This is the third part of a Los Angeles Times series about the growth of the world’s population and the problems it poses. Part 1 appeared Wednesday on Page A1; part 2 appeared Thursday on Page A2.

XIAMEN, China —

A 6-year-old girl with a bob haircut sat alone on an enormous wraparound couch, dwarfed by the living room furniture and a giant flat-screen TV.

As she flicked the remote in search of cartoons, her parents pointed proudly to the recessed lighting and high ceilings. Then they proceeded with an official tour of their three-story house with white marble floors, oversized windows and a granite entryway flanked by a Corinthian column.

All of this was paid for with a $100,000, interest-free loan from the Chinese government, an incentive to keep the family’s size “in policy.” For these residents of a rapidly developing rural area, that meant sticking to two girls and giving up the chance to have a son.

The husband, Zhang Qing Ting, an electrical technician, said living in a modern subdivision for in-policy families beats the usual cramped apartments with no garages. He and his wife, Chen Hui Ping, a factory worker, will also be eligible for cash payments when they retire.

“Many of my friends envy me,” Zhang said, mopping sweat from his neck as a dozen local officials and family planning bureaucrats looked on. The couple had been given a day off work to showcase the benefits of their restraint to two foreign journalists.

Jin Jing, chairman of Chao Le village, summed up the message: “If you practice family planning, you can get this kind of reward.”

A massive experiment

For more than three decades, the most populous nation on Earth has been running a massive social experiment, using elaborate incentives and penalties to limit family size.

The aim was to banish hunger and raise living standards, and by many measures the results have been impressive. By reducing the number of dependents per household and freeing more women to enter the workforce, population control efforts have helped lift hundreds of millions of people out of poverty and contributed to China’s spectacular economic growth.

Prosperity has exacted a steep environmental toll, however.

The colossal industrial expansion of recent decades has depleted natural resources and polluted the skies and streams. China now consumes half the world’s coal supply. It leads all nations in emissions of carbon dioxide, the main contributor to global warming. Pollutants from its smokestacks cause acid rain in Seoul and Tokyo.

China’s experience shows how rising consumption and even modest rates of population growth magnify each other’s impact on the planet.

Middle-class living

The country’s population of 1.3 billion is increasing, even with the controls on family size. What is driving the growth is that hundreds of millions of Chinese are still in their reproductive years. On such a huge base, even one or two children per couple adds large numbers — an effect known as population momentum.

Moreover, the Chinese are living better overall: consuming more food, energy and goods than ever. One-fourth of the population — the equivalent of everyone in the United States — has entered the middle class.

The U.S. consumes much more per person. But with a population four times larger, China has a greater collective appetite — and a greater ecological impact — than any other country.

Within China, signs of environmental damage are pervasive: massive fish kills, lung-searing smog, denuded landscapes. They have stirred popular discontent and the beginnings of greater official concern for curbing pollution and preserving natural resources.

How this drama plays out is not merely China’s concern. Because of the nation’s sheer size, the rest of the world has an enormous stake in the outcome.

The legacy of Mao

China’s massive population is a legacy of Communist Party Chairman Mao Zedong, who strove to increase the ranks of the Red Army by encouraging large families and banning imports of contraceptives and declaring their use a “capitalist plot.”

In the late 1950s and early 1960s, a series of famines claimed tens of millions of lives. The suffering left an enduring awareness that the country couldn’t sustain unlimited population growth.

As Mao’s power waned in the 1970s, other Chinese leaders applied the brakes. Free contraceptives were made widely available. Couples were encouraged to marry later, wait longer to have children and have fewer. In less than a decade, fertility plummeted from nearly six children per woman to fewer than three.

To drive the birthrate down further, Deng Xiaoping imposed the “one-child policy” in 1979. It led to mandated abortions and other abuses by zealous enforcers.

Today, there are many exceptions to the rule: Rural couples and ethnic minorities, for instance, can have two or more children. Although compulsory abortions have been forbidden, families must pay steep fines for having more children than allowed.

Nowhere is the scale of the country’s transformation more vividly displayed than in its cities. Hundreds of millions of Chinese are moving from farms to urban centers to seek jobs and middle-class lifestyles.

In Shanghai, whose population of 23 million exceeds that of Australia, high-rises sprawl in all directions until their silhouettes slip from view, obscured by brown haze.

China, by varying estimates, has more than 100 cities with 1 million or more residents, compared with nine in the United States. The number of million-plus cities will reach 221 within two decades, according to the McKinsey Global Institute, an economics research firm. More than a dozen will have populations of 25 million or more each.

China isn’t hustling just to satisfy the demand from the United States and other countries for cheap merchandise. Increasingly, it is bent on meeting the needs of its own people.

More and more, it is being forced to confront the environmental consequences.

Prosperity’s dark side

A red truck arrived with a squeal of brakes and a swirling cloud of black dust.

The driver peeled back a tarpaulin to reveal his cargo: coal to stoke one of the massive electricity plants in Shanxi province.

The procession of dump trucks continued around the clock, leaking coal dust that piled up along the road like drifts of black snow. Squat cooling towers hissed like fumaroles. Slender smokestacks disgorged white-gray smoke carried east by the breeze, toward Beijing and beyond.

In nearby Datong, a thick gray-brown haze clings to the city like a dark mist, obscuring the tops of high-rises.

A half-day’s drive south is the ancient city of Linfen, identified by the World Bank six years ago as the most polluted city on Earth.

The city, once known for its fruit and flowers, is now infamous for respiratory illnesses and the shroud of smog that regularly blots out the sun.

China likes to consider itself the world’s factory. Yet it has also become the world’s smokestack.

Tendrils of soot extend across the Pacific. On some days, almost 25 percent of the pollutants in the air above Los Angeles originated in China, the Environmental Protection Agency has found.

Under international pressure, China has cracked down on some of its dirtiest plants, mainly to reduce soot or pollutants like sulfur dioxide, which causes acid rain and aggravates asthma and heart disease. After the World Bank’s rebuke, officials in Shanxi province closed some of the illegal coal mines in Linfen and its dirtiest coal-fired furnaces.

China relies on coal to meet about two-thirds of its energy needs. Despite major investments in solar, wind and nuclear energy, coal consumption continues to climb.

Although China has the third-largest reserves in the world, it is reaching around the world for more. It overtook Japan this year as the world’s largest coal importer, drawing mostly from Indonesia and Australia. Its imports are expected to double by 2015.

Those trends are worrisome to climate scientists, who say that to avoid a potentially catastrophic rise in global temperatures, worldwide carbon dioxide emissions must be cut in half by 2050.

For that to happen, China’s emissions would have to peak by 2020, said Nobuo Tanaka, former director of the Paris-based International Energy Agency, which advises governments on energy issues. But by China’s own projections, its output will rise at least 50 percent from current levels before peaking around 2035.

It would be all but impossible for other nations to compensate for such an increase, Tanaka said.