A Review of the National Academies’ 2025 Report on the Environmental Effects of Nuclear War

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Introduction: Forty Years After “Nuclear Winter,” A New Reckoning

In 1983, a team of scientists including Richard Turco, Owen Toon, Thomas Ackerman, James Pollack, and Carl Sagan—collectively known by the acronym TTAPS—published a paper that fundamentally altered the global perception of nuclear war. Titled “Nuclear Winter: Global Consequences of Multiple Nuclear Explosions,” the study introduced a terrifying new dimension to the atomic threat. It argued that the direct effects of nuclear detonations—the blast, the heat, the radiation—were only the prelude. The true cataclysm would be climatic. The vast firestorms ignited in burning cities and forests would inject hundreds of millions of tons of smoke and soot into the stratosphere, forming a persistent shroud that would block sunlight, plunge continental temperatures below freezing for months, and trigger a global agricultural collapse leading to mass starvation. This “nuclear winter” hypothesis, bolstered by a landmark 1985 National Research Council (NRC) report, transcended scientific debate and entered the public consciousness, contributing to a renewed push for arms control and helping to end the Cold War’s escalating nuclear arms race.  

Forty years later, the world is a profoundly different place. The bipolar standoff of the Cold War has given way to a multipolar landscape of nuclear-armed states. Global arsenals, while smaller, have evolved in yield, accuracy, and strategic purpose. Simultaneously, our scientific understanding of Earth’s climate system, atmospheric chemistry, and the intricate web of global interconnectedness has matured beyond what was conceivable in the 1980s. These shifts render the original “nuclear winter” calculations both foundational and obsolete, creating an urgent need for a modern reassessment of the threat. It is into this context that the National Academies of Sciences, Engineering, and Medicine (NASEM) has released its 2025 report, Potential Environmental Effects of Nuclear War. Commissioned by the U.S. Congress, this exhaustive study is not merely an update but a necessary re-evaluation of an existential threat for a new era.  

The NASEM report’s greatest structural strength lies in its central organizing principle: the “causal pathway”. This rigorous framework provides a logical and comprehensive methodology for analyzing an immensely complex problem, tracing the cascading sequence of events from the initial detonation of nuclear weapons, through the dynamics of the resulting fires and their emissions, to the transport of smoke into the atmosphere, the subsequent disruption of the physical climate system, the collapse of global ecosystems, and finally, the devastating societal and economic breakdown. By following this chain of causality, the report moves beyond a singular focus on climate cooling to examine a host of interconnected, compounding disasters.  

This critical review will argue that while the 2025 NASEM report provides the most comprehensive and scientifically sober assessment of nuclear war’s environmental effects to date, its primary contribution lies not in providing definitive answers, but in rigorously mapping the profound, compounding uncertainties at every step of the causal chain. It successfully shifts the debate from a singular focus on “nuclear winter” to a more nuanced understanding of cascading systemic failures, ultimately arguing that the greatest risks may lie in the poorly understood dynamics of fire, food, and societal fragility. Following the report’s own causal pathway, this review will deconstruct its findings at each stage, from the reimagining of nuclear scenarios to the final, grim analysis of human consequence, to evaluate its ultimate contribution to the public and policy discourse on preventing the unthinkable.

Reimagining Armageddon – Scenarios for a Modern Nuclear Exchange

The specter of nuclear war has long been defined by the image of a massive, globe-spanning exchange between superpowers. The foundational studies of the 1980s were predicated on this vision, modeling scenarios that reflected the colossal arsenals of the United States and the Soviet Union at the peak of the Cold War. The 1983 TTAPS study’s baseline case involved a 5,000-megaton (Mt) exchange, while the 1985 NRC report analyzed a 6,500 Mt war, involving tens of thousands of warheads. The NASEM 2025 report makes its first significant contribution by decisively moving beyond these historical precedents, presenting a suite of scenarios grounded in the geopolitical and technological realities of the 21st century.  

