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.