The Environmental Impacts of Overpopulation

Abstract

Overpopulation’s central role in environmental degradation is intermittently challenged. This article assesses the impact of mounting demographic pressures on six critical global sustainability challenges: deforestation, climate change, biodiversity loss, fishery depletion, water scarcity, and soil degradation. By synthesizing findings from hundreds of peer-reviewed studies, the article offers a comprehensive review of the effects of expanding human populations on the most pressing current environmental problems. Although the rate of population growth worldwide is slowing, human numbers are expected to continue increasing on Earth until the end of the century. Current research confirms that overpopulation causes substantial and potentially irreversible environmental impacts that cannot be ignored if international sustainability policy is to be effective.

1. Introduction

Unlimited growth on a planet with finite resources is impossible [1]. During the 1960s and 1970s, overpopulation was a prominent concern among environmentalists, for many, a paramount priority. Rapidly expanding global population placed unsustainable pressure on natural resources, exacerbating environmental degradation, and increasing poverty [2]. Books like Paul Ehrlich’s best-seller The Population Bomb (1968) [3] and the Limits to Growth (1972) [4] emphasized the dire consequences of unchecked population increase, prompting widespread advocacy for population control measures. In a 1971 seminal article in Science, Ehrlich along with John Holdren posited the “Impact Law”, which identified population as one of the three essential contributors to environmental impacts, along with affluence and technology [5].

More than fifty years after ecologists first highlighted the dominant role of population growth in environmental degradation, many experts continue to see overpopulation as the single greatest driver of ecological damage on Earth [6,7,8]. For instance, in a 2017 “Warning to Humanity”—cosigned by more scientists than any journal article in history—15,364 researchers cautioned that rapid population growth was a “primary driver” behind many ecological and even societal threats. They cautioned, “By failing to adequately limit population growth… humanity is not taking the urgent steps needed to safeguard our imperiled biosphere” [9].

By the 1980s and 1990s, however, the focus of environmental organizations began to shift away from overpopulation as a central issue in the global sustainability agenda [10]. This change was driven by several factors, including criticisms that overpopulation rhetoric often targeted the Global South, unfairly blaming developing countries for global environmental harm [11]. The 1994 Cairo International Conference on Population and Development (ICPD) marked a significant turning point in the global population discourse [12]. The conference decision prioritized women’s rights, reproductive health and development [13] over explicit efforts to reduce population growth [14]. After decades of opposition to international family planning initiatives [15], the Vatican played a crucial role in shaping the outcomes of the conference, strongly resisting any language that could be interpreted as endorsing contraception, abortion or coercive population control measures [16].

The Vatican’s influence was bolstered by alliances with conservative Muslim, Catholic-majority and developing countries, who shared concerns about the ethical implications of population control policies [17]. Feminist organizations also advocated for a shift in orientation, calling for a focus on women’s empowerment, reproductive rights, and access to education and healthcare, rather than on population reduction per se [18]. The resulting retreat among many environmentalists and green NGOs from meaningful engagement with scientific studies substantiating the causal relationship between human population pressure and environmental degradation [19] proved to be enduring.

In 2013, British political scientist Diane Coole identified five reasons for the historic decline in advocacy for sustainable population policies and why so many environmental activists came to “disavow the population question” [20]. Among these were fatalism regarding the inevitability of demographic growth; false optimism that high birth rates would eventually resolve themselves; and skepticism about whether population growth was even a problem at all. Chief among Coole’s explanations was “population shaming”, where supporters of sustainable population strategies were accused of racism or embracing eugenics [21].

Advocates for demographic stability and sustainable population policies were aggressively assailed as not really being interested in protecting the environment, but rather disingenuously seeking to constrain reproduction amongst peoples of color [22]. For instance, when the Sierra Club debated whether to take a stronger stance on U.S. immigration in the 1990s due to its environmental ramifications, critics reproached the NGO for veering into nativist or racist territory [23]. Such hostile claims continue to the present [24]. In the face of such malicious allegations, many environmentalists lacked the resolve to stand their ground [25]. Instead, environmental organizations found it politically expedient to pivot and concentrate on the role of consumption patterns, particularly in wealthy nations, as a central driver of environmental degradation, downplaying the role of population [26].

While consistently acknowledging the important role of consumption in environmental degradation, sustainable population advocates continue to advance evidence-based arguments that most environmental problems are ultimately driven by population increase [27]. Using a metaphor popularized by Stanford ecology professor Paul Ehrlich, they perceive population and consumption as two sides of a rectangle: regardless of which side is longer, the total area—representing aggregate environmental damage—remains unaffected [28].

While Western environmentalism may have lowered the profile of overpopulation in the sustainability discourse, the ecological implications of demographic growth have not changed. The rate of population increase has slowed since its peak in the 1960s, but the absolute number of people on the planet continues to rise [29]. Between 2011 and 2023, human population grew by one billion in just twelve years, compared to the fifteen years it took to increase from 3 to 4 billion between 1960 and 1975 [30]. The United Nations projects that the world’s population will continue to incrementally increase, from 8.2 billion in 2024 to 10.2 billion by the mid-2080s, stabilizing around the end of the 21st century [31]. Environmental damage functions, however, are increasingly recognized as non-linear [32,33]. It is not surprising, therefore, that the magnitude of the associated adverse environmental impacts is also expected to intensify [34,35].

UN estimates, like other projections envisaging an imminent end to global population growth [36], involve many optimistic assumptions about future fertility declines in the Global South, assumptions that are challenged as excessively sanguine by many demographers [37]. Demographic models predicting stability consistently ignore waning support for family planning that threatens global fertility declines [38]. In challenging UN demographic methodology, critics chide the historic timidity of the United Nations in confronting controversial demographic issues. For instance, the UN Sustainable Development Goals do not even mention population stabilization as an explicit objective, focusing instead on other causes of environmental degradation [39].

For many countries, the local consequences of overpopulation remain too acute to ignore. With varying degrees of success, nations have implemented effective policies designed to stabilize their populations. For example, voluntary population policies in Asian countries like Singapore [40], Thailand [41], Iran [42] and Bangladesh [43] have reduced fertility levels to replacement levels or below [44]. Cognizant of past famines and concerned about the consequences of rapid population growth on societal wellbeing, from 1980 to 2016 China implemented a draconian “one-child policy” that faced widespread international condemnation for human rights violations [45]. In Sub-Saharan Africa, several countries, such as Botswana [46], Rwanda [47] and Kenya [48], have seen meaningful drops in population growth rates by improving access to contraception and encouraging smaller families [49]. Education, particularly for girls and women, is also highly correlated with lower fertility rates [50]. Greater demographic stability makes sustainability challenges more tractable.

These examples demonstrate that with sufficient political will and carefully designed interventions, rapid population growth and its environmental consequences can be mitigated. But these policy trends are hardly universal. In the absence of a global sustainable population consensus, during recent decades demographic pressures have continued to undermine environmental progress. This suggests that international initiatives must once again prioritize population stabilization to address the root causes of ecological degradation.

The present review of research published in recent years highlights the severe environmental impacts of population pressures on a broad range of media. These include six of the world’s most pressing ecological challenges: deforestation, climate change, biodiversity loss, fishery depletion, water scarcity, and soil degradation. The implication is unequivocal: meaningful ecological progress cannot be achieved without prioritizing population stability as a cornerstone of international and domestic policy.

It is duly noted that population pressures play an important role in many other local ecological challenges as well: air pollution [51,52,53], solid waste [54,55,56], noise pollution [57,58,59], inland water availability [60,61,62], water contamination [63,64,65], natural resource shortages [66,67,68], ocean acidification [69,70,71], transport of exotic animals through increased global trade [72,73,74], eutrophication [75,76] and many other environmental insults are driven by the demands of expanding human populations. Moreover, demographic pressures are directly associated with a host of other social maladies, from psychological stress [77,78,79], depression [80,81] and violence [82,83,84,85,86] to traffic congestion [87,88,89] and disease [90,91,92,93,94]. Rapid population pressures contribute to massive food insecurity [95,96], with one in eleven people globally and one in five in Africa still facing hunger [97] or routinely suffering from insufficient calories [98,99]. In most future scenarios that include population growth, food shortages are expected to remain a global scourge through 2050 [100].

