This story by Gabe Spitzer, our Humanosphere collaborator and podcast producer, is published at KPLU.org

The planet could be much more crowded by the end of the century than previously thought, according to a new report by University of Washington researchers.

That contradicts a general consensus that world population growth is likely to stabilize before long. The population has been expected to rise from the current seven billion or so to about nine billion, before leveling off and possibly declining.

But new projections, based on new statistical models, suggest the numbers will not tail off after all. Instead, statistician and sociologist Adrian Raftery said we could hit 11 billion and counting by century’s end.

“I was quite surprised. I was expecting that conventional wisdom would be borne out,” he said. “But it seems once one brought to bear new, more recent data, that actually there was a substantial change.”

That new data suggests reproductive rates aren’t decreasing as quickly as thought – especially in Africa, where the population could quadruple by the year 2100. Overall, the study concluded there is an 80 percent chance the population will fall between 9.6 billion and 12.3 billion.

Graph. (A) UN 2012 world population projection (solid red line), with 80% prediction interval (dark shaded area), 95% prediction interval (light shaded area), and the traditional UN high and low variants (dashed blue lines). (B) UN 2012 population projections by continent. Science

Raftery said this estimate used much more rigorous methods than the old models, which relied to some extent on opinion. In the past, he explained, experts would interpret the available data to come up with projections, which they would they use to set up the models that made future projections. Now, he said they use something called Bayesian statistics to cut out the middleman.

Raftery noted the ballooning population was a major concern up until the 1990s, when the models started showing growth would taper off.

“It has kind of fallen off the world’s agenda to some extent. And I think these recent results suggest that this may have been premature,” he said, adding that population growth is a key input to problems like climate change, disease and environmental degradation.

The study, done in conjunction with United Nations demographers, is published today in the journal Science.