Elon Musk wants to make humans a multiplanetary species. Colonization of Mars has been the main goal of the entrepreneur’s multi-billion dollar company SpaceX since its founding two decades ago.

The billionaire has in the past argued that we need a Plan B if Earth finds itself irreversibly damaged through climate change, overpopulation, a third world war or an eventual mass extinction event.

While the jury’s still out on what will ultimately render our planet uninhabitable, in January Musk tweeted a new concern: “population collapse.”

Contrary to what you might think, there may be some truth to his worry — the world's population could decrease after the second half of the century, according to a 2020 studypublished in The Lancet by researchers from the Institute for Health Metrics and Evaluation (IHME).

That means there might not be enough people for Musk's Mars colonization plan — which the SpaceX CEO acknowledged himself in the same tweet: "If there aren't enough people for Earth, then there definitely won't be enough for Mars”.
 

Predictions by the IHME suggest that by 2064 the global population will reach its peak at 9.73 billion, followed by a decrease of almost a billion by the end of the century. These findings show a very different picture from previous projections by the United Nations, which estimate a steady increase to around 11 billion by 2100. Musk called the UN numbers "utter nonsense” in the same Twitter post.

What's behind the decrease?

Women play a crucial role in these estimates. The population decline is attributed mainly to decreasing birth rates – women are having fewer kids. The researchers pointed out that changes in the birth rate are mostly explained by an ongoing trend of better and easier access to education and contraceptives.

But why does this happen? Evidence from Denmark could shed light on the causes. According to a 2018 study by the National Bureau of Economic Research, women's earnings dropped drastically after childbirth while men's stayed the same.

Some countries have tried to tackle these challenges with incentive programs like paid maternity leave, employment protection, child care and economic aid. A good example given by the authors is Sweden, which increased its birth rates from 1.5 in the late 1990s to 1.9 in 2019.

The researchers were also concerned that the lower birth rate’s tight relation to women’s increased attainment of education and contraceptives could pose a real threat to their freedom and rights. Instead of implementing programs to lessen the financial blow many women experience when they become mothers, some countries might, on the contrary, prohibit or restrict access to reproductive health services to prevent population declines. They mentioned past examples like Romania in the 60s and the Soviet Union, which used restrictions and bans to try to increase fertility rates.

Good and bad outcomes

Although Musk might not like a less populated world, it wouldn't necessarily be a bad thing, IHME researchers said. They noted it could provide relief to the environment because fewer people use fewer resources and generate fewer carbon emissions. But the authors are clear that although a decline in the population could benefit the planet, it is not a solution against climate change.

Some countries like Japan, Spain and Ukraine could face a halving or more of their population by 2100. China is predicted to fall from its current 1.4 billion people to roughly 700 million, despite the 2015 end to the one-child policy.

But not all countries would see a decline in their population. The models showed that sub-Saharan Africa, North Africa and the Middle East were the only regions expected to have a higher population in 2100 than in 2017.

On average, high-income Western Europe countries would reach a population peak before 2040, much earlier than the predicted 2064 global peak. In the case of Germany, the population would peak at 85 million by 2035 and then decrease to about 60 million by 2100.

The predictions also point to a decreasing working population in some countries. This would have significant economic consequences, researchers said – like lower GDP growth rates.  Combined with a growing number of people entering retirement age, a smaller workforce poses hard fiscal challenges to public health and pension programs. In this regard, immigration could help, wrote the authors. World population growth to diminish, spurring geopolitical power shift.

The study mentions that countries that are able to manage to keep their working population through migration, like Canada, Australia and the USA, could prosper.

Estimates like these are, of course, limited to researchers’ current understanding of population indicators. They may not be able to tell the future, but they can shed some light on it.