Take changing diet structure, for example. As income has risen for the Chinese, they began to eat more meat products, but Lu said that breeding livestock is much more polluting and energy intensive than planting.

"However, China still lags far behind most industrial countries like the US in consumption of meat or aquatic products, eggs and milk," he said.

According to Yuan, the Chinese government has recognized the potential for overconsumption of resources, and has adopted policies and taken steps to encourage a "green" economy and lifestyle.

For example, it has shut down energy- and pollution-intensive industries; has discouraged car buying through such measures as limiting license plates; has encouraged garbage sorting, water and electricity conservation; and banned the free distribution of plastic bags nationwide.

World fertility

Despite the problems the world faces, the 7 billionth child has a better chance of surviving past age 5 than a decade ago, said Noeleen Heyzer, undersecretary-general of the UN and executive secretary of the Economic and Social Commission for Asia and the Pacific.

Yuan said that one-third of the world's countries, largely developed ones, have fertility rates below 2.1. That is the average number of children born to a woman over her lifetime, and it's the minimum rate to renew the population.

"Most developed countries, usually with lower fertility rates, are expecting a diminishing and aging population while the developing ones would instead see a stable rise in population, less aging," he said.

The world's fertility rate has been decreasing since the 1960s, easing global population pressure, Yuan said. On the other hand, population numbers keep rising. The UN has forecast that world population will reach 9.3 billion by the middle of this century.

China has succeeded in reining in its fast growth of population. In the 1990s, about 25 million people were born each year. The average today is 16 million.

Chinese statistics indicate the population, currently 1.34 billion, will peak at 1.45 billion in 2030 and will account for one-sixth of the world's population. That's a significant decrease from the one-third share of population held by China in the late 1660s and early 1700s, Yuan said.

According to the UN, India will overtake China as the most populous country in the world by 2050. The United States will be the only developed nation among the 10 most populated.

If the world seems more crowded, it may be because population will reach 7 billion on Monday, according to United Nation projections. The UN says this global milestone presents both an opportunity and a challenge for the planet. [Carlos Barria/Reuters]