At the Cannes Film Festival this summer, many attendees reveled at the "Top Gun" reboot, a throwback to the past. But on the sidelines a smaller crowd witnessed something more solemn: the possibility of a dark and tragic future. "Plan 75," a film by Japanese director Hayakawa Chie, explores the potential dangers of her country's aging society, where nearly one-in-three people are currently 65 or older. Set in a near-future dystopia, the film depicts a nation whose healthcare and pensions systems have become so overburdened by the elderly that the government aggressively markets a policy to pay for final bucket list items and then euthanize anyone over 75. While technically the stuff of science fiction, demographers say the film arrives at a time when humanity really is aging. The global fertility rate has decreased by half since 1960. In countries responsible for 85% of the world's gross domestic product – the United States, Germany, Japan, even China and India – births have fallen below the “replacement rate,” meaning that unless offset by immigration, population will begin to decline as older generations depart. The United Nations calculates the world population will now peak in 2084, before starting to fall by the century's end.

In a world where economies are designed around growth and social systems depend on the young supporting the old, forward thinkers are beginning to wonder what comes next. Consider Elon Musk, Tesla CEO and business magnate, now most prominent among their ranks. “Population collapse due to low birth rates is a much bigger risk to civilization than global warming,” Musk wrote on Twitter this summer. “Mark these words.” But is he right?

Population concerns are nothing new

For centuries, humans have pondered the ideal size of humanity. But experts warn such efforts usually end in folly, and that our species has within its grasp solutions to prosper whether populations rise or fall. “It's up to us and how the world responds,” said Lauren Johnston, a professor at the University of Sydney's China Studies Centre and economic demographer. For much of the last few centuries, those fretting about overpopulation have had the spotlight. In 1798, English scholar Thomas Malthus published an influential essay that laid out an idea known as the “Malthusian trap,” which holds that population growth inevitably exceeds food and other resources, leading to famine and poverty. The work inspired anxiety in England and helped lead to the first national census of England, Scotland and Wales. Such concerns echoed loudly in 1968, when Stanford University professor Paul Ehrlich and wife Anne Ehrlich published "The Population Bomb," a book that predicted global famine leading to the deaths of hundreds of millions of people within decades. But most experts say such predictions have not come to pass. Particularly in the past 50 years, a “Green Revolution” in agriculture has used new farming methods to reap more calories per acre of land, leading world hunger to decrease even as the population doubled. Although studies show such practices have created additional problems – driving water pollution, contributing to climate change, and perhaps even decreasing the nutritional value of food – Johnston points out that many nations are now facing the opposite of starvation. “In most countries there has been a sufficiently productive response to population growth that there hasn't been a famine,” Johnston said. “Now there's obesity.”

Underpopulation on the horizon?

As concern over having too many mouths to feed has waned, an opposing one has risen: too few people to work. That's an especially obvious worry in China, which infamously implemented a one-child policy in 1980 to address exponential population growth projections. Its current population of 1.4 billion remains the world's largest. But realizing the aging trajectory of its society, in 2016 China eliminated the policy and has also limited pensions and social programs for the elderly, Johnston said.

Many other nations are or soon will be facing similar challenges. To maintain a steady population without immigration, a nation has to achieve a fertility rate of 2.1 children per woman, experts say. But the fertility rate is just 1.7 in China and Brazil, 1.5 across the European Union, and 0.8 in South Korea, the lowest of any country, according to the World Bank. The rate is 1.6 in the United States, where the population is still rising only due to longer lifespans and immigration, which is projected to outpace natural births by 2030. Globally, it's primarily African nations like Nigeria, where the fertility rate is 5.2, that are contributing to population growth. But as those nations develop, some experts expect fertility rates to fall as well, contributing to the possibility of unprecedented global population decline. “There's never been anything close to a parallel,” Johnston said. Some experts are ringing alarm bells on what that could mean for societies. In their book "Reversal: Ageing Societies, Waning Inequality, and an Inflation Revival," economists Charles Goodhart and Manoj Pradhan warn of mounting fiscal crises, "as medical, care, and pension expenditures all increase in our ageing societies." Nations could wind up burning the candle at both ends: as a higher percentage of people become retirees they require more public resources, while at the same time the taxable working population shrinks. Problems could be exacerbated as rates of Alzheimer's and other costly elder illnesses increase, while labor shortages create inflationary pressures. As countries face these challenges, their societies and politics could destabilize. "Our view of the future is not encouraging, but it is coherent and plausible," Goodhart and Pradhan write.

So Musk is right?

