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 marble floors, oversized windows and 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 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 Huge 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.

But prosperity has exacted a steep environmental toll.

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.

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

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.

China's huge population is a legacy of Communist Party Chairman Mao Zedong, who strove to increase the ranks of the army by encouraging big 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. It 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.

One-child policy

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.

Those who work for the vast family-planning bureaucracy take pride in what they see as their contributions to China's prosperity.

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 are obscured by brown haze.

On the family-planning tour, Beijing organizers spoke of Yancheng, with a mere 8 million people, as a backwater. It's a former salt-harvesting town on the northern bank of the Yangtze River near the coast. Bustling shopping districts, new office buildings, housing projects and other development extend in every direction.

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.

The misty hills of Yunnan province were once covered in pine trees. He Shigui remembers the hush of the fragrant forest when he was a boy.

All that changed when Mao instructed farmers to cut down the trees to grow food for a hungry nation.

"conquer nature"

Mao's slogan was "Man must conquer nature," and He Shigui and other farmers responded. By the 1990s, most of the trees along the Yangtze River were gone, along with the roots that once held the soil in place, creating a natural sponge.

He, his face now as furrowed as a field, still remembers the stormy day in 1998 when rainwater raced down the denuded hillsides, contributing to a disaster downstream.

"Of course I feel bad," said He, walking across the remains of a clay slope on his farm that washed away that day. "There was a flood here, and there was a flood there. It was the same water."

The inundations across central China drowned more than 3,000 people and destroyed 13 million homes.

The catastrophe also changed the way China viewed trees.

In the last decade, the government has spent more than $90 billion to plant trees across millions of acres and pay farmers to nurture them. But the solutions are not always as straightforward as planting trees.

The world's smokestack

A truck arrived with a squeal of brakes and a swirling cloud of black dust. The driver peeled back a tarpaulin to reveal 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. Slender smokestacks disgorged white-gray smoke carried east by the breeze, toward Beijing and beyond.

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 air pollutants 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.

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 CO2 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. But by China's own projections, its output will rise at least 50 percent from current levels before peaking around 2035.