As always, America is a work in progress. Any day now, the Census Bureau will announce that the U.S. population has reached 300 million, 39 years after it passed 200 million (1967) and 91 years after it hit 100 million (1915). What is more interesting and less appreciated is that, according to Census projections, it will climb to 420 million by 2050. Virtually alone among big, advanced countries, America is experiencing significant population growth. This is a sign of either impending calamity or enduring vitality. I’m in the vitality camp, but I admit that it often seems a close call.

Aging and immigration — the big population trends — are vexing. By 2030, the 65-and-over population will be about 20% of the total, up from about 12% in 2000, says Census. That will involve staggering costs for Social Security and Medicare. Meanwhile, the Census projections assume annual immigration of about one million, roughly the present level. That will transform the nation’s ethnic profile and could reshape its politics and culture. By 2050, Hispanic-Americans will be almost 25% of the total, double their share in 2000, projects Census. Asian-Americans also will double their share to 8% in 2050. Over the same period, non-Hispanic whites drop from 69% to 50%. Blacks stay around 13% to 14%.

Indeed, immigration has already had a profound effect. Demographer Jeffrey Passel of the Pew Hispanic Center estimates that about half of the last 100 million Americans are immigrants and their American-born children. Without them, the population would now be 247 million, he says. The Hispanic-American population would total 16 million instead of 44 million. Asian-Americans would number two million, not 13 million.

But history suggests that America will change them more than they change us. Our national character and culture are enormously powerful and resilient. In the 1830s, the Frenchman Alexis de Tocqueville — author of “Democracy in America” — identified some American traits, including rampant materialism, religious fervor (and competing denominations), and fierce patriotism. In 1830, the population was 13 million. But if he returned today, he would find the same traits despite massive changes in people andtechnology. We adapt, even while bedrock principles and attitudes endure.

But I could be wrong. Population growth has raised two serious concerns. One is environmental. It is that we are creating overcongested communities that will demand energy and — particularly in the Southwest — water that won’t be there or will be there only at an exorbitant price. Population growth will cause an economic and social backlash.

Perhaps. But this is a big country, and much of it is still empty. There’s plenty of coal. If Southern and Western metro areas become too crowded or costly, maybe people will return to Cleveland and Milwaukee, where water is plentiful and housing prices are low (median prices: $139,000 and $228,000). Still, population growth shows why curbing energy use — and greenhouse gas emissions — is so hard. The population is projected to increase 40% by 2050. Simply to keep total energy demand steady would require each American, on average, to make correspondingly deep cuts in present energy use.

The second concern involves immigration — and its possibly explosive combination with aging. Up to a point, America’s willingness to accept immigrants is a sign of confidence that promotes economic growth. But our careless approach to immigration is creating social problems. Most Hispanic immigrants are poor and have low skills. Their average weekly wages (now $389) are only two-thirds of the average for all workers ($577). The predominance of poor workers frustrates future assimilation. It’s hard to move into the middle class. All this makes immigration seem threatening to millions of Americans, who visualize their country being overrun by an alien underclass.

The potential mixing with aging is obvious. Paying the retirement benefits of baby boomers could easily require federal tax increases of 30% to 50%. Even without immigration, younger workers may object to such steep burdens, especially because many retirees will be richer than workers. Now add the impact of immigration. A growing part of the labor force will consist of Hispanic- and Asian-Americans. Most won’t have relatives on Social Security and Medicare. They may wonder why they should pay so much to support somebody else’s wealthier parents. The politics could get ugly.

We might have mitigated all these problems. We might have controlled the border better and favored more highly skilled immigrants with better assimilation prospects. We might have reduced boomers’ Social Security and Medicare costs by limiting benefits for younger and wealthier retirees.We might even have curbed our energy appetite. But we have done none of these things. So if population growth backfires, we will have only ourselves to blame.