The report puts forth four plausible baseline scenarios designed to facilitate sensitivity analyses in future modeling efforts. These scenarios are organized by scale and reflect a world where the nuclear club has expanded and strategic doctrines have evolved :  

1. Large-Scale Strategic Exchange: A conflict involving 2,000 warheads, primarily between major nuclear powers like the United States and Russia. This number, while devastating, is an order of magnitude smaller than the scenarios of the 1980s, reflecting the significant arsenal reductions since the end of the Cold War.  
2. Moderate-Scale Strategic Exchange: A conflict involving 400 warheads, envisioned as a potential exchange between the United States and a rapidly modernizing power such as China.  
3. Small-Scale Regional Exchange: A conflict involving 150 warheads, based on the widely studied India-Pakistan scenario. This case, using less than 1% of the global arsenal’s explosive power, has become a focal point of modern research.  
4. Very Small-Scale Use: The detonation of a single warhead, perhaps as a desperate act of political signaling or terrorism.  

This recalibration of scenarios is a crucial step toward relevance. As the report notes, global nuclear stockpiles have fallen by approximately 90% from their Cold War peak, and advances in missile guidance have allowed for substantial reductions in the warhead yields required to achieve military objectives. The inclusion of a regional India-Pakistan scenario as a central case study, building on the work of researchers like Alan Robock and Owen Toon, signifies a fundamental shift in threat perception. The potential for global climatic catastrophe is no longer the exclusive domain of superpowers. The NASEM report implicitly demonstrates that a conflict involving a fraction of the global arsenal could trigger global famine, fundamentally altering the strategic calculus and showing that even “minor” nuclear powers now hold the potential for global-scale disruption. This represents a democratization of existential risk, a chilling reality of the modern nuclear age.  

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Table 1: Evolution of Nuclear War Scenarios. This table contrasts the large-scale Cold War scenarios with the more varied and generally smaller scenarios used in modern research, highlighting the shift in focus toward regional conflicts and their potential for global impact. (Data from )  

Beneath these scenarios lie critical assumptions about military strategy. The report adopts a “50% usage factor,” positing that an adversary is unlikely to employ its entire arsenal at once, holding some weapons in reserve or losing some to maintenance or preemptive strikes. This is a more plausible assumption than 100% employment and is consistent with past NRC studies, though it remains a significant simplification of the complex, unpredictable dynamics of nuclear escalation. Similarly, the report’s assumption of a mix of urban and ex-urban military (“counterforce”) targets reflects modern strategic thinking, which has moved away from the purely punitive “countervalue” city-targeting of some earlier models.  

However, the report also identifies a fundamental weakness in the very foundation of these calculations: the models used to predict the immediate effects of a nuclear blast. Much of the analysis still relies on the empirical formulas laid out in the 1977 publication The Effects of Nuclear Weapons by Glasstone and Dolan. The NASEM committee finds that these models, derived from atmospheric tests over flat terrain, are highly idealized and likely overpredict the ground range of thermal and blast effects in the complex three-dimensional environments of modern cities, where buildings can create “shadows” that block thermal radiation. This uncertainty about the initial area of ignition and destruction is a critical source of error that propagates through every subsequent stage of the analysis. The report’s recommendation to develop high-fidelity, three-dimensional models is therefore not a minor technical suggestion, but a call to rebuild the analytical foundation upon which all other predictions rest.  

The Urban Inferno – Deconstructing the “Source Term”

The entire cascade of environmental effects detailed in the NASEM report is overwhelmingly driven by a single phenomenon: smoke from massive, uncontrolled fires. The direct effects of a nuclear detonation, while locally apocalyptic, are secondary in the global context. It is the soot-laden plumes from burning cities and industries that have the potential to alter the planet’s climate. Consequently, the “source term”—the initial quantity, composition, and altitude of this smoke—stands as the most critical and, troublingly, the most uncertain variable in the entire causal pathway. The NASEM report’s meticulous deconstruction of this uncertainty is one of its most vital contributions.  

The calculation of the source term depends on a chain of variables, each with its own significant error bars. The process, adapted from wildfire emissions modeling, can be expressed as a simple product: the total mass of a pollutant emitted is the area burned, multiplied by the fuel load per unit area, multiplied by the fraction of that fuel that actually burns (combustion completeness), multiplied by an emission factor for the specific pollutant. The report’s analysis reveals deep uncertainties at each link in this chain.  