Nonetheless, the six aforementioned global environmental problems on which this article focuses are unique because their association with population increase is so significant. Moreover, frequently, the damage incurred is irreversible or unlikely to be ameliorated, as long as rapid demographic growth continues. The clear consensus emerging from current research confirms the severe environmental consequences caused by overpopulation.

2. Population Pressure’s Effect on Deforestation

Deforestation remains one of the world’s most vexing and alarming ecological challenges, inter alia because of its association with biodiversity decline [101]. Historically, deforestation has been most pronounced in regions where population pressures outpaced technological advancements in farming [102,103]. Over time, timber demand for construction, military [104], agricultural and consumer products exacerbated the phenomenon [105]. Governments have been slow to develop sustainable strategies for managing forest resources: in practice, only about 6.5% of forests worldwide are effectively protected [106], resulting in widespread land clearing. According to National Geographic, most of the Earth’ natural forests have been destroyed by human activities, leaving less than a third intact [107]. More moderate estimates suggest that about a third of the Earth’s woodlands has been lost [108,109]. Regardless of the precise figure, there is little disagreement that the phenomenon is massive and continues apace.

The international community has long recognized the gravity of the issue. In 2021, more than a hundred countries publicly proclaimed their commitment to ending deforestation by 2030 [110]. Data from recent studies, however, indicate that the phenomenon continues to grow worse. Research from the University of Maryland and the Global Forest Watch reported in 2024 that planet Earth lost roughly 37,000 square kilometers of primary forest. This represents a 3.2% increase in the magnitude of deforestation compared to the previous year. Tropical forests remain particularly vulnerable, with over 3.6 million hectares (9 million acres) of primary forest destroyed annually—an area roughly the size of Switzerland [111]. Figure 1 shows global forest loss trends over the past 22 years and the glaring failure of humanity to save the woodlands of the planet.

It is important to distinguish between “forest loss”, which includes woodland destruction from natural causes like wildfires or pests (where regeneration often occurs), and “deforestation”, which refers to the permanent conversion of woodlands to other uses [112]. For instance, in 2022, global tree cover loss increased by 24%, primarily due to unprecedented wildfires in Canada that affected 80,000 km² of woodlands [113]. It is likely, however, that most of these forests will grow back, with natural succession processes eventually restoring the original assemblage and vegetation. Deforestation directly caused by human proliferation is fundamentally different and considered by many experts to be irreparable [114]. As tree regeneration is a slow process, once the natural vegetation is gone, this basic resource rarely returns [115]. The dramatic forest loss that has occurred during the past two decades in countries with growing populations like Indonesia (30.8 million hectares) and Brazil (68.9 million hectares) is probably irreversible [116].

Today, in many developing countries, forests continue to be cleared at alarming rates. In all of them, growing population offers the best proximate explanation for these worrying trends. But the phenomenon is also aggravated by pervasive poverty. Increased demand for food, especially in subsistence economies, drives this trend [117], creating a broader context for deforestation processes. While affluent societies consume more resources per capita than poorer ones, their demographic stability frequently enables them to avoid clearing forests for survival, with tree cover often actually increasing in developed countries [118]. Wealthier communities in developed countries also tend to have more tree cover than poorer ones [119]. Empirical evidence confirms that developed countries with stable populations have achieved greater progress in afforestation and reforestation initiatives than developing nations [120].

In contrast, population growth coupled with poverty in developing nations frequently result in unsustainable land use practices [121]. For example, a study in Malawi calculated that a 1-percent increase in population growth increases deforestation rates by 2.7 percent, due to the increased demand for agricultural land [122].

Table 1. Population loss in countries with highest percentage forest loss: 2001–2022.

Country	Percent Forest Lost (2001–2022)	Population Growth (%)
Paraguay	29%	32.60%
Indonesia	19%	28.50%
Argentina	17%	22.90%
Mozambique	15%	91.20%
Brazil	13%	21.30%
Tanzania	12%	96%
Myanmar	11%	19.50%
Democratic
Republic of Congo	9.90%	110%
Angola	7.10%	123.60%

Sources: World Forest Watch, World Resource Institute; United Nations Population Fund. https://www.wri.org/initiatives/global-forest-watch (accessed on 23 March 2025).

shows a list of the countries with high rates of deforestation alongside population increase between 2001 and 2023.

Forest extirpation is not only a major driver of biodiversity loss, but also increasingly recognized as a major cause of global warming. A research group at the London School of Economics estimates that “Land use change, principally deforestation, contributes 12–20% of global greenhouse gas emissions” [123]. Slash-and-burn agriculture contributes to both the continuous clearing of forest areas and carbon release [124]. At the same time, afforestation has substantial potential for boosting global mitigation efforts [125].

For many communities, especially those who suffer extreme poverty, forests, and the ecosystem services they provide, are of critical economic importance, providing up to 90 percent of local livelihoods [126,127]. Local populations rely heavily on agriculture for subsistence and economic stability [128]. As nations seek to provide food security for their growing populations, additional land is constantly required to produce the necessary calories [129]. Especially in tropical regions where the bulk of deforestation occurs, forests are cleared to create agricultural fields and residential space [130]. Subsequent to the growth of local populations, deforestation and destruction of the ecosystems upon which people have relied from time immemorial accelerate.

The association between population growth and deforestation is particularly dramatic in the Democratic Republic of the Congo. The DRC is home to the world’s fourth largest terrestrial carbon reserve [131]. During the two decades between 2000 and 2020, the country lost 15 million acres (6 million hectares) of its unique, tropical woodlands. This amounts to a full 3.6 percent of total tree cover. As expected in environmental damage functions, when a population is constantly growing, the dimensions of associated deforestation also steadily increase [132]. In recent years, total DRC forest loss has amounted to roughly 500,000 hectares (an area the size of the American state of Delaware) [133].

These alarming statistics can only be understood in the context of the extreme poverty which characterizes life for most Congolese. Its annual income of roughly 640 USD/year per capita is amongst the lowest in the world. With only 20% of Congolese citizens enjoying access to electricity, most rely on wood for cooking and heating, leading to relentless charcoal production and forest depletion [134].

The UN’s Food and Agricultural Organization has long confirmed that a high percentage of families throughout the Global South use firewood for fuel [135]. Trees allow households who lack resources to pay for electricity or fuels to stay warm and to cook. Like many other countries where poverty and population growth are positively associated, in the Congo there is no significant timber industry. Deforestation is not a function of excess resource consumption in the Global North nor the result of massive wood experts to wealthier nations. Rather, the vast majority—as much as 90%—of wood consumed is deemed “informal” [136]. Forests provide “fuel”, and the number of people in the Congo who rely on trees for domestic consumption steadily rises [137]. The increasing use of firewood for cooking and heating by expanding populations in India and Nepal has also left the region surrounding the Himalaya mountains denuded of vegetation, exacerbating erosion problems [138]. In short, economic exigencies exacerbate population-driven deforestation dynamics and accelerate forest loss [139].

Many other countries show similar dynamics. Nigeria lost 14% of its forest cover between 2001 and 2020, during which time its population almost doubled from 122 million to over 206 million [140]. A 2023 analysis in Scientific African summarized dynamics there: “The country’s population growth will also lead to increase in fuel wood demand in the rural areas…. The high reliance on fuel wood as a source of energy in the rural areas will exacerbate deforestation and depletion of carbon sink” [141].

