Overpopulation has been a threat to the planet since long before anyone heard of climate change.
English economist Thomas Malthus first sounded an alarm about the potential for population growth to overwhelm the planet’s natural resources in 1798. The alarm rang again in 1968 with Paul Erlich’s doomsday treatise “The Population Bomb,” and has reverberated since in the background of the climate crisis: All else being equal, more people means more emissions, more hungry mouths, more potential victims of natural catastrophes.
If that’s the case, the collapse of the global birth rate should be encouraging for the fight against climate change: A new set of population projections published on July 14 by the University of Washington’s Institute for Health Metrics and Evaluation (IHME) found that global population will peak in 2064, at 9.73 billion. By 2100, at least two dozen countries—including China, Thailand, Japan, and Spain—will see their populations fall by about one-half.
The trouble is the timeline. The population isn’t shrinking fast enough to prevent a climate crisis.
The global fertility rate has already fallen by half since 1950, and will fall below the “replacement” level of 2.1 in most countries before 2050. But “it’s not until later in the century that you see effects on emissions resulting from changes in fertility happening today,” said Brian O’Neill, an earth systems scientist who directs research at the University of Denver’s Pardee Center for International Futures.
That’s too late to meet the mid-century deadline to zero out emissions.
“When you’re talking about ‘net zero by 2050’ or other near-term climate targets, population is not going to be a substantial contributor to that story,” said O’Neill. “It’s not the most important factor in emissions.”
The power of population
Since Malthus, population alone has proved to be a poor predictor of environmental impact. Innovation can dampen the effects of a growing population: The planet’s carrying capacity has expanded as new technologies stretch resources, and the growing concentration of population in urban areas should also reduce per-capita emissions.
But overall, evidence from the last few decades suggests that a country’s emissions tend to grow even faster than its population—especially in developing countries, says Joel Cohen, director of Laboratory of Populations at Columbia University.
As many countries grow steadily wealthier, their populations tend to use more resources and energy (read: fossil fuels). The number of individuals per household is decreasing, upping per-person energy consumption. Wealthier populations also live longer, as the IHME analysis finds, and older households tend to consume more energy on utilities but less on transportation.
A country’s overall emissions impact comes down to the relationship between its economic growth, available technology, and population—and that relationship looks different in every country. In China, economic development tripled per-capita carbon emissions since 1994, according to the World Bank. During the same period in the US, they fell 15% during the same period.
Many of the countries with the biggest total carbon footprints are expected to hit peak population relatively early, ahead of the global peak. That’s likely a positive signal for emissions—although, as O’Neill said, not until the second half of the century.
Meanwhile, some of the countries most vulnerable to climate change will continue to see population growth until late in the century, potentially putting hundreds of millions more people in harm’s way.
In low-income countries that are at risk for climate shocks and food insecurity, the demand for staple foods is highly sensitive to population growth, said Keith Wiebe, a researcher at the International Food Policy Research Institute. And the addition of nearly 2 billion people over the next few decades will dramatically boost the amount of land needed for food production—by an area twice the size of India by 2050, according to the World Resources Institute.
The biggest source of uncertainty
The numbers in IHME’s projections will be familiar to many climate scientists, O’Neill said. They’re similar to a set produced by the Wittgenstein Centre for Demography and Global Human Capital that have been used for years to predict food security and sea level rise. Their models are unlikely to change.
But the data that feeds them could. The Wittgenstein models are produced using five scenarios (called “shared socioeconomic pathways”) that make different assumptions about the future, including factors like levels of intergovernmental cooperation, conflict, public health, income inequality, and the pace of technological development.
It’s those things—not the overall size of the global population—that governments and individuals have control over. They’re the levers we can pull to stave off the worst of climate change’s impacts.
Kristie Ebi, a researcher at the University of Washington’s Center for Health and the Global Environment who helped develop these scenarios, said IHME’s projections most closely adhere to a middle-of-the-road scenario: middling levels of cooperation, conflict, and tech investment.
For the climate crisis, middling won’t be enough—especially with the addition of 2 billion people.
When scientists design models to predict the impacts of future climate change, Ebi said, “the smallest source of uncertainty is what happens with climate change itself. The biggest sources of uncertainty are what we as humans are going to be doing over the next few decades.”