First is the problem of urban fuel. While the fuel loads of wildlands are relatively well-understood, modern cities are a complex, heterogeneous mix of combustible materials for which systematic inventories are largely absent. The report reviews various methods for estimating urban fuel loads, from land-use classifications to population-density proxies, but finds them all lacking in precision. A city is not a uniform forest; it is a landscape of concrete canyons, asphalt plains, wooden structures, plastic-filled interiors, and vast stores of petrochemicals. The report rightly concludes that historical urban fires, such as those from World War II, are poor analogs for the fires that would consume modern cities, with their vastly different building materials and urban densities. This lack of a robust, validated methodology for mapping urban fuel loads is a foundational weakness in all current models.  

Second is the pivotal question of what kind of smoke is produced. The climatic impact of a smoke plume is determined not just by its mass, but by its optical properties. The key variable is the ratio of light-absorbing particulate black carbon (BC), or soot, to more reflective particulate organic carbon (OC). A plume rich in BC will absorb more solar radiation, heat the surrounding air, and “self-loft” to higher altitudes in the stratosphere, where it can persist for years. A plume dominated by OC will have a lesser climatic effect. The NASEM report highlights a major point of scientific contention: many past nuclear war studies have assumed BC emission factors that are significantly higher than those measured in real-world wildland or even structural fires. For example, some models have used a BC emission factor of 20 grams per kilogram of fuel burned (g/kg), whereas observed values from structure fires are closer to 8 g/kg, and wildland fires are around 0.5 g/kg. This discrepancy is enormous and has profound implications for the severity and duration of any potential climate disruption.  

The entire thesis of a global climatic catastrophe following a nuclear war rests on this fragile, largely unverified foundation of the urban fire “source term.” The NASEM report, while not resolving these uncertainties, performs a critical service by framing them as the single most important area for future research. The severity of a “nuclear autumn” or “winter” is a direct function of the amount and type of smoke reaching the stratosphere. By meticulously detailing the cascading uncertainties in calculating this smoke—from what burns, to how much of it burns, to what kind of smoke it produces—the report reveals that the entire causal chain is precariously balanced on this initial, deeply uncertain estimate. The report’s call to fund an “ecosystem of modeling of urban fuels and buildings” is therefore not a minor technical point, but a foundational challenge to the entire field of study, demanding a far more rigorous empirical basis for its catastrophic predictions.  

The Blackened Sky – From Fire Plume to Stratospheric Veil

For smoke from nuclear-ignited fires to have a lasting, global impact, it must cross a critical atmospheric boundary: the tropopause. The troposphere, the lowest layer of the atmosphere where weather occurs, is characterized by vertical mixing and, crucially, precipitation. Aerosols injected here are typically “washed out” by rain within days to weeks. The stratosphere, the stable, dry layer above, is a different world. With no rain and slow vertical mixing, particles that reach this altitude can persist for years, circling the globe and profoundly altering the planet’s energy balance. A central question for decades has been whether the fires from a nuclear war could realistically inject a significant mass of smoke into this long-lived atmospheric reservoir.  

The NASEM report provides a compelling affirmative answer, drawing on powerful real-world evidence from massive wildfires. A major criticism of the original nuclear winter theory was the speculative nature of the smoke injection mechanism. Today, that mechanism is no longer theoretical. The report details observations of pyrocumulonimbus (pyroCb) events—fire-driven thunderstorms—that act like atmospheric elevators, lofting smoke directly into the lower stratosphere. The 2019-2020 Australian wildfires serve as a powerful case study. This single fire event injected an estimated 1.1 teragrams (Tg) of smoke into the stratosphere, an amount comparable to a moderate volcanic eruption, providing an “empirical test case” that validates the fundamental physics of pyro-convection.  

Furthermore, modern research has confirmed a secondary mechanism that extends the smoke’s atmospheric lifetime: self-lofting. As the dark, black carbon particles in the smoke plume absorb solar radiation, they heat the surrounding air, causing the entire plume to become buoyant and rise further into the stratosphere. Observations from the Australian wildfires showed smoke self-lofting from an initial injection height of 14 km to over 30 km in a matter of months. This process, which was not adequately represented in early models, means that smoke can reach higher, more stable altitudes and persist for much longer than previously thought, prolonging its climatic effects.  

