Potential Environmental Effects of Nuclear War (2025)
Chapter: 8 Conclusion
8
Conclusion
This report presents a framework through which the potential environmental effects of nuclear war can be evaluated. Chapter 2 presents four nuclear war employment scenarios ranging from a single detonation to larger exchanges of nuclear weapons and the expected physical effects from the denotations. Chapter 3 discusses the potential fuel loading and fire dynamics and means by which emissions are determined. Chapter 4 addresses plume rise and fate and transport of aerosols and gas-phase chemistry, particularly in the stratosphere. Chapter 5 describes potential impacts to physical earth systems with a lens of response time to particulate loading in the stratosphere. Chapter 6 uses these climate perturbations to assess effects on ecosystems. Finally, Chapter 7 highlights pathways through which the effects on physical earth systems and ecosystems affect society, economics, and community resilience.
Whereas our knowledge of and ability to simulate climate system dynamics is relatively robust, we have substantially less process understanding and modeling capabilities for other segments of the causal pathway explored in this report—namely, nuclear weapons employment scenarios, fire risk and behavior, ecosystem dynamics, and societal and economic pathways. Nuclear detonation scenarios and societal and economic impacts, in particular, are dominated by complex human behavioral responses, with the paradox being that the largest needs for policy development, prevention, and adaptation reside in the subject matter with largest uncertainty. Uncertainties are also compounded across the entire causal pathway described in this report, and, combined with the lack of data, process understanding, and validated modeling capabilities fundamentally constrain the quantification of precise environmental and societal and economic outcomes from any given nuclear war scenario.
The size and extent of the set of recommendations presented in this report is daunting, as are the uncertainties involved in determining which of these recommendations are most pressing or consequential to improving the modeling and prediction of the effects of a nuclear conflict. As is described in earlier
chapters, however, there are clear segments along the causal pathway in which uncertainties are relatively greater, namely in sections related to fire and emissions, where there are many specific questions related to the physical processes of fire and emissions, and in sections related to ecosystems services and socioeconomic effects, where human behavior and decision-making come into play. Moreover, as is also noted in earlier chapters, overall uncertainty accumulates and propagates over the entire causal pathway. These observations may collectively offer insight into where research priorities may lie—however, given the breadth of the disciplines and federal agencies involved in this topic, this report does not offer an explicit scheme for prioritizing or sequencing its recommended actions.
8.1 OVERARCHING CONSIDERATIONS
The world we live in has changed drastically since the original studies that described the concept of nuclear winter in the 1980s. Stockpile sizes, yield, accuracy, military strategy, and the geopolitical actors have evolved. The plausible nuclear employment scenarios of today are far smaller in total warheads detonated and their yields than they were in the earlier studies. The dynamics of society have also changed. We live in an era of globalization and interconnectivity, with complex supply chains and dependencies spanning the continents. The average surface temperature of Earth has increased by approximately 1ºC since 1980 (Lindsey and Dahlman, 2024). Over the same period, the population of the world has doubled with increased urbanization (Ritchie et al., 2023). Construction materials have changed, with different types and standards, and different building content.
Our scientific understanding of natural and human systems1 has significantly advanced as well. The understanding of stratospheric ozone chemistry in the presence of smoke has evolved to address organics present rather than soot only. Emissions produced by fires are better characterized and quantified, as are their impacts (e.g., tropospheric and stratospheric chemistry, physical and optical processes, atmospheric impacts and downwind impacts). There have been advancements in process-level and whole-systems research (observation and modeling) and integrated systems approaches (e.g., multisectoral dynamics). Analyses evaluating the potential impact on stratospheric ozone and photosynthetically active radiation (PAR) resulting from a nuclear detonation are novel and the regional and global societal and economic impacts thereof are not well understood. Our efforts to understand the response of ecosystems to changes in climate have received enormous attention from the scientific community driven by the recognition to understand the immediate effects of global climate change.
Put together, these differences warrant a comprehensive review of the potential environmental effects of nuclear war with rigorous validation and intercomparison of multiple models and robust exploration of the assumptions, for which the committee identified the following overarching Finding and Recommendation.
FINDING 8-1: There is insufficient modeling effort, organization of modeling (based on choice of employment scenario; wargaming), and comprehensive testing of models, and a need to acknowledge:
- the importance of integrating teleconnected impacts and risks;
- the importance of integrating geographic context, the key underlying conditions, and structural vulnerabilities that interact with the environmental effects;
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1 In the context of this report, a “system” refers to a set of interacting elements that can be physical, biological, social, or economic in nature, and which function together as a larger whole. A key attribute of the various systems described in this report (such as the climate system or an ecosystem or societal system), is the deep interconnections and interdependencies that exist between their sub-components, as well as between individual systems. The complex and unpredictable behavior of natural and human systems arises from dynamic interactions and feedbacks, occurring across scales, that can create cascading effects and unforeseen outcomes in response to an initial shock. A “systems approach” to understanding the environmental and societal and economic effects of nuclear war thus requires us to consider the interconnectedness of the social, economic, political, and environmental factors that create the potential for harm to ecosystems, people, and society.
- the importance of a multisectoral dynamics approach, and our ability to model amplification or amelioration across systems and sectors, ability to model shocks, and nonlinearities; and
- the importance of understanding human behavior and decision making and human response as a determinant of risk.
