The U.S. Census Bureau has projected a possible population of one billion Americans a century from now, in the decades bracketing 2100. That would be more than triple the current population of about 304 million. Initially advanced in 2000 as an upper-limit scenario, the estimate is increasingly viewed as plausible because of continuing high immigration, high birth rates for immigrants and their descendants, and increased life-expectancies in the population as a whole.

If so, the U.S. would account for a rising share of the world’s population over time. Today the 304 million figure is not quite five percent of the world’s estimated 6.7 billion people. But since the world population is projected by the U.N. to top out at between 9 and 10 billion by about 2070 as birth rates come down everywhere, the U.S. share could rise substantially to as much as 10 percent of the total.

U.S. population growth rates above one percent a year stand in sharp contrast to Japan and Europe, whose population counts are leveling off or even decreasing (as in Russia and Italy). Since the difference derives largely from high rates of immigration into the U.S., if immigration rates fell, population projections would go down accordingly. But in the meantime, a separate 2008 extrapolation of current trends to 2050 attributes over 80 percent of the projected U.S. increase of 142 million people to immigrants and their offspring.

Will such continued population growth put a strain on the American economy? Whatever the social frictions immigration may bring, slower economic growth is probably not the issue. Consider, for example, another projection from a survey of economists in 2005 conducted by Robert Whaples of Wake Forest University. Most of those surveyed expect continued U.S. economic growth per capita over the next 60 years of about 1.8 percent a year, or a little lower than the rate for the past decade. At this pace, the 2006 per capita income of $44,200 would triple over the next 60 years and by 2100 would reach $236,400 in terms of today’s dollars.

If these numbers sound farfetched, consider that per capita income in the U.S. rose nearly seven-fold between 1900 and 2000. Can anything comparable take place in the 21^st century? Most of the economists surveyed would answer yes: economic growth will continue, and so will the doubling of average incomes every 40 years or so.

Granted, the current oil and commodities crunch may make such optimistic predictions look naive. As the geographer Jared Diamond opined in January in the New York Times, resource constraints could prevent India and China (whose per capita incomes remain only about 10 percent of those in the U.S.) from ever reaching American living standards. This, even if the U.S. stood still in terms of population and living standards. In his view, radically altered patterns of consumption and conservation will have to be reached in the U.S. and elsewhere, given the earth’s resource limitations.

He may be right, but judging by the survey, not many economists would agree with him. What they might suggest instead is that rising average incomes in the U.S. could lead to falling birth rates among immigrants and their descendants. But that would be a more gradual adjustment, in the fullness of time.

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