The report reviews the current generation of atmospheric models, such as the Weather Research and Forecasting (WRF) model coupled with fire modules (WRF-Fire), which are beginning to simulate these complex dynamics. These models show that plume height is highly sensitive to a range of factors: it increases with the intensity and area of the fire and with ambient atmospheric moisture (which releases latent heat as it condenses in the plume), but it decreases with strong ambient winds that can shear the plume apart. The committee rightly recommends moving away from the simplistic approach of assuming an injection height and toward fully coupled atmosphere-fire models that can simulate plume rise dynamically.  

The real-world observations of massive wildfires have thus provided powerful, albeit imperfect, empirical validation for the core physical mechanisms of stratospheric smoke injection. This lends a new and sobering credibility to a theory once criticized as purely speculative. While the precise scale and composition of smoke from nuclear fires remain deeply uncertain, the mechanism for its long-term atmospheric impact is no longer just a theory; it is an observed and increasingly well-understood phenomenon.

A Planet in Shock – Modeling the Physical Climate Response

Once a veil of smoke is established in the stratosphere, it triggers a cascade of disruptions across the physical Earth system. The NASEM report reviews the state-of-the-art modeling of these impacts, revealing a threat profile that is both more complex and more severe than the simple cooling envisioned in the 1980s. The physics of radiative transfer is well understood: the smoke particles, particularly black carbon, absorb and scatter incoming solar radiation, dramatically reducing the amount of sunlight reaching the Earth’s surface. This leads to rapid and profound surface cooling, especially over continental interiors, which lack the thermal inertia of the oceans. The report notes that the relationship is not perfectly linear; at very high smoke concentrations, the cooling effect can become saturated as the plume becomes essentially opaque. This cooling is accompanied by a major disruption of the hydrologic cycle, as a cooler surface leads to less evaporation and a more stable atmosphere, causing a significant global reduction in precipitation.  

However, the most significant modern finding reviewed in the report is the potential for massive destruction of the stratospheric ozone layer. This introduces a cruel paradox to the nuclear war scenario. The original “nuclear winter” narrative focused exclusively on the effects of cold and dark. The NASEM report, synthesizing recent modeling by researchers like Charles Bardeen, details a devastating second-order effect that would emerge as the smoke begins to clear. The mechanisms are twofold. First, the immense heating of the stratosphere caused by the sun-absorbing smoke particles—warming of more than 50°C is projected in some scenarios—dramatically accelerates the chemical reactions that destroy ozone. Second, the smoke particles themselves provide vast surface areas for heterogeneous chemical reactions that convert stable chlorine compounds into ozone-destroying radicals, a process validated by observations of ozone depletion following the Australian wildfires.  

The projected consequences are staggering. A large-scale exchange could lead to a global average ozone loss of 75%, while even a regional conflict could cause a 25% loss, with these effects persisting for over a decade. As the smoke that initially blocked sunlight finally dissipates after several years, the severely damaged ozone layer would allow harmful ultraviolet-B (UV-B) radiation to flood the Earth’s surface. This creates a dual threat: an initial phase of cold and dark (“winter”) followed by a secondary phase of intense, DNA-damaging UV radiation (“UV summer”), compounding the environmental stress on any surviving ecosystems. This dual-shock scenario represents a far more complex and potentially more lethal threat than cooling alone.  

The report also examines the slower, longer-term responses of the planet’s systems with the most inertia: the oceans and the cryosphere. The surface cooling would disrupt the ocean’s large-scale circulation patterns, such as the thermohaline circulation, with effects that could last for decades or longer. The cooling would also lead to a significant expansion of sea ice and terrestrial snow cover. This creates a powerful positive feedback loop: the brighter, icier surface reflects more sunlight back to space, further amplifying and prolonging the initial cooling, a phenomenon known as albedo feedback. While current models do not show this feedback triggering a runaway, permanent ice age for the scenarios considered, it demonstrates how the climatic disruption could persist long after the last smoke particle has fallen from the sky.  