Population pressures also drive the expansion of cities, towns and infrastructure such as highways, schools, and hospitals, with urban development often occurring at the expense of forests. In regions where rural–urban migration is widespread, sprawl often pushes into previously forested lands [142]. Even in rural areas, forests are often cleared to build new settlements and the roads that service them. Loss of woodlands typically reflects the immediate need to house more people, rather than any increase in per capita consumption levels [143]. A recent analysis suggests that between 1970 and 2010, an estimated 125,000 km² of land was converted worldwide to urban land uses. Many of these lands contained forests [144]. Land conversion rates have been particularly high in India (where population doubled from 550 million to 1.2 billion during these years) [145] and Nigeria (where population tripled from roughly 55 million people to 161 million) [146]. Land conversions explain one of the reasons why food insecurity and land degradation are associated with high population growth rates [147].

Affluence and high consumption levels surely play a role in global deforestation, indirectly, through demand for wood-related commodities. But it is the need to accommodate more people living near forests that primarily drives large-scale deforestation today. Population pressure leads to land clearance for agriculture, urban expansion, and the extraction of basic resources, making it the most immediate driver globally of deforestation hotspots. It is not just developing societies who suffer when the ecosystem services that forests provide are depleted. A study published in Nature Scientific Reports estimates that the probability that humanity will survive the present rate of deforestation without facing a catastrophic collapse is less than 10% [148]. There are few adverse environmental impacts so consistently associated with demographic growth.

3. More People, More Greenhouse Gas Emissions

The association between population growth and climate change has long been recognized [149,150,151,152]. Figure 2 shows the parallel growth in global population and greenhouse gases over the past 24 years. A growing population requires more energy for its expanded electricity, transportation, food and industrial systems. Even when individual consumption levels are low, the cumulative effect of large numbers of people using energy for heating, cooking, traveling and electricity leads to significant emissions. In many regions whose populations are rapidly increasing, fossil fuels such as coal, oil, or diesel remain the primary energy sources. Given today’s global economy, with continued reliance on these carbon-intensive sources of greenhouse gases, population growth contributes significantly to global warming, sea level rise, melting ice caps, and extreme weather events [153].

In recent years, the academic literature has been inundated with studies confirming the strong association between population growth and rising greenhouse gas emissions in developing countries. Studies in forty-four African countries [154], southern Asia [155], India [156,157], China [158] and Israel [159,160] consistently demonstrate that population increase is driving the steady rise in greenhouse gas emissions at a time when a broad international consensus calls for a 43% reduction in emissions by the year 2030 and 60% in 2035, relative to 2019 levels [161].

Every child is born with a carbon footprint—one that is perpetuated and expanded across subsequent generations. Stated simply, “more people create more greenhouse emissions”. The implications were underscored in a pivotal 2017 study evaluating the relative contribution of individual actions to climate mitigation. Wynes and Nichols calculated that having “one less child” was almost fifty times more effective in reducing individual carbon footprints than other actions such as cycling, vegetarianism, foregoing flights, clothes dryers, etc. [162]. This is particularly true today in Western societies, where per capita emissions remain high, even as present trends suggest that global emissions in many developed countries are steadily dropping [163,164].

Israel’s experience demonstrates the difficulty of meeting global mitigation targets with a rapidly growing population. In the leadup to the 2015 Paris climate accord, countries began to submit their Nationally Determined Contributions. Israel was the only OECD nation to base its emission goals on “per capita” greenhouse gas reductions. The format gave an illusion of conscientiousness. Despite achieving a reasonable drop in per capita emissions, Israel’s average annual population growth is approximately 2% [165]. Accordingly, even if it would have met its target, aggregate national carbon emissions would double over time [166]. A comprehensive 2024 analysis calculated the relative impact of different mitigation scenarios, combining technological and behavioral changes in Israeli emission reductions from electricity, transportation, water, food, construction and fuel for heat. The study concluded that “even when implementing an advanced scenario that combines major technological and behavioural changes, the nation’s mitigation goals will not be achieved given the current demographic trend” [167].

When the United Nations Framework Convention on Climate Change was signed in 1992, developed countries were estimated to be responsible for 75% of global emissions, while developing countries accounted for just 25% [168]. Thirty years later, significant differences persist in per capita emissions between the developing and developed countries. (For instance, according to the World Economic Forum, Africa, with 16% of the global population, is estimated to contribute only 4% of current global emissions [169].) But these asymmetrical dynamics are rapidly changing: as Western countries attempt to meet their international commitments, per capita emissions and aggregate carbon footprints are dropping in the Global North. Conversely, emissions in the Global South are rapidly increasing, indicating the start of a convergence between developing and developed countries [170,171]. Historic distinctions between wealthy and poor nations will be increasingly less salient over time.

This shift in carbon portfolios is expected to become far more dramatic over the coming decade. It is already reflected in the EU’s Energy Projections for African Countries. The assessment projects that by 2065, African countries will release 3.4 billion tons of CO₂ [172]—more than Europe does currently—and far less than the continent is expected to release forty years hence when Europe’s population may actually be smaller than its present size [173]. It is also worth noting that the carbon released as a result of deforestation in developing countries is typically underreported or, at times, not reported at all [174]. As described in the previous section, in many low- and middle-income countries, deforestation trends do not show signs of slowing down. For many developing countries, the release of the vast carbon reservoirs stored in these woodlands into the atmosphere is an order of magnitude greater than any other category of greenhouse gas emissions [175]. Yet, in calculations conducted pursuant to the Paris Climate Agreement, these are not fully calculated in national inventories [176,177].

Table 2. % increase in GHG emissions and population growth vs. % drop in GHG emissions: 2005–2023.

Country	2005 Mton CO2eq	2023 Mton CO2eq	% Change	Population In Mil. 2005	Population (Mil)-2023	% Change
Cambodia	27.3	48.7	78.4	13.2	17.0	28.8
Chad	35.6	98.3	176.1	10.0	18.2	82.0
Burkina Faso	20.3	34.4	69.5	13.8	23.2	68.1
Benin	8.1	16.7	106.2	8.1	13.7	69.1
Bolivia	31.9	55.2	73.0	9.3	12.3	32.3
Brazil	1025.0	1300.0	26.8	186.7	216.4	15.9
Colombia	158.0	224.0	41.8	42.2	52.0	23.2
China	8191.0	15,943.0	94.6	1304.8	1425.6	9.3
Chile	91.1	121.4	33.3	16.1	19.6	21.7
Egypt	256.0	335.9	31.2	79.0	114.4	44.8
India	2120.0	4133.0	95.0	1154.6	1428.6	23.7
Indonesia	649.0	1200.0	84.9	228.8	277.5	21.3
Jordan	24.7	33.4	35.2	5.6	11.3	101.8
Mexico	624.0	712.0	14.1	105.4	128.4	21.8
Israel	71.8	79.5	10.7	6.7	9.1	35.8
Tajikistan	11.8	21.4	81.4	6.9	10.1	46.4
Tanzania	46.5	89.8	93.1	39.4	67.4	71.1
Turkey	327.8	606.4	85.0	68.7	85.3	24.2
UAE	155.5	267.8	72.2	4.2	9.5	126.2
Vietnam	226.4	524.1	131.5	83.1	98.8	18.9
Country	2005 Mton CO2eq	2023 Mton CO2eq	% Change	Population In Mil. 2005	Population (Mil)-2023	% Change
United States	7123.6	5960.8	−16.3	296.8	339.9	14.5
United Kingdom	672.9	379.3	−43.6	60.3	67.7	12.3
Germany	983.7	681.8	−30.7	81.2	83.2	2.5
France/Monaco	542.9	385.5	−29.0	60.5	64.7	6.9
Italy	580.4	374.1	−35.5	58.1	58.8	1.2
Spain	452.7	285.3	−37.0	43.6	47.5	8.9
Japan	1401.1	1041.0	−25.7	127.7	123.2	−3.5
South Korea	573.3	653.8	14.0	47.8	51.7	8.2
Denmark	66.0	41.8	−36.7	5.4	5.8	7.4
Sweden	70.9	49.1	−30.7	9.0	10.6	17.8
Portugal	83.2	53.0	−36.3	10.5	10.2	−2.9
Netherlands	221.3	150.7	−31.9	16.2	17.6	8.6
Belgium	143.0	106.3	−25.7	10.5	11.6	10.5
Greece	128.1	69.2	−46.0	11.1	10.3	−7.2
Finland	73.7	43.4	−41.1	5.2	5.5	5.8
Austria	96.2	72.9	−24.2	8.2	8.9	8.5
Ireland	76.0	57.8	−23.9	4.1	5.0	22.0
Norway	58.6	56.7	−3.2	4.6	5.4	17.4
Cuba	44.1	39.4	−10.7	11.2	11.1	−0.9
Czech Republic	153.4	114.4	−25.4	10.2	10.4	2.0
Slovenia	23.5	15.9	−32.3	2.0	2.1	5.0

