From the emergence of Homo sapiens, it took roughly 300,000 years before one billion of us populated the Earth. That was around 1804, the year morphine was discovered, when Haiti declared independence from France, and when Beethoven first performed his Third Symphony in Vienna.

We’ve added our most recent one billion more just since the first term of U.S. President Barack Obama. A mere dozen years after reaching seven billion, the planet most likely will surpass eight billion people sometime around mid-November, the United Nations estimates based on its best demographic projections.

The actual timing, however, is uncertain. In parts of the world, census data is decades old. During COVID-19 it was virtually impossible for some countries to record every death. Even sophisticated computer models may be off by a year or more. It’s not as if anyone has done a global person-by-person head count.

But the UN is declaring November 15 as the “Day of Eight Billion” because there is no mistaking the import of this moment. Humans everywhere are living longer, thanks to better health care, cleaner water, and improvements in sanitation, all of which have reduced the prevalence of disease. Fertilizers and irrigation have boosted crop yields and improved nutrition. In many countries, more children are being born, and far fewer are dying.

Of course, the challenges we face as the world’s population continues to rise also are significant. Pollution and overfishing are degrading many areas of the oceans. Wildlife is disappearing at an alarming clip, as humans wipe out forests and other wildlands for development, agriculture, and commercial products made from trees. A changing climate driven by a global energy system that is still overwhelmingly powered by fossil fuels is fast becoming the greatest threat in history to biodiversity, food security, and access to water for drinking and farming. And that’s with the number of people we already have.

The risks and opportunities of our population boom and parallel resource crisis depend largely on decisions we’ve not yet made. Which will control our future more—the billions of mouths we’ll have to feed, or the billions more brains we could employ to do so?

“The exact impacts on future human life, I think, are still somewhat yet to be determined,” says Patrick Gerland, who oversees population estimates for the United Nations' Department of Economic and Social Affairs.

“So far, the overall experience is that the world has been successful in adapting and finding solutions to our problems,” Gerland says. “I think we need to be somewhat optimistic.”

But he quickly concedes that climate change is a powerful threat. “Simply maintaining the status quo and doing nothing is not an option,” he says. “Whether we like it or not changes will be happening, and the situation will not improve by itself. There is a need for current and future interventions.”

In the meantime, our overall population explosion belies vastly different types of demographic change taking shape around the globe. And the world’s top demographers don’t agree on just where our population is headed from here.

Population changes vary dramatically 

The world is facing the likelihood of huge population explosions and collapses at the same time. The most significant just happen to be on opposite sides of the planet.

Perhaps as soon as this year, for the first time in two millennia, China will no longer be Earth’s most populous country, as India finally surpasses it. Even before China’s one-child policy, which went into effect in 1980, “births in China have been declining almost continuously,” Gerland says. In the 1970s alone, the birth rate dropped by half. With increasing opportunities for better education and careers, more women are delaying childbirth, and there already are fewer of childbearing age.

These trends accelerated during the pandemic. There were 45 percent fewer children born in 2020 than in 2015. China’s birth rate is now far lower than that of the United States.

Even with one of the longest life expectancies of any country, at 85 years, China’s population of 1.4 billion is expected soon to begin declining—in fact that decline may already have started. The workforce has been shrinking for a decade. As it is, there are barely two workers supporting every retiree or child. In the next quarter century, the country will likely see 300 million people over the age of 60, straining government resources, according to a report in Nature. Health care costs are expected to double.

In Africa, on the other hand, trends are moving rapidly in the other direction. Across the Sahel, population is expanding rapidly. Nigeria’s median age is just 17, less than half that of China. Birth rates there are falling, too, but remain 20 times higher than in China.

Food security is already a concern. More than one-third of the country lives in extreme poverty, a greater number than any other country, including India, which is six times larger. A third of all households include one adult who must skip meals at times for the family to survive.

Currently at 216 million, the country’s population by some estimates could quadruple by the end of the century. By then it could have more people than China, which has 10 times more land. But that all depends on childbirth rates. All these projections are driven by assumptions, and the reality could be much different.

The biggest driver of falling birth rates is education, especially for girls. A decade ago, researchers determined that increasing access to education could slow global population growth by one billion by mid-century. How much and how fast we expand those educational opportunities over the next several decades are among the important unanswered questions that will determine how many of us will be living on Earth as we approach 2100.

Predicting the world’s population is complex

Gauging population in the near-term isn’t terribly controversial. “The majority of the people that will be alive in 2050 are already alive today,” Gerland says.

The UN, a group of researchers at the University of Washington in Seattle, and other experts in Vienna, Austria, tend mostly to agree on what the next quarter century holds. Based on past events, at least, few expect another deadly global pandemic quite so soon. Despite crises like the war in Ukraine, neither do demographers yet foresee planet-wide mass migration by mid-century. Most experts see the population topping nine billion roughly by then.

After that, projections vary greatly. A few years ago, the UN estimated that by 2100, the globe’s population could balloon to 11 billion. Earlier this year, it revised those estimates downward, to about 10.4 billion, thanks to progress in reducing the average number of children born per family. At the International Institute for Applied Systems Analysis, in Vienna, researchers in 2018 projected the population could rise to 9.7 billion in 2070 and then fall back to around 9 billion by century’s end. They used different assumptions, largely by asking global experts to weigh in. “The main story is not just about fertility but about progress in fighting child and infant mortality,” says Anne Goujon, population program director for IIASA.

Meanwhile Seattle’s Institute for Health Metrics sees population peaking at roughly 9.7 billion in 2064, but dropping down to 8.8 billion, possibly less, by century’s end. Populations could fall by half across nearly two dozen countries, including Bulgaria and Spain. A lot of the difference is based on a complex method the researchers use to estimate future birth rates.

In addition to the differences between models, all of the researchers agree that efforts so far to incorporate climate change into future population projections have been inadequate. In part that’s because the potential effect largely depends on how quickly the world reduces greenhouse gas emissions. But part of the difficulty also lies in assessing climate impacts. Extreme heat could make parts of the Middle East, sub-Saharan Africa, and India uninhabitable. Storms could worsen food security. How will people respond to sea-level rise in heavily populated coastal regions?

“No one is doing this in the right way at the moment,” says Stein Emil Vollset, who oversees IHME’s population estimates.

And aside from global population estimates, climate change and politics also will likely greatly influence migration between countries. Population in the U.S. and Western Europe has been largely sustained by immigration, but it has become a political hot button. Other countries with declining populations, such as Japan, have been even more reluctant to welcome immigrants.

Yet the lopsided trends, between booming and declining populations, exacerbated by climate change, will almost certainly increase migration pressure almost everywhere.

“The only way we can get out of this demographic imbalance,” Vollset says, “is well-managed international collaboration.”