The Silent Spring – Cascading Ecological Collapse

The profound disruptions to the physical climate system would inevitably cascade into the biosphere, triggering an ecological collapse on a global scale. The NASEM report systematically reviews the potential impacts on terrestrial, marine, and freshwater ecosystems, painting a grim picture of a planet struggling to support life.  

The core of the ecological threat is the failure of photosynthesis, the process that underpins nearly all life on Earth. The combination of drastically reduced sunlight (specifically, photosynthetically active radiation, or PAR), plummeting temperatures, and altered precipitation patterns would lead to a massive global decline in Net Primary Production (NPP)—the rate at which plants convert solar energy into biomass.  

On land, the most immediate and devastating consequence for humanity would be the collapse of agriculture. The report reviews modeling studies projecting catastrophic declines in the yields of major staple crops like maize, wheat, rice, and soy. Growing seasons would be drastically shortened or eliminated entirely across the mid-latitudes of the Northern Hemisphere, where most of the world’s food is produced. The combined effects of killing frosts, darkness, and drought would lead to widespread crop failure for multiple years, triggering a global famine of unprecedented scale.  

The oceans, which might seem insulated from terrestrial fires, would face a similar catastrophe. The reduction in sunlight reaching the ocean surface would cause a collapse in the productivity of phytoplankton, the microscopic marine plants that form the base of the entire marine food web. This would ripple upwards, leading to a massive decline in global fisheries—a critical source of protein for billions of people. The report reviews modeling that suggests a potential restructuring of phytoplankton communities, but the overarching conclusion is a severe reduction in the ocean’s capacity to produce food.  

A critical caveat, honestly acknowledged by the NASEM committee, pervades all these biological projections. Our current ecosystem and agricultural models, such as the Agricultural Model Intercomparison and Improvement Project (AgMIP), were developed and calibrated to simulate the biological responses to a world of gradual warming, not one of abrupt, severe, and prolonged cooling. The report explicitly states that “nuclear war would cause sudden cooling with little opportunity for organisms and society to adapt, making it difficult to accurately predict ecological responses by simply inverting warming studies”. This reveals a profound “epistemic mismatch” in our scientific tools. We are attempting to forecast the consequences of an event for which our primary predictive instruments are not designed. The models cannot fully capture the physiological shock to plants and animals that have no evolutionary adaptation to such a rapid and extreme environmental shift. This crucial admission means that the projections of crop failure and ecosystem collapse, as catastrophic as they appear, should be viewed as preliminary estimates from models operating far outside their validated domain. The true biological devastation could be even worse than they predict.  

The Human Consequence – Famine, Fragility, and Societal Breakdown

The final links in the causal chain are the societal and economic consequences, where the environmental devastation translates into human catastrophe. It is here that the NASEM report makes arguably its most important contribution to the modern discourse, reframing the ultimate threat of nuclear war away from the direct casualties of the blasts or even the climate change itself, and toward the catastrophic failure of the complex, fragile, and globally interconnected systems that underpin modern human civilization.  

The primary mechanism for global human catastrophe identified in the report is not freezing or darkness, but famine. Synthesizing recent landmark studies, the report details how the projected collapse of agriculture and fisheries would lead to a calamitous shortfall in global caloric production, potentially lasting for more than a decade. The work of researchers like Jägermeyr, Xia, and Robock, reviewed in the report, suggests that even a “limited” regional nuclear exchange between India and Pakistan could slash global average caloric production by an amount that would trigger a global food crisis, placing billions of people at risk of starvation.  

Crucially, the report emphasizes that globalization and our tightly integrated global food system act as powerful amplifiers of this risk. The concept of “societal teleconnections” is used to explain how a localized shock can have global consequences. A nuclear conflict does not need to cause global climate change to cause global famine. A war that disrupts agricultural production in a few key breadbasket nations—such as the United States, Russia, or Ukraine—could trigger a collapse in the global food trade. This would lead to skyrocketing prices and severe shortages in import-dependent nations, particularly in Africa and the Middle East, even if their local climates remained physically untouched. This insight reveals that one of the greatest vulnerabilities of modern civilization is not just its dependence on a stable climate, but its dependence on the uninterrupted flow of goods across the planet. A regional nuclear war could sever these flows, turning a climate crisis into a global systemic collapse.  

