that figure has moved into double digits.

Mark Z. Jacobson, an engineer at Stanford University in California, found that the entire world could operate on 100 percent renewable power by 2050.

Yet such scenarios would involve an extraordinary push. Dr. Jacobson’s plans would require, among many other actions, that 156,000 wind turbines be built off American coasts in the next 35 years, and twice as many on land. In 20 years of effort, European countries have managed to build about 3,000 offshore turbines.

Dr. Jacobson cited a scientific paper that calculated the oil and gas industry has been building 50,000 new wells a year in North America since 2000. He said building tens of thousands of turbines a year would be well within the industrial capability of the United States.

“We think it’s technically and economically feasible,” Dr. Jacobson said. “It ultimately does come down to political will.”

Dr. Jacobson has often cited the United States’ mobilization during World War II as an example of what can be done by a determined society. But to some other experts, that argument speaks to the political and economic impracticality of his plans.

“I just don’t see a World War IIstyle mobilization happening for anything other than a world war,” said Jesse Jenkins, an energy analyst at the Massachusetts Institute of Technology.

The scenarios laid out by the Deep Decarbonization Pathways Project call for substantial amounts of renewable power. But these scenarios also suggest that the energy transition would be easier and cheaper with additional technology options, including some that are disliked by the environmental movement.

For instance, in some countries with growing power demands, like China, the research found that nuclear power would be essential for staying within a strict emissions budget.

The research also suggests that, to meet strict targets, some countries might need to keep burning coal or natural gas while capturing the carbon dioxide emerging from smokestacks, compressing it and injecting it deep underground.

Governments have discussed the need for this technology, known as carbon capture and storage, for decades. But they have put little effort into developing it. Germany, among the most determined countries in battling global warming, has largely decided not to pursue it.

Perhaps the most compelling finding of the Deep Decarbonization Pathways Project is that governments could easily mishandle the energy transition by failing to plan far enough ahead. Most countries are setting 10- and 15-year targets that can be met with incremental changes.

Yet that almost ensures that the toughest problems, like perfecting the carbon- capturing technology, will be tackled too late to meet the long-term goal of zero emissions, the researchers found.

While the best technology mix varies by state or country, the researchers found some themes in common.

To achieve the emissions goals, the entire economy, including transportation, needs to be electrified as much as possible. That might mean cars running on batteries, but it could also mean cars running on hydrogen.

The implication, either way, is that the internal- combustion engine that has powered cars since the 19th century is a technological dead end in the 21st century.

There is an urgent need to make electric cars better and cheaper, and to roll out charging stations by the millions.

Another potential dead end, the research suggests, would be an overreliance on natural gas. Gas is a lower- carbon fuel than coal, and switching power plants to run on gas can achieve big emission reductions in the short run.

But the deep- decarbonization research suggests that gas has to go away within a few decades. Heavy investment in natural gas now could wind up undermining the long-term goals.

The California governor, Mr. Brown, argues that if states and cities can figure out how to move first on climate, they can essentially drag their national governments behind them.

In some countries, state and local governments control the building codes, and could toughen energy rules for new buildings. Governments also buy millions of cars, and switching fleets to electric cars would help expand the market and reduce costs through economies of scale.

“California, doing more than any other state in America, is setting a pace, and will step it up,” Mr. Brown said. “But we have to be part of a larger movement to really get global warming under control.”