Sources: European Commission, EDGAR—Emissions Database for Global Atmospheric Research. 2025. https://edgar.jrc.ec.europa.eu/l= (accessed on 23 March 2025).

highlights the association between population growth and rising greenhouse gas emissions. All countries whose greenhouse gas emissions meaningfully rose between 2005 and 2023 had growing populations, most with demographic increase of over 20%. Countries whose annual emissions dropped during this period typically had stable populations and—apart from Ireland—population growth below 20%.

More than two-thirds of the countries participating in the United Nations Framework Convention for Climate Change do not even acknowledge the role of demographic trends in their emissions profile, much less the need to consider interventions to address them [178]. As one analysis published in Science concluded, in the global climate discourse, population has been “left out in the cold” [179].

4. More People, Less Biodiversity

In 1993, 168 countries ratified a convention on biological diversity which set out to reverse the relentless extirpation of the natural world [180]. The agreement reflected growing recognition that beyond nature’s intrinsic value, it provides invaluable ecosystem services, such as crop pollination, water filtering, air purification and natural medicines [181]. Despite this international agreement, more than thirty years later, the crisis has only deepened. The World Wildlife Fund’s 2024 Living Planet Report, which tracks population changes in more than 5000 vertebrate species, reveals a staggering 73% decline in monitored wildlife population between 1970 and 2020 [182].

Presented as a sterile statistic, the drop is difficult to apprehend. A closer look at a few charismatic large mammals may be more instructive: In 2023, only 70,000 giraffes remained in the wild, a 40% decline in only three decades [183]; the world’s tiger population has dropped from 100,000 to an estimated 3500 [184]. Almost all primate populations in the wild are shrinking, with certain species, such as the cross-river gorilla, teetering on the brink of extinction, with only 200–300 surviving in the wild [185]. In nearly all cases, human encroachment and associated habitat loss are the proximate reasons for the decline [186]. For instance, the east-lowland gorilla has lost 87% of its original habitat, with its numbers dropping by 50% in recent years [187].

Population pressure manifests in a variety of ways. A high-resolution assessment of the human footprint using a 1 km² scale measured biodiversity impacts from built environments, crop lands, pasture lands, night lights, railways, major roadways, and navigable waterways. Each specific pressure was linked to threats of species endangerment identified by the International Union for Conservation of Nature (IUCN) Red List. The study concluded that “Human impacts on threatened vertebrates are widespread, extending across 84% of Earth’s terrestrial surface” [188].

The 2019 report by the Intergovernmental Panel on Biodiversity and Ecosystem Services (IPBES) may be the most comprehensive assessment of the state of global biodiversity to date [189]. Reflecting three years of work by 145 scientists from 50 countries, the authors perused 15,000 publications and summarized the critical importance of biodiversity: 75% of agricultural production is dependent on the pollination of natural systems; 70% of cancer drugs are synthesized from natural products; 60% of carbon dioxide produced by humans is absorbed by natural systems; and 2 billion people rely on wood products for energy. The report attracted international attention for its precise estimate of the magnitude of the loss: some 25%—or 1 million—animal and plant species are at risk of extinction.

The report identifies five primary drivers of biodiversity loss: habitat destruction and fragmentation [190]; overexploitation or hunting [191]; pollution [192]; invasive species [193]; and climate change [194]. While the report suggests possible responses, such as stricter enforcement of poaching laws and better regulation of hunting protocols, it did little to draw international attention to population pressures underlying the biodiversity crisis.

The primary mechanisms of anthropogenically driven biodiversity loss involve habitat fragmentation [195]. As human populations expand, demand for land increases for agriculture, urban development, and infrastructure. This fragments once continuous habitats into smaller isolated patches, reducing their size and isolating wildlife populations, impeding species’ ability to access resources, reproduce, and migrate. Fragmented habitats are also more vulnerable to invasive species, pollution, and climate change. Research aggregating findings in five continents over 35 years showed that habitat fragmentation reduces biodiversity by as much as 75%, impairing key ecosystem functions, decreasing biomass and altering nutrient cycles. Once a habitat is divided, species loss averaged 20% after one year but exceeded 50% after a decade in the smallest, most isolated fragments [196]. According to one analysis, preventing fragmentation could avert the impending extinction of millions of species [197].

Fragmentation also disrupts ecological processes such as seed dispersal and pollination, which are vital for maintaining healthy ecosystems [198]. Moreover, smaller and more isolated populations are more prone to extinction due to inbreeding, genetic drift, and reduced adaptability to environmental changes. A recent study published in Nature Sustainability emphasized the global scale of fragmentation, linking agricultural expansion and urbanization with habitat loss, especially in biodiversity hotspots located in Southeast Asia and Central Africa. These areas, rich in endemic species, are losing their biological diversity at an alarming rate. Without comprehensive land use planning and conservation strategies, the combined effects of population pressures and habitat fragmentation could result in the loss of up to 50% of species in affected regions by the end of the century [199].

One longitudinal study in the Brazilian Amazon, conducted as part of the Biological Dynamics of Forest Fragments Project, demonstrated how fragmentation drastically reduces species richness over time. The study found that small forest fragments supported fewer species of birds, mammals, and insects (especially solitary species) than larger, continuous forests. Fragmentation’s effects are compounded by other anthropogenic threats, such as logging, hunting, and especially fire, creating an even greater peril for Amazonian biota [200].

The precise pathology of habitat destruction varies geographically, but the general dynamics are similar worldwide: As populations expand, natural habitats are cleared to make way for farmland, housing, and infrastructure. This leads to their fragmentation, and “island dynamics” where there is not enough space for organisms to survive. In densely populated regions, people rely heavily on natural resources for survival [201]. Hunting is not a “sport” but the key to survival [202]. The same is true for harvesting of timber for fuel, fish for protein, and wildlife for food or trade. These activities are driven by immediate exigencies to meet basic local necessities rather than global markets in affluent lands. In regions with rapidly growing populations, small-scale farming and hunting impacts accumulate across a large population, resulting in widespread biodiversity loss in poverty-stricken communities [203].

Countries experiencing the most acute biodiversity loss share these dynamics. As an island nation, Madagascar is home to unique ecosystems, with 90% of local species being endemic, existing nowhere else on the planet. Lemurs, tenrecs and fossas are just a few examples of charismatic local species. As the island came to be colonized, an array of large mammal species began to disappear [204]. Ecological damage became particularly acute during the past century, as population surged by 800% from 4 million in 1950 to 32 million in 2024 [205] and the country lost an estimated 80% of its natural areas [206]. A recent report about Madagascar in Science confirmed that “many species have perilously reduced population sizes”, while there may be many undocumented extinctions, especially among taxa that are poorly studied [207].

