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.