Researchers at Flinders University in Australia have published a peer-reviewed paper arguing that the planet can sustainably support roughly 2.5 billion people, a figure that sits far below the current global population of nearly 8 billion. The finding, published in Environmental Research Letters, arrives as projections suggest the world population could keep climbing for decades, sharpening a long-running scientific dispute over how many humans the Earth can actually carry without degrading the systems that keep everyone alive.
What the Flinders University Model Found
The study, led by ecologist Corey Bradshaw, applied a Ricker logistic model to historical data on human population growth and ecological impact. That model, commonly used in fisheries biology to describe how populations behave under resource constraints, was fitted to what the researchers call the “facilitation phase” of human expansion, the era when population growth and resource use were still broadly in balance. The result: a maximum sustainable population of about 2.5 billion people.
According to the paper, the planet entered a negative phase of ecological overshoot in 1962, eight years before a global biocapacity deficit began in 1970. That timeline matters because it pins the start of overshoot not to the oil crises of the 1970s or the industrial boom of the postwar years, but to the early 1960s, when the global population was roughly 3 billion. “It means that adding more people will only accelerate climate change and pollution,” Bradshaw said in a university news release.
Decades of Disagreement on Carrying Capacity
The 2.5 billion figure is striking, but it does not exist in a vacuum. Scientists have debated Earth’s human carrying capacity for decades, and the estimates vary enormously depending on what assumptions go into the model. A landmark 1995 paper by Joel Cohen, published in Science, surveyed dozens of prior estimates and found a range stretching from under 2 billion to more than 11 billion, depending on diet, energy use, and equity of distribution. That paper, indexed at PubMed, remains one of the most cited works in the field and established that carrying capacity is not a fixed number but a function of collective human choices.
More recent work has narrowed the range somewhat while keeping it well below current population levels. A 2022 scientists’ warning paper found that analysts’ estimates of sustainable population size vary between 2 and 4 billion people. Economist Partha Dasgupta, writing in a review published in late 2024, estimated that the sustainable world population enjoying 20,000 international dollars in annual income would be 3.3 billion. That figure assumes a moderate standard of living, not extreme affluence, which makes the gap with today’s population even harder to dismiss.
Food, Nitrogen, and Hard Physical Limits
One reason carrying-capacity estimates cluster so far below 8 billion is that food production depends on finite inputs. A quantitative study examining nitrogen constraints on global agriculture found that the amount of reactive nitrogen available for fertilizer places a ceiling on how many people the food system can feed without wrecking soils and waterways. That preprint on nitrogen operating space links fertilizer limits directly to population ceilings, reinforcing the 2-to-4-billion range from other methodologies.
Some researchers push back on these boundaries. A 2020 study reported by the University of Chicago concluded that feeding 10 billion people is physically possible and could be done sustainably, provided farming practices shift dramatically toward efficiency and waste reduction. The tension between these two positions is not really a contradiction. The optimistic scenario assumes sweeping changes in how food is grown, distributed, and consumed, changes that have not materialized at scale. The pessimistic scenario describes what happens if current patterns persist. Both can be correct about different futures.
Where Population Is Actually Headed
The practical problem is that global population is not declining toward 2.5 billion or even 4 billion. According to the Flinders University release, the global population is likely to peak somewhere between 11.7 and 12.4 billion people by the late 2060s or 2070s if current trends continue. That projection, based on United Nations demographic models, would put the world at roughly five times the sustainable threshold identified by the Ricker model.
The distance between those two numbers, 2.5 billion and 12 billion, is the core tension. It means that even aggressive fertility reduction, which some demographers advocate through expanded access to education and contraception, would not bring the population anywhere near the sustainable range within this century. A Yale Environment Review analysis noted that effective interventions will take both technological and social innovations, as well as substantial effort, to align population trajectories with ecological limits.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