Haiti, the poorest country in the Western Hemisphere today, historically was home to one of the hemisphere’s richest ecosystems. Although data may be outdated, the country contains between 5000 and 5600 species of vascular plants, of which 37% are endemic, along with over 2000 species of fauna, 75% of which are also geographically unique [208]. Haiti’s rapidly growing population has led to devastating biodiversity loss. In 1950, the country was home to 3.2 million people and forest cover was roughly 50%. In 2023, Haiti’s population was 11.6 million, roughly 400% larger, leaving roughly 1% of original primary forest intact [209]. The need for agricultural land, combined with the reliance on wood for fuel, has led to the clearing of almost 80% of the country’s woodlands [210,211], bringing numerous species to the brink of extinction [212]. Research confirms that environmental degradation in Haiti is driven primarily by population density and lack of alternative energy sources [213].

Papua New Guinea lies on a sizable area of 462,243 km², with over 20,000 km of coastline. Its extraordinary biodiversity is attributed to abundant water resources (more than 5000 lakes) and one of the world’s largest mangrove systems with 8000 km of wetlands, lagoons, coral reefs and atolls, along with 100 offshore islands [214]. With over 28 million hectares of tropical rainforests, it hosts the third largest rainforest in the world, which, due to its relative isolation and high altitude, is considered a biodiversity treasure [215]. As a result, Papua New Guinea is home to one of the richest assemblages of vertebrates on Earth. Its forests harbor at least 1786 species of birds, mammals (including 10 of the world’s 12 species of tree kangaroos), reptiles, and amphibians. This is estimated to be over 5 percent of the world’s total biodiversity—with many species yet undiscovered and unclassified.

The country’s unique biodiversity, however, is in “freefall”. A 2022 summary by the United Nations reports that 66% of populations of known animal species in the country are decreasing, with 623 plants and 481 animals currently listed as either critically endangered, endangered or vulnerable. A fifth of local mammal species are defined as threatened [216]. Again, the underlying driver of this loss is the exponential growth in humans [217,218].

In 2024, Papua New Guinea’s population was roughly 10.5 million people, roughly seven times the 1.5 million counted in the 1950 census [219]. The country’s forests face mounting pressures by the growing number of people, enlarging agroforestry-related land clearing, mining and logging activities, along with expanded subsistence agriculture [220]. In a 2013 report by the Papua New Guinea National Research Institute, the government was forthright about these dynamics and what should be done:

“The government of Papua New Guinea has recognised that the population growth rate and distribution of PNG’s population has become more unsustainable. With the population doubling approximately every 27 years, pressure on the available natural and human resources continue to increase dramatically as well as the need for increased demographic investment and service delivery. This is considered a major stumbling block for the achievement of responsible sustainable development.” [221]

In a recent retrospective evaluation, the British Royal Society estimated that between 1996 and 2008, 60% of total global biodiversity loss for bird and mammal species occurred in just seven countries, Indonesia, Malaysia, Papua New Guinea, China, India, Australia and the USA, where the majority has occurred on the islands of Hawaii [222].

Table 3. Population increase in countries where most biodiversity loss occurred: 1986/1996 to 2008.

Country	Population Growth 1996/2008	Population Growth 1986/2008
Indonesia	18.18%	40.69%
Malaysia	33.98%	78.06%
Papua New Guinea	51.06%	102.86%
China	8.31%	33.00%
India	22.69%	51.32%
Australia	16.48%	32.50%
USA	13.23%	28.95%

Sources: British Royal Society, 2022; United Nations, World Population Prospects, 2024 [31,222].

shows the population growth in these countries during those 12 years. As habitat decline and biodiversity loss is a prolonged process, population increase during the 22 years between 1986 and 2008 was also calculated. The majority of biodiversity loss on Earth has occurred in countries experiencing exceptionally rapid population growth, with average annual increases ranging from 1.3% to 4%.

By contrast, the Nature Conservation Index ranks the effectiveness of countries in protecting the natural environment [223]. Economic capacity is not a particularly good predictor of high performance. Of the top-twenty ranked countries, seven are developing countries (four are in Africa), with per capita GDP below USD 10,000. While conservation policies are critical for success, so is population stability: women living in all of the “conservation champions” have total fertility rates below three children per family, and with the exception of Australia, all are trending towards demographic sustainability.

5. Overpopulation and Overfishing

Overfishing refers to the practice of harvesting fish from oceans, rivers, and lakes at rates that surpass their natural replenishment levels. Because more fish are caught than are replaced through natural reproduction, fish populations around the world are plummeting [224]. With the number of people in the world growing and demand for fish doubling by mid-century [225], critical thresholds are being crossed. What emerges is a clear causal relationship between the rise in human populations and the decline in fish populations [226]. Unsustainable extraction not only threatens marine ecosystems, but could ultimately lead to the eventual collapse of entire fisheries [227].

The world’s natural marine environments serve as a quintessential, overexploited “commons”, described in Garret Hardin’s seminal 1968 essay, The Tragedy of the Commons. Hardin identified fisheries as particularly susceptible to overexploitation in a world where human population is expanding: “Maritime nations still respond automatically to the shibboleth of the “freedom of the seas”. Professing to believe in the “inexhaustible resources of the oceans”, they bring species after species of fish and whales closer to extinction” [228].

Since this warning was published, fish stock depletion has escalated drastically, driven by global population growth and the associated increased demand for animal-sourced, protein-rich food [229]. The expansion of industrial fishing fleets, equipped with advanced technologies such as deep-sea trawling, GPS tracking, and fish aggregation devices (FADs), has further contributed to overfishing [230]. Dramatic declines in large predatory fish, like tuna and cod [231], offer indicators of broader ecosystem destabilization [232].

Overfishing is evaluated using benchmarks established by international and regional bodies which categorize fish stocks as underfished, maximally sustainably fished, or overfished [233]. One widely used metric is Maximum Sustainable Yield (MSY), representing the highest quantity of fish that can be harvested annually indefinitely without depleting the stock. Other indicators, such as spawning biomass and fishing mortality rates, provide evidence-based insights into stock health [234]. Standards are set, like those established under the Marine Stewardship Council certification program, adding a layer of scrutiny, and promoting accountability within the global fishing industry [235]. These benchmarks and certifications are valuable for monitoring but, in practice, have not been able to stem the negative global trends driven by human population pressures. Rather, they allow for higher resolution when reporting fishery declines.

The United Nations’ Food and Agriculture Organization monitors global fish stocks and its data reflect the magnitude of the ongoing collapse. According to the organization’s official figures, in 1960, when there were 3 billion people on planet Earth, roughly 10% of the world’s natural fish stocks were classified as overfished; by 2019, with a global population of 7.7 billion, this percentage tripled to roughly 34%. Figure 3 offers a graphic presentation of the steadily decline. The FAO’s 2022 report reveals that “maximally fished stocks now account for 57.3 percent of total stocks, while underfished stocks, a mere 7.2 percent” [236].

These figures often rely on national reporting, which for a variety of reasons, historically, has been prone to significant underestimations [237]. This produces a phenomenon known as “phantom recoveries”, where fish stock are actually in decline and overfished, but misinformed fishery managers continue to maintain current fishing quotas or even expand catch levels [238]. If current trends persist, remaining fish populations in the oceans may experience irreversible declines, posing a severe threat to global food security, especially in developing countries [239].

Freshwater systems, which comprise only 0.01% of global water resources and 0.8% of the Earth’s surface, are even more vulnerable than oceans to overfishing due to their limited size and capacity [240]. Lakes and rivers are more constrained in their capacity to support large-scale fish populations, making them highly susceptible to overfishing, pollution, habitat destruction, and water diversion. These systems, which support roughly 1 million species—6% of known life forms—have seen dramatic population declines [241]. Between 1970 and 2014, freshwater fish populations declined by 83%, a rate surpassing that of marine systems [242].

