As the world population increases, climate changeDisease, war, resource shortages and other crises threaten to dramatically reduce the Earth’s carrying capacity for humanity – the maximum number of people who can live sustainably on our planet. A new study suggests that if a global catastrophe were to occur today, we could see rapid population decline in the coming decades.
The findings, published May 22 in the journal Chaos, Solitons & Fractals, they show that if Earth’s carrying capacity were reduced to about 2 billion people right now, the global population could decline by 50% by 2064. In other words, within about 40 years, humanity could shrink from a projected population of about 8 billion to 10 billion people to 4 billion to 5 billion people. The authors reached that conclusion using a new mathematical model that unifies key global population growth regimes over the past 12,000 years.
“In the paper we highlight that this is not a forecast, but rather an illustrative mathematical scenario intended to show how sensitive population dynamics can be to abrupt environmental or social changes,” said co-author Alessio Zaccone, professor of physics at the University of Milan. wrote in a statement. “We emphasize that the current trajectory remains relatively stable and does not imply an imminent collapse.”
Rethink population growth
Zaccone and his late colleague Kostya Trachenko, who worked as a theoretical physicist at Queen Mary University of London until his passed In May 2025, they used their model to revisit the famous “doomsday” scenario proposed by Heinz von Foerster and his collaborators in 1960. In that paper, calculated that the world’s population would approach infinity in 2026 if it continued to grow at the rate it had done over the past two millennia.
That doesn’t mean there would be an infinite number of people on the planet. Rather, it means that the rate of population growth would accelerate continuously, without limit. Of course, this scenario did not occur, since the birth rate has decreased. But according to Zaccone, “the underlying mathematics of galloping growth can still reappear under certain conditions.”
The study models different scenarios for future population growth. While calculations indicate that the current global trend will not lead to the “catastrophic singularity” that von Foerster and his colleagues wrote about, they also show that a major environmental crisis could lead to rapid population decline and potentially collapse.
“Under the deliberately conservative worst-case assumption, that the Earth’s sustainable carrying capacity suddenly fell to around 2 billion people, our model predicts a rapid decline in the world’s population, with humanity potentially halving around the year 2064,” explains Zaccone.
The calculations also show that, under a separate runaway growth scenario, global population growth accelerates toward a mathematically unsustainable singularity (i.e., a point where population growth exceeds sustainable limits) around 2078, effectively updating the previous “end of the world” projection.
Understanding “Doomsday”
Zaccone is confident the model works because he compared it to empirical population data from various historical eras. He found that it successfully reproduced both phases of “compressed exponential” growth (such as the industrial-era population boom) and the slower “extended exponential” growth regime that has been taking place since around 1970.
It is important to remember that the model does not make predictions about future population growth, but rather provides a picture of what could happen under various conditions. Still, it is not out of the question that an extraordinary catastrophe (such as a nuclear winter, a major pandemic, or extreme climate collapse) or the confluence of multiple global crises could reduce the Earth’s carrying capacity to 2 billion people in the relatively near future.
This is, of course, an unlikely worst case scenario. But Zaccone hopes this new model will offer a unified framework for exploring possible futures as threats to humanity intensify.
