Reuters/Enrique Marcarian.

Spanish researchers are making the case that the world will in fact be less crowded in 2100 than it is now.

For the past two decades, demographers have generally agreed that global population growth will continue to inch steadily higher in the coming century, raising concerns about everything from pollution to housing to the world's water supply.

But a new study out of Spain suggests those estimates may be way off—we're talking several billion people off—and that the earth's population could instead peak as soon as 2050. Applying a mathematical model to global population trends, these researchers believe that there will be fewer people living on earth in 2100 than there are today.

In 2011, the United Nations population division predicted a global population of 10.1 billion by 2100, an increase of nearly 50 percent from the earth's current population of 7 billion. In the U.N. models, only the low-fertility curve showed the possibility for population decline in the 21st century.

But scientists at the Autonomous University of Madrid and CEU-San Pablo University say their estimates, developed by using techniques from high-level physics to analyze UN population data between 1950 and the present, match that low-fertility curve. That path, indicated with boxes in the graph below, shows global population peaking in 2050 slightly above eight billion, and then falling back to 6.2 billion by the end of the century, the same as the total world population back in 2000.

This sort of decline in global population would obviously dramatically affect the future of the world's food supply, its water use, global carbon footprint, urbanization, and nearly every other aspect of life on earth.

Recent UN forecasts have also indicated that 70 percent of the world's population in 2050 will live in cities, which has left many demographers concerned that cities in the developing world will face enormous humanitarian crises in the next few decades.

But what if global population in 2050 is not the middle-rate 9.2 billion, but the low-rate 8.2 billion? The situation could be entirely different, with the potential urbanized population dropping significantly from previous estimates of 6.4 billion.

Population projections try to predict the battle between two trends: the drop in the mortality rate and the decline in the birth rate. UN population estimates, national and global, have documented the progress from a high-mortality, high-fertility society to one with low mortality and low fertility [PDF]. Declining fertility has followed declining mortality, meaning that national populations have tended to rise before they stabilize and, eventually, fall.

This research, published in the journal Simulation in February, indicates that the decline in the birth rate (prompted by declining fertility rates) will catch up to the decline in mortality rates faster than in the middle-line UN forecast, which would make the 1980s the time at which the world population was growing the fastest.

There is some evidence in support of the UN's low-fertility rate prediction. In 1992, the UN estimated that world population would hit 10 billion by 2050. In more recent predictions, that milestone has been pushed back to the last decade of the 21st century.

What if it's never reached at all?

Top image: Mar del Plata, Argentina. Reuters/Enrique Marcarian.

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