Population growth, particularly in coastal regions, has significantly intensified pressure on global fish stocks. Regions such as Southeast Asia [243], the Mediterranean and Black Seas [244], and much of West Africa [245], where fishing remains a major economic activity, have witnessed particularly severe declines in fish populations. The combination of rising human demand for protein and modern technologies have intensified fishery-related conflicts and led to more aggressive, destructive fishing practices [246]. A range of 17 anthropogenically associated stressors affect both the quantity and quality of wild-caught and farmed fish (“blue foods”) [247], highlighting the cumulative pressure facing coastal communities worldwide [248].

The Philippines, for example, an archipelagic nation with a population of over 100 million, exemplifies how rapid population growth exacerbates overfishing. Coastal regions in the Philippines that are heavily reliant on fish for sustenance are struggling to cope with a 55% growth in population during the first quarter of the twenty-first century [249]. Provincial data reveal the adverse effects on fishermen’s livelihoods as fisheries steadily decline [250]. In regions like the South China Sea and the Philippine Sea, fish stocks have dropped by more than 70% since the 1960s due to unsustainable practices like “fishing down the food web”, where smaller fish, lower in the food chain, are targeted after larger species become depleted [251,252].

International interventions, such as stricter trawling regulations, have attracted attention in recent years [253], but their effectiveness in maintaining stability have thus far been underwhelming [254]. The vast majority of global fish stocks that have been assessed need rebuilding, with significantly reduced exploitation required to reverse the collapse of vulnerable species [255]. Marine reserves, offering sanctuaries for fish populations, have been hindered by enforcement challenges and economic dependence on fishing [256]. A vicious cycle ensues where, in the absence of abundance, local fishing industries adopt destructive practices like blast and cyanide fishing, alongside aggressive trawling, that further degrade marine habitats, making recovery difficult [257].

Peru, one of the world’s leading fishmeal producers, also faces a severe overfishing crisis, largely due to the increasing global demand for its anchoveta stocks [258]. Anchoveta, a small pelagic fish, forms the backbone of Peru’s fishery sector, and is used primarily in fishmeal production to support livestock and aquaculture [259]. Since 1980, the country’s human population has doubled, leading to overfishing that caused anchoveta stocks to decline by more than 50% between 2000 and 2020 [260]. Recently, fishing activity in these coastal areas has in fact dropped off, after the remaining adult anchoveta populations dwindled below economically viable levels [261].

Senegal is just one of many African countries that has experienced dramatic declines in fish stocks [262] as the local human population doubled from 9 million in 2000 to 18 million in 2023 [263]. The implications are profound in a country where fish and seafood represent more than 40% of the animal protein intake and where one in six people work in the fisheries sector [264]. Long-term trends in the trophic level of exploited species and total catches are discouraging, reflecting mounting human pressures and increased fish catches. This pressure is particularly manifested at the top of the food web, suggesting that “fishing down the food web” is quite advanced in the country’s marine ecosystems, with white grouper, an especially popular local catch [265], virtually disappearing [266]. Results of stock assessments suggest a 63% reduction in Senegal fish populations compared to their pristine state [267]. The collapse is driven by a growing local demand along with competition from foreign fishing fleets [268]. Senegal’s local fishing communities have been hit hardest by the depleted stocks, with the volume of catches by traditional wooden fishing canoes falling 58 percent in just a decade. Artisanal fishermen find it increasingly challenging to sustain their livelihoods [269]. To survive, desperate Senegalese fishermen often cross illegally into Mauritania, sparking tensions and a harsh enforcement response by the Mauritanian coastguard that has led to fatal incidents [270]. Ineffective local monitoring and enforcement against illegal, unreported, and unregulated fishing further strains depleted marine resources [271].

Aquaculture offers a partial solution by supplementing natural marine supplies, but it introduces its own environmental consequences [272]. The overfishing crisis ultimately stems from an imbalance between supply and demand. A 2024 review in Science outlines key measures for reversing these trends: allowing fish to grow and reproduce before capture, using environmentally low-impact fishing gear, establishing no-take zones to preserve genetic diversity, and maintaining functional food webs by reducing fishing of forage species like anchovies, sardines, herring, and crustaceans like krill. The strategy is grounded on a simple yet vital principle: “take out less than is regrown” [273]. But such measures are difficult to implement when growing populations rely on local fisheries as a primary source of food [274].

6. The Population Threat to Global Water Security

Water scarcity is intrinsically associated with population growth, as expanding human populations require more water for drinking, agriculture, industry, and sanitation [275]. Globally, water demand has surged due to urban expansion, industrialization, and intensified agricultural production to meet the food needs of larger populations [276]. With human numbers doubling on Earth between 1970 and 2020, demand for freshwater resources for domestic use increased globally by 600% [277].

In the mid-1980s, Malin Falkenmark, serving as the State Hydrologist for the Swedish Meteorological and Hydrological Institute [278], developed objective standards for managing water scarcity [279]. In an influential article with colleagues, she introduced the Water Stress Index, which established a simple framework that has since been widely accepted [280]. The standard defines water scarcity based on water availability per individual, setting the baseline at 100 L per person per day.

To understand a country’s overall hydrological capacity, the standard is typically aggregated to a national scale. A country with less than 1700 m³ of water per capita annually is considered “water-stressed”; below 1000 m³, the country experiences “water scarcity”; and below 500 m³, it faces “absolute water scarcity” [281]. Falkenmark’s equation uses human population as a denominator. This underscores the central role of population size in characterizing scarcity: water stress is defined simply as too large a population per unit of water available from the water cycle.

The United Nations Food and Agriculture Organization reports that water resources in most places in the world are under increasing stress. Population growth is identified as the primary driver, followed by rising incomes and changing lifestyles and diets [282]. According to a 2023 World Resource Institute Assessment, the most vulnerable regions are the Middle East and North Africa, where 83% of the population is exposed to extremely high water stress [283]. Historically, the region has seen a dramatic demographic increase. The resulting hydrological pressure has even led to political unrest and violent protests by citizens dissatisfied with government policies amid worsening water shortages [284]. Syria is a conspicuous example of a Middle Eastern country where water stress led to widespread agricultural failure with disastrous results [285].

Groundwater resources, which recharge slowly, are particularly vulnerable to over-extraction [286]. It is increasingly common for countries facing hydrological shortfalls to rapidly withdraw water from aquifers that took millennia to accumulate. In the long run, such “overdrafts” are unsustainable. Hydrological deficits are occurring all over the world, with the demand for water outpacing the natural replenishment rates of freshwater resources. Lake Chad offers a cautionary tale: During the 1960s, it ranked as the world’s sixth largest inland water body (with an area of 25,000 km²). With the doubling of population in its riparian states (Cameroon, Chad, Niger and Nigeria), the lake shrank to less than 2000 km² by the 1980s, losing more than 90% its original area [287]. A recent evaluation of Lake Chad’s condition published in Scientific Reports ends with the warning that the modest water remaining is threatened due to “increasing pressure on resources in consequence of rapid population growth in Sahel” [288].

India exemplifies the challenges posed by water scarcity in a densely populated country. As the population has grown from 500 million in 1970 to over 1.4 billion in 2023, predictably, demand for water has intensified. Agriculture consumes roughly 85% of India’s freshwater resources, and shortages in the rural regions have become acute [289]. Overextraction and anthropogenic activities taking place alongside dense population centers have caused contamination and overutilization of key rivers, including the sacred Ganges [290]. At the same time, groundwater levels in regions like Punjab and Haryana are critically low due to over-pumping for irrigation [291]. With 18 percent of the world’s population, but only 4 percent of Earth’s water resources, the World Bank warns that India’s perennial water shortages will soon grow more severe [292]. Already, over 600 million people in India experience high to extreme water stress [293]. While the 1960 Indus Water Treaty between India and Pakistan has helped avert violent conflict over the shared water resources until now, there are cases where water shortages become a source of conflict.