Not so fast, says Daniel Kammen, a professor of sustainability at the University of California, Berkeley and former Science Envoy to the U.S. State Department. While aging societies do pose possible challenges in the future, Kammen says the world is facing a current full-blown crisis right now: climate change. And adding more people to the Earth's population will only further complicate humanity's lagging efforts to fight global warming, experts say. “There's no ideal number, but certainly I would say there are too many people on our planet for our current lifestyle,” Kammen said. Kammen believes the entire conversation about population is a red herring, a view commonly held among population experts. Instead, he says the focus should be on whether or not countries are wisely using resources. That's when the wealth of nations like the U.S., and not their population, come into focus. A study in the journal Nature Sustainability this year found that the world's wealthiest 10% of people produce 47% of its carbon emissions, compared to just 10% of emissions for the entire bottom half of the economic ladder. To put it another way, World Bank data shows the average Nigerian's carbon footprint is 0.6 metric tons each year. With the globe currently emitting about 34 billion metric tons of CO2 annually, that means it could currently support 58 billion people if they had a Nigerian carbon footprint. On the other hand, the average American uses 14.7 metric tons of CO2 each year, meaning the world could support just 2.3 billion people if everyone had an American footprint. The same effect can be seen within countries. While many Americans believe that population-dense cities hold the most blame for carbon emissions, work from Kammen and his colleagues show the carbon footprints of urban Americans are actually substantially less than rural residents, with suburban residents surpassing both. That's true both on a per capita basis and in total: about half of U.S. carbon emissions come from suburban settings, while less than a third come from urban. Ultimately, Kammen said, the question is how to reduce resource footprints, especially in wealthy nations. The smaller they get, the more people the planet can support. “While it sure seems like there are a lot of people on our planet, our individual impact is much more measured by the ways in which we amplify or minimize our footprint,” Kammen said. “If you make it about population, you avoid how critical our patterns of consumption are.” Experts also say the challenges of population decline are not insurmountable. Johnston says it will come down to smart planning and cooperation. If populations do peak and fall, governments can mitigate the repercussions by sharing resources more equitably. That will likely include sacrifices among the older generations. Not with their lives as "Plan 75" depicts, but through higher retirement ages and adjustments to pensions and benefits. Other experts note that it may be possible to maintain productivity levels with fewer people, through increased education or even possibly with the assistance of technologies like Artificial Intelligence and automation. In the end, people of working ages may also need to sacrifice in the form of higher taxes. But such a future will inevitably look different than the world we live in now, and Goodhart and Pradhan warn a lot will be riding on whether or not societies accept such changes. "We doubt that politicians, facing rising health and pension costs, will be prepared or able to raise taxes enough to equilibrate the economy via fiscal policy," they wrote.

Population 'cures' can be worse than population collapse

While population decline comes with challenges, experts warn that attempts to reverse course are often at best ineffectual, and at worst hateful and destructive. After all, they note, the basis of population decline is personal freedom. Reiner Klingholz, a population researcher and author based in Germany, notes that smaller families and a more developed lifestyle often go hand-in-hand. As a society becomes wealthier and more educated, its fertility rate invariably falls. That's particularly tied to women's education and empowerment. When women become more educated, both professionally and on sexual reproduction, they are presented with life choices beyond homemaker and often choose to have less children, experts say. Development also brings increased wealth, which creates societies that are overall healthier and happier, even if the fertility rate is lower. “Look at Sweden and Denmark,” where fertility rates stand at 1.7, Klingholz said. “People are very happy in these countries.” Also troubling: Concerns about population decline often boost xenophobia. In the United States, "Great Replacement Theory" – an unfounded conspiracy that political leaders are intentionally replacing white Americans with non-white immigrants –   has moved from extreme right-wing circles into mainstream discourse. Perhaps nowhere is this tension more apparent globally than in Hungary, where the government of Prime Minister Viktor Orban is now offering about $30,000 and a raft of subsidies on homes and cars for Hungarian families with at least four children, while opposing new immigration. “Instead of just numbers, we want Hungarian children. Migration for us is surrender,” Orban said in 2020. Such rhetoric stands in stark contrast to most economists, who according to Goodhart and Pradhan, value immigration as a tool to offset population decline and boost a country's workforce and productivity. Attempts to instead fix population decline through economic policies like tax incentives often fail due to the ties between women's empowerment and lower fertility rates, said Per Espen Stoknes, director of the BI Centre for Sustainability and Energy at the Norwegian Business School. “Men can't tell women how many children they should have,” Stoknes said. “It's not really about the issue of (resources). It's really about what kind of life do women want for themselves?”

A happier future?

Johnston says that in the end, population decline doesn't have to be a crisis. Ultimately, as with climate change, it comes down to wise resource allocation. If humanity can cooperate and efficiently distribute resources through immigration and economic policies, it could build a world with where people are fewer but more educated, and in which productivity and ingenuity still flourish. But that's a big "if." “It might be so much healthier if there's a smaller population overall, but much more cooperation,” Johnston said. “If China goes from 1.4 billion people to 800 million, but people go from peasants to middle class, how on Earth is that going to be a bad shift?” Kyle Bagenstose covers climate change, chemicals, water and other environmental topics for USA TODAY. He can be reached at kbagenstose@gannett.com or on Twitter @kylebagenstose.