The next 100 million and the face of America

"The tipping point has been reached there," says Henderson. "I feel very hopeful that the evolution to the solar age could happen much quicker than we might have expected because it's being driven by so many stress points, from global warming to water shortages to desertification."

By mid-century, she predicts: "Cars will be getting 100 m.p.g. if they're still using gasoline instead of fuel cells. That's definitely a no-brainer. Cities and towns will get more and more compact as these sprawling suburbs end up being too costly and inefficient."

That vision for the future contrasts sharply with Mr. Kotkin's. But given current political, economic, environmental, and social trends – especially the unknowns about world energy supplies – it is likely to be just as valid.

Meanwhile, the US population clock keeps ticking: Every 13 seconds somebody dies. Every 31 seconds there's another immigrant – legal or illegal. It adds up to a net gain of one person every 11 seconds, or about 8,000 every day. It took 39 years to add the most recent 100 million; the next 100 million will take a couple of years less than that.

The US population growth rate is expected to decline a bit by mid-century. Still, by then the numbers will have increased to some 420 million, according to official calculations. Critics of US immigration policy say the number could be significantly higher.

"If Congress should end up ducking the issue of immigration reform and maintaining the status quo of mass legal and illegal immigration, our population is projected to still continue its rapid growth," warns the Federation for American Immigration Reform in a recent report. "Our projection is for a population of between 445 and 462 million residents depending on the assumptions used."

But societal changes tied to population are more than numbers.

As the racial and ethnic mix among Americans shifts in the decades ahead, public attitudes are likely to change as well. In some ways, they already are.

For example, between 1986 and 2003, the share of adults who approved of interracial marriage rose from 70 percent to 83 percent, according to a Roper Reports study. This trend is especially true among young Americans. A 2002 Gallup survey showed that just 30 percent of adults 65 and older approved of marriage between blacks and whites. But among people between 18 and 29, 86 percent said they had no problem with interracial marriage.

"The fact that today we see young people intermarrying more, interracial dating much more common – all of that I think portends that we're going to become much more ecumenical in the way we look at things than we were in the past," says William Frey, a demographer at the University of Michigan and the Brookings Institution. "I think we'll have much more tolerance for people of other backgrounds, cultures and languages, points of view, and religious and belief systems."

What's certain is that there will be a lot more Americans.

• Last of five parts. Previous installments in this series appeared Sept. 12, 19, and 26, and Oct. 3.

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