One often-cited example involves Syria: demographically driven water scarcity there has been a significant factor in its political instability. Since receiving independence until the advent of its recent civil war, Syria’s population grew by roughly 700%, from 3.2 million in 1946 to 22.7 million people in 2011 [294]. This dramatically reduced per capita renewable water availability [295], from an abundant 5500 m³ level in the mid-twentieth century to below 700 m³ per person, a level at which local communities are considered water-stressed [296]. Depletion of groundwater resources and salinization of farmlands were already identified as a critical issues before the unprecedented 2007 drought and subsequent dry years [297].

While multiple factors contributed to the Syrian Civil War, many analyses emphasize successive years of droughts that led to the collapse of local agriculture, forcing migration to urban areas and ultimately leading to inter-ethnic violence [298,299]. Other explanations highlight Turkey’s unilateral diversion of Euphrates water [300] or inherent vulnerabilities in Syria’s disparate agricultural systems [301]. Typically, there are many forces at play beyond natural resource shortages that contribute to extreme social instability and warfare. Though hostilities are rarely attributable to a single factor, rapid population growth and demographic pressures significantly undermined Syria’s water and agricultural resilience, accelerating the social collapse that ultimately led to civil war [302].

Neighboring Jordan’s population increased twenty-fold during the past 75 years from 500,000 in 1948 to 11.5 million in 2023 [303], making it amongst the most water-scarce countries in the world [304]. The government reports that per capita renewable freshwater availability is just 61 m³ annually—only 12% of the minimum threshold set by the Falkenmark Index. The arrival of 1 million Syrian refugees after 2011 further strained resources, contributing to a 40% increase in water demand [305]. Refugees are particularly vulnerable to water scarcity, with the United Nations reporting Jordanian camps facing severe shortages, with inadequate access to basic sanitation and hygiene infrastructure affecting children’s ability to attend school [306].

In practice, Jordan’s acute water shortages translate into highly intermittent water supply to urban residents. Residents of major cities receive water once a week or once every other week [307], relying heavily on rooftop cisterns to store water for basic needs [308]. In rural regions, farmers report only receiving 60% of their expected allocations, leading to rampant water theft and illegal pipeline tapping, as desperate farmers seek to avoid losing their crops [309]. Since 2013, Jordan has begun to mine the ancient waters of the Disi aquifer, which lies 500 m underground near the Saudi Arabia border [310]. Formed roughly 30,000 years ago, this fossil aquifer cannot be replenished. Recent research confirms projections that the resource will either run dry [311] or become too saline to use within a few decades [312]. Agreements with Israel, which has developed a substantial desalination infrastructure [313], may temporarily ease urban water shortages. But Jordan’s rapid population growth and naturally arid conditions suggest that its traditional agrarian economy is not sustainable.

Egypt, once a water-rich nation, now faces severe water scarcity driven by its explosive population growth. Historically reliant on the Nile River for its freshwater, as recently as the 1960s, Egypt enjoyed a water surplus of 20 km³ per year. With its population more than tripling from 30 million in 1960 to 112 million in 2023, Egypt now faces a water deficit of 40 km³ per year. The country must supply water to an additional 2 million people each year, forcing it to import more than 50% of its food and become the world’s largest wheat importer [314]. With a projected population of over 157 million people by 2050 [315], rapid population growth will continue to transform the country from a water superpower to a water pauper.

Natural water resources are shrinking at an alarming rate in the face of mounting demand, with scarcity expected to intensify globally, especially in regions with rapidly growing populations [316]. Climate change is expected to further exacerbate water scarcity by altering precipitation patterns and increasing the frequency and severity of droughts [317]. The World Population Review ranks the most water-stressed countries each year.

Table 4. Population growth in the world’s ten most water-scarce countries.

Country	Population Percent Increase
Kuwait	104.76%
Cyprus	27.00%
Oman	95.83%
Qatar	250.65%
Bahrain	80.72%
Lebanon	15.56%
UAE	143.59%
Saudi Arabia	58.47%
Israel	50.77%
Egypt	47.61%

Source: UN World Population Prospects: The 2024 Revision [31].

lists the ten most “water scarce” countries in 2024. All showed substantial population growth during the past twenty years.

By 2050, more than half of the world’s population will live in water-stressed regions, with sub-Saharan Africa, the Middle East, and parts of South Asia facing the most acute shortages [318]. To meet the projected rise in demand for food, fiber and biofuels, agricultural production will need to increase by 50% relative to 2012 [319]. The anticipated increase in human population will push water demand to unprecedented levels, with almost half of humanity by mid-century expected to face acute shortages as an existential challenge [320].

7. Proliferating Populations, Land Degradation and Desertification

Land degradation refers to the decline in the productive capacity of land and the diminution of its productive potential and value as an economic resource [321]. Contrary to common misconceptions, desertification is not the natural expansion of existing deserts. While there have been innumerable definitions given over the years to the phenomenon, it generally refers to the degradation of land in arid, semi-arid, and dry sub-humid areas [322] where desiccated conditions and extreme temperatures make soil restoration far more challenging than in more temperate climates [323]. Soil erosion, a more generic process, entails accelerated removal of topsoil from land surfaces through water, wind and human activities [324], reducing the land’s ability to sustain crops or vegetation [325]. Both land degradation and desertification are highly associated with increased pressures from populations [326].

Throughout history, land stewardship has been guided by the recognition of carrying capacity and the perils of exceeding natural biological thresholds. Ancient civilizations were quick to observe the detrimental effects of soil fertility loss, caused by the conversion of natural ecosystems into croplands or rangelands [327]. The book of Genesis even describes the dynamics of resource overshoot when the herders who tended the flocks of the Patriarch Abraham quarreled with those of his nephew because “the land could not support them while they stayed together, for their possessions were so great that they were not able to stay together” [328].

The effects of human activities are particularly acute in drylands where people depend heavily on local ecosystems for basic needs [329]. As population pressure begins to physically erode drylands, local ecosystem services, such as food, firewood, forage, fuel, building materials, and water for humans and their livestock, deteriorate. Resulting damage can become irreversible, as soil fertility and ecosystem function are difficult, if not impossible, to restore [330]. Where efforts are made to rehabilitate degraded land, they often require substantial time and resources, with decades passing before even partial recovery of productivity is achieved—if it is achieved at all [331].

In 2005, 1300 experts joined under the auspices of the United Nations to prepare the Millenium Ecosystem Assessment, which examined the state of the planet and its disparate ecological regions. The report identified desertification as the environmental problem affecting more people than any other single ecological challenge. It estimated that the livelihoods of over 1 billion people across 100 countries were threatened by desertification, with nearly 1 billion of the poorest and most marginalized people living on the most vulnerable lands at risk [332]. Since the publication of this seminal report, the population of drylands has increased and the crisis has only worsened. Recent calculations by the United Nations indicate that at least 100 million hectares (240 million acres) of healthy land is “lost” each year to the forces of desertification [333]. The cumulative impact is staggering: desertification adversely affects 36 million square kilometers of land [334]. Present estimates suggest that 500 million people already live in areas that suffer from desertification [335]. The livelihoods of twice that number are threatened by the scourge of land degradation [336].

While specific dynamics vary by region, the general pattern of events leading to land degradation is well documented: as population pressure grows, human activities adversely affecting land productivity intensify [337]. The specific drivers of land degradation and desertification are of course diverse, including overgrazing, deforestation, destructive cultivation and ill-advised irrigation practices [338]. But for the most part, they all emerge after a general succession of events transpires. Populations grow, human demands exceed the land’s carrying capacity, and inappropriate management of land resources ensues. Over time, fertile areas become barren, leaving the soil depleted and desert-like, incapable of sustaining life [339].