Beyond famine, the report details a host of secondary human impacts. The breakdown of public health systems, overwhelmed by blast and burn injuries and unable to cope with subsequent outbreaks of infectious diseases fueled by malnutrition and mass displacement, would lead to millions of additional deaths. The psychological trauma on survivors would be immense and intergenerational. The report anticipates mass displacement on a scale never before seen, severely straining humanitarian resources and potentially leading to widespread social and political instability. Critical infrastructure—power grids, communication networks, transportation systems—would likely suffer cascading failures, accelerating the descent into chaos. While the report discusses the concept of community resilience, it acknowledges that modeling human behavior, governance, and social cohesion under such extreme and prolonged stress remains a frontier of research, fraught with uncertainty.  

Conclusion: A Commendable Blueprint with Critical Caveats

The 2025 NASEM report, Potential Environmental Effects of Nuclear War, stands as a monumental and necessary contribution to our understanding of the gravest threat facing humanity. The committee has produced a methodologically rigorous, comprehensive, and intellectually honest assessment that successfully navigates a deeply politicized and scientifically uncertain field. Its disciplined adherence to the “causal pathway” framework provides a clear and logical structure for dissecting an overwhelmingly complex problem, while its transparent acknowledgment of uncertainty at every step is its greatest scientific strength. The report successfully updates the “nuclear winter” narrative for the 21st century, shifting the focus to more plausible regional scenarios and highlighting the systemic fragilities of our globalized world as a primary vector of catastrophe.  

However, as this review has detailed, the report’s value lies as much in the questions it raises as in the answers it provides. Three critical caveats, or gaps, emerge from its analysis that must inform any future research and policy discussions:

1. The Foundational Uncertainty of the “Source Term”: The entire edifice of climatic and ecological prediction rests on the initial estimate of smoke produced by burning cities. As the report makes clear, our knowledge of urban fuel loads and the specific emission factors for black and organic carbon under these unique fire conditions is profoundly limited. This remains the single largest source of uncertainty in the entire causal chain.
2. The “Epistemic Mismatch” of Biological Models: Our best tools for predicting agricultural and ecosystem responses are calibrated for a world of gradual warming. Their application to a scenario of abrupt, severe cooling is a necessary but deeply flawed extrapolation. The true biological consequences of such a shock may be far outside the predictive capacity of our current models.
3. The Unknowable Human Factor: The report rightly identifies societal response and resilience as critical variables. Yet, modeling human behavior, governance, and the stability of global markets under the unprecedented stress of a global famine remains largely speculative. The potential for hoarding, conflict, and systemic breakdown could amplify the initial environmental shock in ways we can barely imagine.

In the face of these profound uncertainties, the report’s overarching recommendation is both logical and essential: the coordinated development of a suite of Model Intercomparison Projects (MIPs). For decades, mainstream climate science has relied on MIPs to systematically compare models, standardize assumptions, reduce uncertainty, and build confidence in its projections. The field of nuclear war effects research has historically lacked this coordinated framework. Implementing a series of MIPs, as outlined by the NASEM committee, is the only rational path forward to address the critical gaps identified in this report.  

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Table 2: Summary of Key Uncertainties and Research Priorities. This table synthesizes the NASEM report’s core findings on knowledge gaps at each stage of the causal pathway and aligns them with the highest-level research priorities needed to reduce uncertainty. (Data synthesized from )  

Ultimately, the 2025 NASEM report’s final message is not one of apocalyptic certainty, but of profound, unquantifiable, and unacceptable risk. It provides a sober and scientifically grounded warning that even a “limited” nuclear war could trigger a global environmental and societal catastrophe through pathways that are now better understood than ever before. The prevention of such a conflict remains, as it was in the 1980s, the single most essential imperative for human survival.

Note: This review has been prepared with the aid of Gemini AI Pro with Kevin Parker’s input and oversight.