A new study released this week by the University of Washington and the United Nations confirmed that the world population is likely to continue growing throughout the 21st century.

The number of people on Earth should reach 11 billion by 2100, which is 2 billion higher than previous estimated, according to a University of Washington news release.

The researcher's paper was published on Sept. 18 in the journal Science. It 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, according to the release. "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 outlines the most recent UN population data released back in July. This is the first U.N. population report to include modern statistics, also known as Bayesian statistics, which combines all available information to generate better predictions.

Most of the expected growth will take place in Africa, where population is predicted to quadruple from 1 billion today to 4 billion by the end of the century. This is expected because birth rates in sub-Saharan Africa have not been going down as fast as expected, according to the news release.

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 locations around the world are expected to see less of a change.

Asia, which has 4.4 billion, is projected to peak at around 5 billion people in 2050, and then decline. Populations in North America, Europe, and Latin America and the Caribbean will stay below 1 billion each, according to the study.

"Earlier projections were strictly based on scenarios, so there was no uncertainty," said first author Patrick Gerland, a demographer at the U.N., according to the release. "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."

Projections are based on two things: fertility rates and future life expectancy. Previous techniques mainly revolved around expert opinions for how those trends were expected to change.

For newer projections, researchers used statistical methods to combine government data and expert forecasts for mortality rates, international migration, and fertility rates.

Earlier reports represented uncertainty by using scenarios in which women would have 0.5 children more or less than the experts' predicted.

"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.

There is an 80 percent probability that the population in 2100 will be between 9.6 billion and 12.3 billion, according to the new method.

"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."

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