These sizable increases represent an unprecedented global demographic upheaval, according to David Bloom, Clarence James Gamble Professor of Economics and Demography at the Harvard School of Public Health, in a review article published July 29, 2011 in Science.

Over the next forty years, nearly all (97%) of the 2.3 billion projected increase will be in the less developed regions, with nearly half (49%) in Africa. By contrast, the populations of more developed countries will remain flat, but will age, with fewer working-age adults to support retirees living on social pensions.

"Although the issues immediately confronting developing countries are different from those facing the rich countries, in a globalized world demographic challenges anywhere are demographic challenges everywhere," said Bloom.

The world's population has grown slowly for most of human history. It took until 1800 for the population to hit 1 billion. However, in the past half-century, population jumped from 3 to 7 billion. In 2011, approximately 135 million people will be born and 57 million will die, a net increase of 78 million people.

Considerable uncertainty about these projections remains, Bloom writes. Depending on whether the number of births per woman continues to decline, the ranges for 2050 vary from 8.1 to 10.6 billion, and the 2100 projections vary from 6.2 to 15.8 billion.

Population trends indicate a shift in the "demographic center of gravity" from more to less developed regions, Bloom writes. Already strained, many developing countries will likely face tremendous difficulties in supplying food, water, housing, and energy to their growing populations, with repercussions for health, security, and economic growth.

"The demographic picture is indeed complex, and poses some formidable challenges," Bloom said. "Those challenges are not insurmountable, but we cannot deal with them by sticking our heads in the sand. We have to tackle some tough issues ranging from the unmet need for contraception among hundreds of millions of women and the huge knowledge-action gaps we see in the area of child survival, to the reform of retirement policy and the development of global immigration policy. It's just plain irresponsible to sit by idly while humankind experiences full force the perils of demographic change."

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