This is how immigration will change Europe over the rest of this century

The debate over immigration has thrown politics on both sides of the Atlantic into confusion.

In the United States, Republicans are divided over the nominee their voters have chosen for the presidency and his incendiary comments about migrants. Meanwhile, as Britain prepares to leave the European Union, the British prime minister has said he will step down, and the leader of the opposition there just lost a vote of confidence among members of his own party.

Control over immigration was a major issue in the campaign over whether Britain would remain in the E.U. Polls closed there last week just hours after the U.S. Supreme Court declined to intervene in the controversy over President Obama's immigration policy.

Demographers project more and more immigrants for decades to come. Last Thursday's events raise troubling questions about the ability of political institutions in the developed world to cope with their arrival.

Annual net immigration into Europe is projected to increase steadily from current levels for another 20 years. This year, just over 1 million immigrants will arrive in the Europe, according to Eurostat, the statistical agency of the E.U. That figure will reach an apex of nearly 1.5 million in 2036, the agency projects.

If current trends hold, immigration to Europe will not subside below its current level until 2069. The continent will have seen net immigration of 77 million people in that time. The figure includes immigrants to the 28 members of the E.U. as well as Iceland, Norway and Switzerland.

By 2080, these migrants and their progeny will have increased the population of the E.U. by 121 million, relative to what the continent's population would be then without any immigration. The result will be a Europe that is substantially more diverse than it is today.

For example, the number of European Christians is projected to decline by about 18 percent, to 454 million, between now and the middle of the century, according to the Pew Research Center. The center predicts that the number of European Muslims will nearly double, to about 71 million.

To be sure, these are only projections. If Europe's economic woes continue, the continent will be less attractive to potential immigrants in Asia and Africa. If the population in Asia and Africa does not grow as fast as forecasters anticipate, there will be fewer migrants as well.

European governments could also try to reduce levels of legal immigration by preventing migrants from crossing their borders and deporting those who do illegally. Efforts to reduce immigration have only a mixed record of success, however. When one crossing is policed, migrants inevitably find other routes to their destinations.

In the United States, there is evidence that enforcement along the border with Mexico might even have worsened the problem. Some experts on immigration argue that had the federal government allowed Mexican laborers to move freely across the border, they would have traveled back and forth alone, between work and their families. But because U.S. authorities made the journey costly and dangerous, some say, many migrants decided to cross the border just once, taking their families with them.

By one estimate, instead of reducing the number of migrants living illegally in the United States, enforcement along the Mexican border has increased that population by 44 percent by encouraging workers to take their families with them on the journey north. Some researchers dispute these figures. There also is no consensus that the manpower and resources dedicated to controlling the border have reduced illegal immigration.

Europe's efforts to control illegal migration might be more successful, in part because the Mediterranean is a natural barrier and because fewer people are trying to go to Europe. An average of 1.6 million immigrants arrived in the United States every year between 2000 and 2005, Pew estimates.

The rate of immigration has declined since then and is not expected to accelerate as it is in Europe. All the same, the share of the U.S. population born outside the country should continue to increase, according to Pew, from 14 percent today to 18 percent five decades from now.

The organization projects that future immigrants and their progeny will increase the U.S. population by 103 million in 2065 relative to what the population would be without additional immigration.