The population of the United States officially passed the 300 million mark on Tuesday (October 17). While it took almost two centuries for the U.S. population to reach 200 million, it took just 40 years to add the last 100 million.
The number of people living in America almost tripled in the 20th century, with most of that growth coming just since the 1960s. The United States is now the third most populous nation in the world, behind China and India.
A baby is born in the U.S. every 14 seconds. The current U.S. population growth rate -- about one percent annually or three million new mouths to feed every year-- is moderate compared to many countries in the developing world. Still, the growth is fueling public debate about the pressures a larger population will put on America's environment, energy resources and social fabric.
"The more people there are the more you need to divide already scarce resources," says Janet Larson, the director of research at the Earth Policy Institute in Washington, DC. "Having a larger population means those people are using the same limited amount of water, using the same transportation infrastructure. Those 300 million people are also all going to want to use energy so that also has an impact on the environment."
Larson says the timing of this population milestone is also significant. "It's interesting that we are going to hit this 300 million landmark right before midterm elections, when immigration is a very hot topic. So I imagine that we are going to hear more and more about population from politicians in terms of immigration."
The U.S. has experienced population spurts before, most notably right after World War Two. Thomas Buettner, chief of Estimation and Projections with the United Nations Population Division, says America's so-called "baby boom" was triggered by a an expanding economy, improved health care and rising immigration rates in the late 1940s and early 1950s.
"The population change is the result of the momentous change of the conditions of living on earth," he says. "It is driven by, first, an increased ability of people to manage their health and fight diseases and, second by the increasing ability to plan their family sizes."
The fertility rate in the United States has decreased in the last four decades and stands today at just slightly over two births per woman. Though that's higher than most Western European countries, demographers consider this to be a replacement rate for the U.S. population. "The biggest growth is immigration, much of the growth is coming from people coming into the country," says the Earth Policy Institute's Janet Larson. California and Texas are the most populated states but the U.S. East Coast has the highest density of population.
Larson says many of the immigrants coming to the U.S. are from Latin America. "They have more children than the average American, and that's helping to drive the population growth."
America's fast-growing population poses many social and political challenges, and, for that reason, provides incentives to better manage scarce resources, according to United Nations demographer Thomas Buettner. "We have, I think, much of the knowledge to do that. I think the challenge is now for the politicians to react to that and put it into place and also to convince the population to conserve energy and also be responsible with resources."
But according to Buettner, the most serious challenge the United States and other developed countries face is not population growth but population aging. More than 12 percent of the U.S. population is 65 or older. This sizeable group of elderly citizens, Buettner says, will have a profound impact on social services and the U.S. economy in the next decades.