Chinese skylines are defined by construction cranes and the din of jackhammers. China produces 50 percent of the world's cement [pdf]—the next largest producer, India is responsible for just 6 percent—to build seemingly endless tracts of high rises, railroads, parking lots, highways, airports and shopping malls. But all booms end—and China's may end sooner than most people think.

A study released on April 27 by Lawrence Berkeley National Lab's (LBNL) China Energy Group says that this high-growth phase of China's development could wind down in less than two decades. China's energy use will cease to be a great unknown, threatening and shape-shifting as a dragon, and become something that is, if not tame, at least simpler to predict.

The LBNL forecast is the first such survey that attempts to come to grips not only with China's energy and resource needs during the current era of rapid development, but also attempts to pinpoint when that era will end. "Most conventional forecasting techniques use a process that assumes the future is going to look like the past—they establish certain relationships between energy and economic activity, and project forward what will be energy uses in the future," explains LBNL staff scientist and report co-author David Fridley. (He is referring specifically to energy forecasts by the International Energy Agency (IEA) and two scenarios prepared by the Energy Research Institute (ERI), a Beijing think tank affiliated with the Chinese government.) Yet the relationship between economic growth and energy demand is not just a straight line. "Nothing can grow exponentially forever."

Current thinking says China's energy demand will only go up and up throughout this century. The LBNL report, however, projects that after rising steeply for the next 15 years, China's energy growth curve will bend and begin to plateau; demand will then peak by 2050 at between 4.5 billion and 5.5 billion tons coal equivalent (depending on the government's success implementing various energy efficiency targets). That is considerably less than the amount if you simply extended other surveys forward. The report forecasts that energy use per person in China will rise to about 40 percent of what Americans consume.

The insights underlying the LBNL forecast are simple. First, the study simply extends further into the future than many forecasts. (The International Energy Agency's most recent "World Energy Outlook" only looks to 2035 and shows no sign of China's energy demand beginning to level off.) Second, their research takes account of what China Energy Group director Mark Levine calls "saturation points." That is the point at which energy demand in a given sector levels off. In a country where still more than half the population is rural, many people have yet to purchase even basic appliances. "But once you have a refrigerator, you don't need another," he says. The concept is simple, but actually integrating it into forecasting requires looking at the dynamics of each sector, and gathering data that can be hard to come by in China. "What makes our model unusual is that we actually account for the specific number and size of refrigerators, televisions, air-conditioners, and more per household," he says. "If you don't do that, you can't know there's a saturation point."

Take, for instance, the energy needs of China's massive infrastructure build-out. Currently 70 percent of China's energy demand is attributable to industry, and of that, fully 40 percent goes into the production of cement and steel. Yet at some point, China won't need to build as many new highways, bridges and parking lots. To envision the future, the LBNL team examined the total mileage of road networks in developed countries with comparable population densities. Then they mapped out just how many additional roads they think China will need—and estimated when the convoys of cement trucks will at last slow down.

China will then be more like developed countries, in which energy demand is flat —or rising at about 1 percent a year—even as standards of living continue to increase. The need for new infrastructure will have diminished, while the replacement of old technologies with newer energy-efficient ones will offset rising consumption. (From 1975–1986, energy in the U.S. did not increase at all, even while GDP increased 35 percent, Levine says.) China's city skylines will no longer feature rows of cranes.

A slower growing China has implications for climate scenarios. In 2010, China pumped about 3 billion tons coal equivalent into the atmosphere. The IEA extrapolates this would hit 5.5 billion tons by 2030 and would double by 2050. China's ERI shows about 6.1 billion tons coal equivalent in 2035. But the LBL study finds China's total carbon output, after roughly plateauing from 2030 to 2035, could be as low as 4.5 billion tons coal as far out as 2050. For China's carbon emissions to increase 50 percent between now and then is no small problem, but it's less of a problem than previously thought. "Will China overwhelm the world with its greenhouse gas emissions?" says Levine. "No, not necessarily."

Of course, the outlook is still dire—a 50 percent rise in China's energy demand and greenhouse gas emissions is tremendous. By identifying a future horizon point, the study may help international climate negotiators press Beijing to commit to absolute targets for reducing carbon emissions, which Beijing has staunchly resisted in favor of relative targets linked to domestic GDP.

The planet's most populous country is now laying the foundation for its future. The window of opportunity is very small. The time when China's planners, and their international advisors, can take steps to ward off future catastrophe—by ensuring that all those shiny new buildings, power plants and transportation systems are as energy efficient and environmentally sound as possible—is now, before the cement has dried.

Christina Larson, a contributing editor at Foreign Policy magazine, has reported extensively on environment and energy from across China. She is also a fellow at the New America Foundation.