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.”

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