The changing nature of the population also has environmental consequences.
"Today's baby boomers – 26 percent of the population – are the largest, wealthiest, highest resource-consuming of that age group ever in the nation's history, and they have unprecedented environmental impact," says Ms. Markham.
The generation's preference for bigger houses and bigger cars – and the proliferation of them – are gobbling up more resources and creating more pollution, according to a recent study by the Center for Environment and Population. For example:
•Land is being converted for development at about twice the rate of population growth. When housing, shopping, schools, roads, and other uses are added up, each American effectively occupies 20 percent more developed land than he or she did 20 years ago.
•Nearly 3,000 acres of farmland are converted to nonagricultural uses daily..
•Each American produces about five pounds of trash daily, up from less than three pounds in 1960.
•While the US is noted for its wide open spaces, more than half of all Americans live within 50 miles of the coasts where population density and its environmental impact are increasing.
That concentration poses special challenges for areas near the coast, like Portland, where land is rapidly being gobbled up. The city's population, which is now a bit over half a million, is fairly stable. But surrounding population pressures are great. The metropolitan area grew about 30 percent during the 1990s to just over 2 million. It's projected to grow to 2.6 million by 2010 and to 3.1 million by 2025.
Some groups worry that Portland's growth will undermine its environmental sustainability.