As the global population rises, climate change, disease, war, resource strain, and other crises threaten to drastically reduce Earth’s carrying capacity for humanity—the maximum number of people that can sustainably live on our planet. A new study suggests that if a global catastrophe struck today, we could see a rapid population decline over the next several decades.
The findings, published May 22 in the journal Chaos, Solitons & Fractals, show that if Earth’s carrying capacity dropped to around 2 billion people right now, the global population could decline 50% by 2064. In other words, within about 40 years, humanity could shrink from a projected population of roughly 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 regimes of global population growth over the past 12,000 years.
“In the article we stress that this is not a forecast, but rather an illustrative mathematical scenario intended to show how sensitive population dynamics may be to abrupt environmental or societal changes,” co-author Alessio Zaccone, a 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 imminent collapse.”
Rethinking population growth
Zaccone and his late colleague Kostya Trachenko, who worked as a theoretical physicist at Queen Mary University in London until his passing in May, 2025, used their model to revisit the famous “doomsday” scenario proposed by Heinz von Foerster and collaborators in 1960. In that paper, they calculated that the global population would approach infinity by 2026 if it continued to grow at the rate it had for the past two millennia.
That doesn’t mean there would be an infinite number of people on the planet. Rather, it means the rate of population growth would accelerate continuously, without limit. Of course, this scenario did not come to pass, as the birth rate has declined. But according to Zaccone, “the underlying mathematics of runaway growth can still reappear under certain conditions.”
The study models different scenarios for future population growth. While the 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 a deliberately conservative worst-case assumption that Earth’s sustainable carrying capacity suddenly dropped to around 2 billion people, our model predicts a rapid global population decline, with humanity potentially halving by around the year 2064,” Zaccone explains.
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 earlier “doomsday” projection.
Understanding “doomsday”
Zaccone is confident that the model works because he compared it with empirical population data across various historical eras. He found that it successfully reproduced both “compressed exponential” growth phases (like the industrial-era population boom) and the slower “stretched exponential” growth regime that has been taking place since around 1970.
It’s important to remember that the model is not making predictions about future population growth but rather offering a picture of what could happen under various conditions. Still, it’s not out of the question an extraordinary catastrophe (such as a nuclear winter, a major pandemic, or an extreme climate collapse) or the confluence of multiple global crises could reduce 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 that this new model will offer a unified framework for exploring possible futures as threats to humanity intensify.
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