The U.N.'s Population Division projected that the world will reach just over 10 billion by 2100, but this only shows us what would happen if today's demographic trends follow specific paths.
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New U.N. Projections Show World Population Will Hit 7 Billion in October

Today, the United Nations announced that the world's population will reach an historic 7 billion people on Oct. 31, 2011. World population hit 1 billion people in 1804. It took 123 years to add the next billion, but less than a century to cruise past the next four billion -- from 2 billion people in 1927 to 6 billion people in 1999.

The U.N.'s Population Division also projected that the world will reach just over 10 billion by 2100, a number that contains many uncertain assumptions about the future. Demographic projections are often mistaken for predictions, but they only show us what would happen if today's demographic trends follow specific paths. And we all know that change is inevitable. We also know that if millions of women continue to have problems accessing contraception, and lack economic and educational opportunities, these numbers could become far greater.

The projections offer only a few scenarios and are based on potential changes in policies, services and behaviors. They do not account for the realities we see on the ground, where in some countries, women are not getting the family planning services they want. The decisions and policies we make today will ultimately determine whether our numbers climb to anywhere from 8 billion to 11 billion by mid-century.

The U.N. projections show some striking changes in countries' projected populations for 2050. Forty countries' populations are projected to at least double in the next 40 years, including Afghanistan, Iraq and Yemen. Nigeria's population for 2050 is projected to jump by 150 percent, from 158 to 390 million.

Many of these increases in projected population, especially those in sub-Saharan Africa, are due to persistently high fertility rates. The projections assume that the average number of children per woman will begin falling in such countries, but this is often a rosy assumption. For example, Nigeria's fertility rate for 2010-2015 was previously projected to be 4.8 children per woman, but has now been revised to 5.4. This difference contributes to a much larger total population by 2050.

Still, the assumptions built into the projections for many high-fertility countries would require major increases in the use of family planning. Nigeria's fertility rate, measured at almost six children per woman in 2008, is projected to fall to slightly over three children by 2050. This is highly unlikely if current trends continue, because only 10 percent of married women use effective contraception. Twice as many women have an unmet need for family planning. Until their health care needs and rights are fulfilled, the demographic future projected for Africa's largest nation seems too optimistic.

The projections may also be optimistic for countries at the opposite extreme, with very low fertility rates. Fertility rates in Japan, Korea and Russia have declined significantly since the late 1980s. Their rates are now 1.4 children or less, which would lead to significant population decline. However, their rates are projected to increase by at least 30 percent by 2050. Very low fertility in these countries is a new phenomenon, but some researchers believe it is linked to gender inequities and difficulty balancing work and family. As it turns out, a lack of opportunities for women may be the driving force behind both very high and very low fertility rates.

The surprising assumptions underlying some countries' fertility rates reflect one of the key features of the population projections. In the past, the projections were constructed using a technique that, in the U.N. medium fertility projection, assumed all countries would move toward a universal fertility rate of 1.85 children per woman. High and low fertility projections only varied from the medium by 0.5 children per woman in either direction.

This universal rate was highly improbable among countries at the demographic extremes, and the U.N. recognized that it did not allow for unpredictable demographic transitions within individual countries. Uganda is a dramatic example of this. The previous Revision of World Population Prospects showed that the country's fertility rate had fallen by about half a child per woman -- less than eight percent -- between 1950 and 2010. Yet the country's fertility rate was projected to decline by 60 percent, to less than three children per woman, by 2050.

Starting with the 2010 Revision of World Population Prospects, the U.N. has shifted to a "probabilistic" model for its medium-fertility variant. This allows the pace of each country's fertility decline to be calculated individually, based on new estimates of historic fertility rates. The new method also assumes that fertility rates will eventually balance out around 2.1 children per woman, a level where couples would "replace" themselves in the population, rather than 1.85. This method allows for much more variance across countries. The projections also now extend out to 2100, and incorporate life expectancies ranging as high as 90+ years.

These U.N. changes add greater nuance to population projections, but they are still far from a crystal ball. Policymakers must take the next step and invest in family planning and education for the projections to ever meet reality.

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