IF you're a young American thinking ahead to when your children are grandparents, you'd better think in terms of a crowd. At current growth rates, United States population could double in just 64 years.
While Asia, Africa, and Latin America typically are thought of as posing the biggest population challenges, the US is a major worry to demographers and resource planners as well.
With 259 million people, the US today is the third-largest country in the world (behind China and India). And among industrialized nations, only two countries - relatively sparse Canada and Australia - are growing faster.
When emigration from other states and countries is factored into the equation, California has been growing more rapidly than India -
and in some years faster than Bangladesh, says Ric Oberlink, executive director of Californians for Population Stabilization.
The problem is not just the US fertility rate (the average number of children per woman), which has inched up nearly 20 percent since the mid-1980s. Consumption patterns and a liberalized immigration policy are having an effect as well.
``It seems that the problem is global and somewhere else, but that's just not true,'' says Mark Nowak, executive director of Population-Environment Balance, a private research organization working to stabilize US population.
``The 259 million people here have a vastly greater impact than, say, 2 billion people elsewhere,'' he adds.
According to State Department estimates, for example, Americans (who make up less than 5 percent of global population) use six times the per-capita world average in energy supplies and produce 20 percent of the carbon dioxide released into the atmosphere.
Writing in The Amicus Journal (published quarterly by the Natural Resources Defense Council), Paul Harrison, who worked for the United Nations Population Fund, estimates that the US contribution to global warming is ``about equal to China's and India's combined - and two and a half times bigger than Africa's.''
In his speech to this month's UN preparatory meeting for the International Conference on Population and Development to be held in Cairo in September, US State Department counselor Tim Wirth referred to ``the difficult issue of wasteful resource consumption and the disproportionate impact the developed world has on the earth's environment.''
THE UN population document argued over by some 170 countries in New York and now headed for Cairo makes several references to ``wasteful,'' ``excessive,'' and ``unsustainable'' consumption.
The relative impact of population and consumption is expected to be a point of contention between developing countries of the South and developed countries of the North.
Immigration is also controversial. There now are about 1 million legal emigrants into the US each year, plus several hundred thousand illegal migrants.
``The prospect of easy immigration in the US ... helps maintain and indeed even increases fertility rates abroad, which is exactly what we don't want to do,'' says David Durham, founder of Carrying Capacity Network, a research group focusing on domestic population. ``We're not doing the sending countries any good by encouraging them not to look at their carrying capacity.''
``We need to adopt replacement-level fertility and immigration,'' says Mr. Nowak of Population-Environment Balance.
Twenty-two years ago, the Commission on Population and the American Future headed by former Vice President Nelson Rockefeller recommended that the US stabilize its population.
Since then, the country's population has grown by about 60 million, the flow of immigrants has swelled, and consumption rates have increased.