A view of New York city to to the north from the 75th floor of 432 Park Avenue October 15, 2014. TIMOTHY A. CLARY/AFP/Getty Images

When ecologist Corey Bradshaw gives public talks about the state of the globe's endangered wildlife, he sees a recurrent phenomenon. At the end of the talk, in the question and answer period, "someone will stand up and say, 'You’ve neglected the elephant in the room -- human population size is the principal problem,'" says Bradshaw, who is director of ecological modeling at the Environment Institute at the University of Adelaide in Australia.

It's easy to see where this idea comes from: There are a lot of humans on the Earth. There are currently more than 7.2 billion of us, and the UN projects there will be 9.6 billion by 2050, and 10.9 billion by 2100. In fact, though estimates vary, something on the order of 6.5 to 14 percent of all the people who have ever lived at any time, ever, are living now. (Stop and think about that for a minute.) And there's no denying these people take up space and consume resources.

So Bradshaw decided to look into this question of whether trying to reduce the size of the global population would help stave off climate change, the loss of species, and other environmental concerns. The resulting research, just out in the Proceedings of the National Academy of Sciences and co-authored by the University of Adelaide's Barry Brook, seriously challenges the idea that greens ought to be campaigning for population control.

On top of the serious ethical problems with trying to restrict the global population, the study also finds a purely practical one: It doesn't even appear possible. "No matter what levers you pull, we have such a huge demographic momentum, there’s no way we can rein in the human population fast enough to address sustainability issues in the next century," says Bradshaw. Or as the study itself puts it, the increase in population over the course of the 21st century is "virtually locked-in"; this means, the authors argue, that population reduction "cannot be argued to be the elephant in the room for immediate environmental sustainability and climate policy."


Credit: United Nations, Department of Economic and Social Affairs, Population
Division (2013). World Population Prospects: The 2012 Revision,
Volume I: Comprehensive Tables ST/ESA/SER.A/336.

To show as much, the researchers used population models to examine nine different scenarios for the global human population over the course of the 21st century. One involved a "humane" approach to population control, in which female empowerment and greater access to contraception lead to better family planning and women having more control over their fertility. Two others modeled a much harsher "one child" policy, implemented either gradually by 2100, or more rapidly by 2045.

Rather grimly, the study also modeled a number of hypothetical disaster scenarios in which large swaths of humanity die due to climate change, pandemics, or wars. In one scenario, Bradshaw and Brook added together the number of people killed in World War I, World War II, and the deadly Spanish Flu (roughly 131 million) and calculated what percentage of humanity that represented at the close of the second World War. The answer was about 5.2 percent. So in one scenario, the study assumed a similar 5 percent die-off of the human population circa 2056 (when that total would be 500 million people out of a projected 9.95 billion living on the globe). Higher catastrophe scenarios, meanwhile, contemplated the staggering death of 2 billion and 6 billion people worldwide. In a climate change scenario simulating food shortages, meanwhile, childhood mortality dramatically increased.

The upshot was that only very unrealistic and extreme scenarios -- 6 billion people suddenly killed in a catastrophic war or pandemic; or a sudden, draconian and globally enforced one-child policy -- dramatically changed the trajectory of population growth by 2100. With greater childhood mortality due to climate change, a dramatic die-off of 500 million people, and a gentle, humane move towards female empowerment, the result was always the same: We wound up roughly where the U.N. currently projects, or around 10 billion people by 2100. "The population has this natural resistance to catastrophe," says Bradshaw.

In some ways, just as significant as the new paper itself is the person who edited it: Stanford University's Paul Ehrlich, author of the 1968 classic book The Population Bombwhich in many ways put population concerns on the map. They have since become a third rail because of the ethical issues-- though Bradshaw emphasizes that he does not support any kind of population reduction except for the voluntary kind that naturally results from female empowerment, leading to greater control of fertility through contraception. "Fertility reduction through family planning is the only humane way to reduce the human population size," says Bradshaw.

By 2050, that approach might reduce the global population by a few hundred million people. That's not nothing, but Bradshaw emphasizes that it's no big dent in population projections. The reason is simply that at present, we're in a phase of population growth that is exponential. "Think about a speeding car," says Bradshaw. "We’re going 150 km per hour." We may slow it down eventually, but there is no way to take away the current momentum.