Why estimate for world population in 2100 is now 11 billion

That's an updated estimate according to United Nations data. The new number comes days before the UN Climate Summit, which will take up a range of issues influenced by world population.

Earth's human population is expected to continue growing through the end of the century, with an 80 percent likelihood that it will reach 10.9 billion from today's nearly 7.3 billion, according to updated estimates from the United Nations.

The projections stand in stark contrast to a widely held idea that the human population would peak in 2050 at about 9 billion.

The revised numbers were published online Thursday afternoon by the journal Science in a paper summarizing data that the UN's Department of Economic and Social Affairs posted on its website in July.

For some analysts who follow global demographic trends closely, the prospect that population growth will watch the 9-billion marker recede in its rearview mirror isn't surprising.

"The reality is that the UN has never said with anything even remotely resembling certainty that the world population would in fact top out at 9 billion in 2050," says Robert Engelman, a senior fellow at the Worldwatch Institute in Washington who focuses on global population issues. For projections that extended beyond 2050, "their median projection has always foreseen continued growth after the middle of the century."

Projections suggesting that this would be the case also have come from outside the UN. In 2001, for instance, an international trio of demographers led by Wolfgang Lutz at the International Institute for Applied Systems Analysis in Austria published an estimate in the journal Nature pointing to an 85 percent chance that global population growth would top out before 2100 but only a 20 percent chance it would peak by 2050. That study suggested that population would peak at 9 billion by 2070 and slip back to 8.4 billion by 2100.   

The latest analysis is the result of efforts undertaken over several years to build into UN population forecasts some sense of how likely the projections are, not just a range of projections based on expert opinion.

Each approach explores trends in life expectancy and fertility rates – essentially the number of births per woman. But the approach applied by the new study's team – led by Patrick Gerland in the population division of the UN's Department of Economic and Social Affairs in New York and Adrian Raftery at the University of Washington in Seattle – uses sophisticated statistical tools to derive its numbers and probabilities.

The latest projection underscores the idea that "population, which had sort of fallen off the world’s agenda, remains a very important issue,” Dr. Raftery said in a prepared statement.

The new numbers are appearing during the final countdown to the UN Climate Summit on Sept. 23. Population is not on the table per se, but a range of issues influenced by population are, notes Jessica Hellmann, a conservation biologist who heads the climate adaptation program within the University of Notre Dame's Environmental Change Initiative in South Bend, Ind.

The new population analysis shows that the largest growth rates are likely to be in Africa, where the population is expected to burgeon from 1 billion today to 4 billion by century's end. Much of that will be driven by population growth in sub-Saharan Africa, where birthrates have not fallen as quickly as many had expected.

Yet this is where some of the poorest countries on the planet are – where energy demand will grow with development and where the effects of climate change will tax economies lacking the resources to effectively adapt.

"Equally important, if not more important than how many people there are, is what energy technology they are using," Dr. Hellmann says. The population numbers reinforce the notion that economic development there "has got to be clean." Helping these countries pay for the technologies they will need is one of the tough issues at global climate talks.

The same holds true for adaptation needs. Based on an adaptation index that Hellmann and her colleagues have devised to rank a country's or region's capacity to adapt to global warming, sub-Saharan Africa has the lowest score of any region on earth.

As with green development, funding to help these and other developing countries adapt to global warming is another thorny issue.

As for climate projections themselves, the UN Intergovernmental Panel on Climate Change already has accounted for a population reaching about 12 billion by 2100 in the emissions pathways it developed for the latest set of climate reports that the panel has been issuing since September 2013.

"We always assume a range of population estimates because population scenarios are inherently uncertain," explains Richard Moss, a senior scientist with the Joint Global Change Research Institute, based at the University of Maryland at College Park, in an e-mail.

The trajectories that those estimates take can be affected by whether a range of social or economic changes occur. Population specialists say the most important of these are increased education and job opportunities for women, as well as improved health care and changes in contraception policies, especially in the developing world.

The new numbers were factored in to the report released this week by the Global Commission on the Economy and Climate on the costs of greenhouse-gas abatement, according to Manish Bapna, executive vice president and managing director of the World Resources Institute in Washington and one of the project leaders for the report.

Climate is only one of several environmental or development issues on which population touches. Another is food security – a topic that researchers are tackling in a variety of ways.

For instance, researchers have been uncovering ways to soup up photosynthesis to increase crop productivity.

In Thursday's issue of the journal Nature, a team led by Myat Lin, a researcher at Cornell University in Ithaca, N.Y., genetically modified tobacco plants to produce cyanobacteria's version of the enzyme Rubisco, which plays the key role in converting carbon dioxide to sugars, with the help of sunlight.  Tobacco plants have their own version of Rubisco. But it's much less efficient in converting CO2 to sugars than the cyanobacterial version.

The engineered Rubisco alone is no silver bullet, researchers say. But in combination with other approaches, it could play an important role. This led Australian National University biologists Dean Price and Susan Howitt to dub the results "a milestone on the road to boosting plant efficiency" in a companion article to the paper describing the results.