WORLD POPULATION TO KEEP GROWING THIS CENTURY , HIT 11 BILLION BY 2100
Using modern
statistical tools, a new study led by the University of Washington and the United
Nations finds that world population is likely to keep growing throughout the
21st century. The number of people on Earth is likely to reach 11 billion by
2100, the study concludes, about 2 billion higher than widely cited previous
estimates
The paper published
online Sept. 18 in the journalScience includes the most up-to-date
numbers for future world population, and describes a new method for creating
such estimates.
"The consensus
over the past 20 years or so was that world population, which is currently
around 7 billion, would go up to 9 billion and level off or probably
decline," said corresponding author Adrian Raftery, a UW professor of
statistics and of sociology. "We found there's a 70 percent probability
the world population will not stabilize this century. Population, which had
sort of fallen off the world's agenda, remains a very important issue."
The paper explains the
most recent United Nations population data released in July. This is the first
U.N. population report to use modern statistics, known as Bayesian statistics,
that combines all available information to generate better predictions.
Most of the
anticipated growth is in Africa, where population is projected to quadruple
from around 1 billion today to 4 billion by the end of the century. The main
reason is that birth rates in sub-Saharan Africa have not been going down as
fast as had been expected. There is an 80 percent chance that the population in
Africa at the end of the century will be between 3.5 billion and 5.1 billion
people.
Other regions of the
world are projected to see less change. Asia, now 4.4 billion, is projected to
peak at around 5 billion people in 2050 and then begin to decline. Populations
in North America, Europe, and Latin America and the Caribbean are projected to
stay below 1 billion each.
The figures largely
support U.N. projections from 2013 but add a new dimension, said first author
Patrick Gerland, a demographer at the U.N.
"Earlier
projections were strictly based on scenarios, so there was no
uncertainty," Gerland said. "This work provides a more statistically
driven assessment that allows us to quantify the predictions, and offer a
confidence interval that could be useful in planning."
World population
projections are based mostly on two things: future life expectancy and
fertility rates. Earlier techniques relied largely on expert opinion for how
those trends were expected to change.
The newer forecast
instead uses statistical methods to combine government data and expert
forecasts for such things as mortality rates, fertility rates and international
migration.
Also, earlier reports
represented uncertainty by using scenarios in which women would have 0.5
children more or less than the experts' forecast. That method, Raftery said,
generates too great a range.
"In a given year
and country the fertility rate might be half a child higher, but the
probability that it would be half a child higher in all countries in all years
in the future is very low," Raftery said.
The new method uses
statistical models to narrow the range, finding an 80 percent probability that
the population in 2100 will be between 9.6 billion and 12.3 billion.
"This paper
brings together the research from the past seven years, and also brings in
recent data," Raftery said. "We can answer questions about future
population growth using standard principles of statistical inference, which has
never really been done before."
Rising population
could exacerbate world problems such as climate change, infectious disease and
poverty, he said. Studies show that the two things that decrease fertility
rates are more access to contraceptives and education of girls and women,
Raftery said. Africa, he said, could benefit greatly by acting now to lower its
fertility rate.
The research was
supported by the U.S. National Institute of Child Health and Human Development
and Science Foundation Ireland. Other co-authors are Hana Ševčiková and Nevena
Lalic at the UW; Nan Li, Danan Gu, Thomas Spoorenberg, Guiomar Bay and John
Wilmoth at the U.N.; Leontine Alkema, a former UW graduate student now at the
National University of Singapore; Bailey Fosdick, a former UW graduate student
now at Colorado State University; Jennifer Chunn, a former UW graduate student
now at Singapore's James Cook University; and Gerhard Heilig and Thomas
Buettner, consultants who formerly worked at the U.N.
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