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The pledges that countries have signaled they will make in Paris over the next two weeks to cut emissions will inevitably fall short of what is needed to solve the problem of climate change. But many political leaders gathering there — including governors, mayors, and provincial cabinet secretaries — are pushing for more aggressive cuts. By the dozen, they are signing a voluntary agreement committing their jurisdictions to faster and deeper reductions in emissions of greenhouse gases than their national governments have promised. “We are not moving fast enough,” Gov. Jerry Brown of California, who is helping to lead the effort, said in an interview. “We’ve got to do more.” All of which raises a provocative question: What would a truly ambitious plan to tackle climate change look like? Despite the intensity of the debate around global warming, the question has long been considered theoretical, and few people have spent much time studying potential steps to “deep decarbonization” — certainly not at the level of detail needed for a concrete plan. Lately, that has started to change. But the recent analyses make clear just how difficult a worldwide transition to a clean energy system is likely to be. “The arithmetic is really brutal,” said Jeffrey D. Sachs, a prominent Columbia University economist. “We’re in such a dreadful situation that every country has to make this transformation, or else this isn’t going to work.” Dr. Sachs helped start what is perhaps the most serious effort to draw up a detailed road map for the energy transition: the Deep Decarbonization Pathways Project, based in Paris and New York. Over the past couple of years, the effort enlisted teams from 16 countries, which account for the large majority of global emissions, to devise such plans. The analysts used conservative assumptions about current technologies and their costs. They also presumed that developed countries would not be willing to make big changes in their way of life — that people would continue to insist on transportation, refrigerators, electric lights and so forth — and that poor countries would keep striving to reach higher standards of living, requiring more energy. The experts also made a point of ruling out energy miracles, such as technologies like nuclear fusion that could help enormously if they became available but are still largely on the drawing board. “If we couldn’t put on a hard hat and go visit a technology in the field, at least in pilot stage, then we didn’t include it in our analysis,” said Ben Haley, a senior consultant at Energy and Environmental Economics, a consulting firm involved in the work. The issue can be overwhelming. The science is complicated. We get it. This is your cheat sheet. With those assumptions, the experts focused on a specific question: Can emissions be cut enough from now to 2050 to meet an international target designed to head off the worst effects of climate change? “It can still be done — barely,” said Guido Schmidt-Traub, the executive director of the Sustainable Development Solutions Network, which helped organize the effort. Perhaps the single most crucial finding of the project is that the technologies available today, while good enough to get a running start on the transition, are probably not good enough to finish it. That means experts who have long argued for a more intensive research program on clean energy have a point. The 16-country analysis suggests that many technologies, like electric cars and offshore wind turbines, have to become cheaper and better. Bill Gates, the Microsoft founder and philanthropist, has long argued for a more intensive focus in energy innovation, and on Monday he announced in Paris that he had corralled a group of billionaires to invest huge sums in developing new technologies. Twenty countries, including the United States, also pledged to double their own investment in basic energy research. In an interview, Mr. Gates said that poor countries would be reluctant to switch to clean energy technology unless it becomes better and cheaper, “and the only answer to that is innovation.” Yet the new research on decarbonization also suggests that the environmentalists who have long called for a rapid expansion of existing clean-energy technologies also have a point: The very process of rolling them out helps to spur innovation, and as they spread beyond niche markets, economies of scale drive down the costs. Solar power offers a stunning example, with costs of the panels plunging 80 percent in the last decade, a direct result of subsidies and other policies meant to create a larger market. In many places, solar power is still more expensive than power produced from fossil fuels, but the difference has narrowed considerably. Wind turbines have been a big winner, too, in recent years. 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. Wind power is so abundant in Texas that one company there is giving away electricity at night. The good news about wind and solar power has inspired claims that they could carry the entire load of the energy transition. Mark Z. Jacobson, an engineer at Stanford University, has drawn attention with a finding 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. In an interview, 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. Each of those, he said, is as complicated as erecting a wind turbine, and building tens of thousands of turbines a year would be well within the nation’s industrial capability. “We think it’s technically and economically feasible,” Dr. Jacobson said. “It ultimately does come down to political will. If people don’t want to do it, it’s not going to happen by itself.”
