An additional 150 people join the ranks of humanity every minute, a pace that could lead our numbers to reach nine billion by 2050. Changing that peak population number alone could save at least 1.4 billion metric tons of carbon from entering the atmosphere each year by 2050, according to a new analysis—the equivalent of cutting more than 10 percent of fossil fuel burning per year.

"Demography will matter to greenhouse gas emissions over the next 40 years," said Earth systems scientist Brian O'Neill of the U.S. National Center for Atmospheric Research, lead author of the analysis, in a statement. "If global population growth slows down, it is not going to solve the climate problem, but it can make a contribution."

O'Neill and his colleagues paired data from national household surveys in 34 countries with a new economic model—the Population Environment and Technology (PET) model—to estimate the impact of various population growth scenarios on greenhouse gas emissions. The model also took into account changes in the makeup of that overall population, based on United Nations data, such as the aging population of Europe as well as the rapidly urbanizing one of India.

That urban growth—roughly half of humanity already lives in cities for the first time in recorded history—may lead to a big increase in greenhouse gas emissions. As urban residents enter the middle class, particularly in China and India, they increase their consumption of energy and transportation. "Urban living can be more energy efficient," the authors write in the analysis published online in Proceedings of the National Academy of Sciences on October 11, but increased income results in "increased emissions."

At the same time, the mellowing that comes with age in industrialized countries could cut emissions from countries such as those in the E.U. by as much as 20 percent. At least that's true if present retirement ages and the like remain the same; "if retirement is postponed," the scientists note, "the emissions-reducing effect of aging that we find here will be lessened."

Overall, curbing population growth could reduce greenhouse gas emissions; reducing peak population to roughly 8 billion, for example, could save 29 percent of expected greenhouse gas emissions. Economic growth seems like one way to accomplish that, considering that rising wealth has historically slowed birth rates. But O'Neill and his colleagues warn that, if fewer but richer people consume more—as current consumption patterns in places like the U.S. suggest—those greenhouse gas savings become increased emissions.

Ultimately, family planning alone—such as the use of condoms and other reproductive health services—in parts of the world with growing populations, including the U.S., could restrain population growth significantly, this analysis finds. It would appear that we're trying, thanks primarily to ongoing efforts to enable women to take control of their own lives through education and other methods. Already, birth rates the world over have halved from an average of five children per women to just 2.6 today—a baby bust replacing the baby boom.

Image: © / Nikada