LOS ANGELES — Ever since the Industrial Revolution brought belching smokestacks, greenhouse gases and human population have climbed in lockstep to ever-higher levels.
And while scientists warn that humanity must dramatically slash future carbon-dioxide emissions to avert extended droughts, floods and other climate catastrophes, they have generally avoided a rigorous examination of how slowing population growth would help. Now, an international team of scientists has done the math.
If global population were to grow by less than a billion by midcentury, instead of by more than 2 billion, as expected, it would be the equivalent of cutting as much as 29 percent of the emissions reductions needed by 2050 to keep the planet from tipping into a warmer, more dangerous zone. By the end of the century, the effect would be up to 41 percent.
That’s similar to some of the other large-scale emissions-cutting strategies, such as doubling the fuel efficiency of 2 billion cars from 30 miles per gallon to 60 or replacing coal power with a 700-fold increase in solar power.
Richard Somerville, a climate scientist at Scripps Institution of Oceanography, said he was impressed that researchers had tackled population growth, an often-taboo topic because of religious and cultural resistance.
“People are not aware of how fast the population has grown and how it’s a multiplier of consumption,” said Somerville, who was not involved in the study. “You don’t see policymakers taking about it in climate negotiations.”
Being published Tuesday in the Proceedings of the National Academies of Science, the study offers a novel way to quantify how changes in human population influence the buildup of greenhouse gases in the atmosphere. Brian O’Neill, senior scientist at the National Center for Atmospheric Research in Boulder, Colo., was the lead author.
Drawing on surveys from 34 countries — representing 61 percent of the world’s population — the analysis looks at how demographic changes affect economic growth and energy use and the resulting pollution.
Today’s population of 6.9 billion is expected to reach 9.1 billion by 2050, according to U.N. demographers. They suggest two alternative scenarios: a path of slower growth that would reach slightly less than 8 billion or a faster one that would reach nearly 10.5 billion.
But it’s not just population size that matters, the study found. It’s also where and how people live.
Growth in the United States has an outsized influence. An average U.S. resident emits four times as much carbon dioxide as a resident of China and 20 times as much as an African.
The U.S. Census Bureau estimates that U.S. population of 310 million will swell to 439 million by midcentury, with most of the growth from immigrants and their children.
More broadly, the scientists found the trend toward urbanization had a large effect on pollution levels. In developing countries, a typical farmer may or may not have electricity back home but generates more pollution after moving into a bustling city with modern amenities.
The most profound influence was urban migration’s indirect effect through economic growth. It’s a twist on the idea that a booming economy, like a rising tide, lifts all boats. In this case, economic growth lifts pollution levels higher too.
“As the economy grows faster, it raises the income for everybody, and people are spending more money and consuming more and emitting more,” O’Neill said. As the population ages, the ranks of retirees swell, slowing economic growth, he said. The result: lower emissions.
When it comes to climate change, O’Neill said some people believe human population is the key, underlying problem. Others believe it’s inconsequential. “We find that the truth probably lies somewhere in between.”
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