By Victoria Markham My daughter and I watched the U.S. Census Bureau Population Clock tick its way, second by second, to the magical 300 million mark this week. At the historic moment early Tuesday, she asked: "Why does that matter?" The answer may be more disturbing than she--or any of us--knows, because the U.S. is a world leader not only in gross domestic product, per-capita income, innovative technologies, and many health and educational standards, but also in a more dubious measure: our environmental footprint. And that can only become more worrisome as our numbers increase.

The U.S. population has the largest per capita environ-mental imprint in the world, with greater impact on many of the planet's resources and ecosystems than any other nation on earth. Indeed, while we represent just 1/20th of the global population, we consume disproportionately higher amounts--at least one-fourth--of practically every natural resource. Meanwhile, the U.S. is the only industrialized country in the world still experiencing significant population growth. This combination of relatively rapid population increase and hefty rates of resource consumption makes for a volatile mix.

To be sure, a large population doesn't necessarily equate to equally large environ-mental consequences. A lot depends on the choices that people make about the way their land is developed (say, "cluster" mixed-use building vs. more spread out "sprawl" development), the types of transportation or energy they use, and the methods employed in local industries or agriculture. And nature has an amazing capacity to bounce back from human assault, often saving us from lasting negative environmental impacts.

But current U.S. population trends, such as rapid growth or density in environmentally vulnerable coastal areas, coupled with high consumption rates of polluting resources--all over a relatively short period of time--are pushing us past some ecological limits. For example, America's current high levels of fossil-fuel burning create carbon dioxide emissions that accelerate climate change; rapid land development costs us 3,000 acres of prime farmland loss daily; and ground and airborne pollutants have made more than 40% of America's rivers and lakes unsuitable for fishing or swimming.

Although we have plenty of empty land in the U.S., that is not the best indicator of how many people we can actually support. The U.S. is now primarily a "metro-nation," with four out of five Americans living in suburban and metropolitan areas. This suburbanization has resulted in sprawl development being the nation's predominant form of land-use change. That has brought a marked increase in both the number of houses (and their average size, the acreage around them, and the resources needed to build, heat, and cool them) and in all the vehicles it takes to get to and from them. That's a big reason transportation has become the nation's fastest-growing, energy-use sector--driving the increased fossil fuel use that has made us the world's largest CO2 emitter. With just 5% of the world's people, we're responsible for 25% of global CO2 emissions.

As we reach the 300 million milestone, a look in the mirror reveals we've become a supersized nation, with a supersized appetite for land, water, energy, and resource consumption. "More of more" is the rule: more people than ever before, more natural resources being utilized to support everyday life, and increasingly, more damage to the natural systems that support us.

But with America's dubious distinction as the most disproportionate user of global resources should come an equally weighty responsibility in dealing with the consequences. Starting here at home, we need to make environmental sustainability a national priority, and American consumers need more readily available environmentally-sound choices that they can afford. In our globalized world, the U.S. should demonstrate renewed international leadership in reducing greenhouse gas emissions and in supporting international family planning. These measures can help restore some balance between people and the environment in the nation and worldwide.

Expanding our nation's consciousness of our per capita environmental impact won't be easy. But maybe that's the growth that America--a nation of 300 million people and counting--really needs. Victoria Markham is director of the Center for Environment & Population.