Overpopulation creates a feedback loop that accelerates land degradation and desertification by intensifying demand for food, water, and living space, pushing human activities beyond traditional arable areas. As the number of people reliant on subsistence and pastoral agricultural systems increases, more and more land is cleared for agriculture and habitation, often resulting in unsustainable farming practices that accelerate soil depletion and erosion. A vicious cycle is created where population pressure pushes productive grasslands beyond their capacity. When they can no longer provide adequate forage for herds, pastoralists move on, utilizing marginal lands which become degraded at an even faster rate [340]. Inappropriate farming techniques, including monocropping and the excessive use of chemical fertilizers, are also associated with rapidly expanding populations, degrading soil structure and biodiversity while further accelerating desertification. Deforestation, another consequence of rising population pressures, exposes soil to wind and water erosion by removing root systems that stabilize land, diminishing the soil’s ability to retain moisture. In arid lands, salinization caused by excessive irrigation on arid lands further compounds these effects by leaving salt deposition that reduces soil fertility [341]. Each of these drivers is intensified by population growth, as rising resource exploitation perpetuates more unsustainable practices, accelerating land degradation and desertification.

Overgrazing is frequently cited as the leading cause of desertification worldwide [342]. Typically, the phenomenon begins with an increase in the number of humans relying on foraging animals for their livelihoods. To support larger herds for meat and dairy production, livestock numbers eventually surpass the carrying capacity of traditional rangelands. The excessive foraging depletes vegetation cover, exposing soil to wind and water erosion. The removal of plant roots reduces soil cohesion, leading to the loss of topsoil, which is vital for retaining nutrients and moisture [343]. Heavy grazing also compacts the soil, reducing its organic carbon, nitrate levels, and moisture [344], further diminishing the land’s ability to support vegetation.

The Tragedy of the Commons paradigm, originally introduced in 1968, describes the dynamics that drive the unsustainable exploitation of shared pasturelands. Mounting population pressure on unregulated commons leads to system collapse [345]. Even after decades of regulation and stock limits, these forces persist [346]. For example, recent studies in Mongolia suggest that herders are aware of the risks of overgrazing. Yet they choose to prioritize short-term economic benefits to sustain growing populations, triggering overexploitation of commonly owned lands. Government policies intending to reach a forage–livestock balance are widely ignored [347]. Over time, overgrazing leads to irreversible damage to the soil [348].

These dynamics are unfolding around the world. In Niger, for example, rapid population growth has exerted enormous pressure on land resources [349]. Over the past 50 years, the population has increased by 500%, from 5 million in 1974 to 27 million in 2023. This growth has caused the degradation of over 60% of arable land, primarily due to overgrazing and deforestation [350]. Smallholder farmers, with few alternatives, are forced to continuously farm and graze on the same land, leading to steady declines in soil fertility [351]. The need for firewood and more arable land has exacerbated deforestation, leaving soil bare and vulnerable to wind erosion, particularly in the arid Sahelian climate where rainfall is modest and unpredictable [352]. Studies report that Niger loses approximately 100,000 hectares of arable land annually to desertification, a trend that threatens both food security and the livelihoods of its population [353].

As populations continue to grow, competition for limited land resources intensifies, leading to shorter fallow periods and increasingly unsustainable agricultural practices [354]. Land that once had the opportunity to recover between cultivation cycles is now farmed continuously, depleting its nutrients and compromising its structure. The expansion of agricultural land into previously forested areas, as described, results in significant deforestation, further exacerbating soil erosion and the loss of biodiversity, which could play an important role in soil replenishment [355]. More than economic development, population growth remains a dominant driver of urban sprawl that encroaches on arable terrain, reducing the amount of productive land available per capita and leading to problems of employment, feeding the urban population and environmental protection [356]. This escalating cycle of land degradation due to population pressures has caused the Earth to lose approximately one-third of its arable land over the past forty years [357].

8. Conclusions

This review summarizes the effect of population pressures on the critical sustainability challenges facing humanity. It suggests that the resulting irreversible damages to disparate ecosystem services are already well underway. Rising standards of living and consumption patterns surely contribute to the magnitude of the impact. But the rapid growth in human population serves as the ultimate driver of a wide array of harmful environmental impacts. In theory, advancements in clean technologies, efficient land management, desalination and sustainable fishing practices could do much to ameliorate adverse outcomes. Present experience, however, indicates that resources for such interventions will not be mobilized. In the face of mounting population pressures, deforestation, biodiversity loss, climate change, overfishing, water scarcity and land degradation will continue to intensify.

The most optimistic estimates published by the United Nations suggest that the world’s population will exceed 10 billion people by the end of the 21st century—an increase of roughly 4 billion from the start of the century. Yet the UN demographic model has a range of possible trajectories, including scenarios where the global population grows by an additional 2 billion, making Earth 20% more crowded than commonly accepted projections [358]. Many experts argue that higher population scenarios are in fact far more plausible, if present policies and attitudes remain unchanged [359].

In a world where ecological and climatic systems exhibit non-linear patterns of damage, the consequences of continual environmental disruption are inherently unpredictable [360,361,362]. The environmental implications of an additional two billion (or four billion) people on the planet are difficult to predict precisely, but the results ecologically will not be favorable. Tipping points will be crossed where climate change becomes catastrophic, ocean acidification decimates marine systems, terrestrial species extinction accelerates, fisheries collapse and land productivity loss becomes irreversible.

At a more micro level, countries with rapidly growing populations will suffer increasingly acute symptoms, undermining local or national efforts to confront environmental hazards like solid waste [363] or air pollution emergencies [364]. While these might not be characterized as part of a “global crisis”, they often pose the most egregious public health insults and immediate challenges for humans at the local level. Rapid population growth also exacerbates social pathologies such as traffic congestion, overcrowded classrooms, courtrooms and hospitals, or even rising violence [365]. Making progress on most of these issues will be extremely challenging as long as rapid population growth continues. The metaphor of a treadmill—where efforts to move forward prove futile against the relentless growth of carbon, water, and land footprints—is frequently invoked, and not without good reason [366].

This article’s focus on the effect of population increase in no way contests the significant contribution that aggregate consumption makes to global ecological decline and climatic disruption [367,368]. Individual and collective ecological footprints, especially in affluent countries, must be dramatically reduced. But dismissing the critical role of overpopulation growth in accelerating global environmental deterioration or evading the issue entirely because it is sensitive or intractable weakens the global response to the most pressing environmental challenges.

There are, of course, many actions that can be taken and many policies that can be adopted to ameliorate the six environmental problems summarized in this article. These include expanding global afforestation efforts [369,370] and the electrification of communities that rely on trees for fuel [371,372]; expediting the transition to a low-carbon economy [373] and adopting a global price for carbon [374,375]; dramatically expanding strategically located nature reserves worldwide [376,377] and creating ecological corridors [378,379]; intensifying aquaculture [380] and strengthening fishery management [381]; increasing sea water desalination [382,383] and wastewater reuse [384,385,386]; and implementing a range of sustainable agriculture practices and soil conservation measures [387,388]. Assessing the potential effectiveness of these interventions and challenges to implementation is a formidable task and beyond the scope of this article.

It is important to note though that the above strategies share a common weakness, they address “direct drivers” or “symptoms” produced by these disparate environmental crises rather than their underlying sources. Addressing the underlying root causes or “indirect drivers” is critical and requires family planning programs and policies—such as those briefly described and documented in first part of this article—that encourage lower fertility. If rapid population growth continues, the ultimate outcomes of most proposed solutions to myriad environmental crises will likely be unsatisfactory.

The UN 2024 World Fertility Report calculates that women today bear on average one child fewer than they did around 1990 [389]. This constitutes an encouraging trend. In some countries, this reflects evolving social dynamics. In many others, however, it is the result of deliberate public policies, robust family planning initiatives and an honest reckoning about the catastrophic implications of unchecked population growth. These efforts have transformed national demographic profiles in disparate places, offering a modicum of stability that now enables meaningful economic and environmental progress [390]. A consensus, acknowledging the central role of population growth in causing the world’s most pressing environmental problems, is a critical starting point. Without addressing this issue, progress towards sustainability will remain elusive and humanity’s ability to confront its most urgent ecological crises will continue to falter.