Early global climate modeling documented substantial differences among different climate models, motivating in-depth model intercomparison projects (MIPs)2 to accurately characterize uncertainties, both structural (model design and components) and statistical. MIPs are motivated by the critical role that robust and well-quantified information plays for decision making by the public and private sectors as they confront climate change. Given the importance of nuclear war to security policy, it is important that a series of MIPs be similarly applied, to ensure an appropriate level of confidence and robustness in the underlying science.
RECOMMENDATION 8-1: Relevant U.S. agencies should coordinate the development of and support for a suite of model intercomparison projects (MIPs) to organize and assess models to reduce uncertainties in projections of the climatic and environment effects of nuclear war. This effort should employ stress testing where appropriate (human factors) and parametric variations, support data availability and integration, and use an enterprise approach with a system of systems.
The current framing of MIPs is summarized in Table 8-1, which is categorized by the process/component, input, modeling needs, modeling community, and outcome, mapping back to specific recommendations from the report. The table is framed around a Coupled Model Intercomparison Project (CMIP)-type MIP, starting (from top to bottom) with rows describing work (or MIPs) that may inform the CMIP, then what the CMIP would investigate, and then what other work (or MIPs) that would be informed by the CMIP.
Because of the vast array of systems interactions involved and the complexity and uncertainty of each system and their interactions and responses to others, the suggested map of MIPs will be most effective if each suggested MIP is able to identify crucial characteristics of its components and the lower and upper bounds of their outputs, to inform a suite of prescribed forcings for the next MIP that is representative of the parameter space needed to be explored. By determining a suite of prescribed inputs for the next MIP to constrain the forcings for different component models, the suggested work can prevent propagation of uncertainties across systems to isolate the sources of uncertainty within the next system and focus on exploring its response to the previous system in a robust way.
8.2 EMPLOYMENT SCENARIOS AND WEAPONS EFFECTS
Chapter 2 constructs four plausible scenarios generated from a military use strategy and various exchanges that could occur based on current geopolitical climates. The intent of this approach is to give the research community a range of reasonably possible scenarios from which they can model environmental effects of nuclear war, acknowledging further need to identify an accepted range of bounding scenarios that can be used and relied on.
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2 CMIPs have been repeatedly carried out as models have become more complete (e.g., including coupled water cycle and ecosystems) and as scenarios for future greenhouse gas emissions have evolved. While generally applied to models, MIPs also have been applied to observational datasets. A catalog of MIPs carried out or underway (available at https://www.clivar.org/clivar-panels/former-panels/aamp/resources/mips) describes over 30 different activities, including for example, an El Nino–Southern Oscillation (ENSO) intercomparison project, an ocean-carbon cycle model intercomparison project, a sea-ice model intercomparison project, etc. MIPs ensure open scrutiny of how different models perform and differences in observations, building trust and credibility for climate science.
TABLE 8-1 Environmental Effects of Nuclear War Model Intercomparison Project
| Recommendation | Process | Input | Research Needs | Research community | Outcome |
|---|---|---|---|---|---|
| 2-1, 2-2* | Employment scenarios |
|
Wargaming and tabletop exercises, perspectives from military experts and policymakers | Military/intelligence | Usable set of plausible scenarios for modeling inputs (Inform CMIP) |
| 2-3 | Weapon effects | Weapon employment as specified in scenarios | Development and validation of 3-D models of nuclear weapon effects | Weapon effects modelers, emergency response planning | Understand interaction of weapon effects (thermal and blast) with emplacement and environmental conditions for improved fire ignition modeling (Inform CMIP) |
| 3-1* | Fuel loading | Mix and range of target area type | Data and focused modeling studies | Fire science and engineering and wildland fire communities | Increased accuracy in predictions of fire ignition, spread, and emissions (Inform CMIP) |
| 3-2, 4-1 | Ignition, spread | Consider potential environmental and physical factors that amplify or dampen | Improved understanding of physics and chemistry of fire ignition and spread; development of fire models for urban environments | Fire science and engineering and coupled atmosphere–fire modeling communities | Increased understanding of fire fluxes, spread, and chemistry (Inform CMIP) |
| 3-3*, 3-4*, 3-5 | Emissions |
|
Better understanding and quantification of composition of emissions from urban and suburban fires | Fire inventory and fire modeling communities | More accurate emissions (e.g., BC/OC ratio), including gaseous chemistry (Inform CMIP) |
| 4-1*, 4-2, 4-3 | Plume rise | Fuel loading, emissions factors; realistic atmospheric conditions, atmospheric physics and chemistry | Coupled atmosphere–fire ignition and spread models, including both fire and atmospheric chemistry and cloud–aerosol interactions; validation with observational data | Coupled atmosphere–fire modeling community | Improved quantification of heterogeneous fire fluxes and understanding of resulting plume height, interactions with the ambient atmosphere, aerosol properties within the plume, and in-plume removal mechanisms (such as wet scavenging) (Inform CMIP) |
| 4-4, 4-5 | Atmospheric chemistry | Emissions, physicochemical characteristics of plumes | Fully coupled models of the troposphere and stratosphere with full chemistry and radiative transfer | Atmospheric chemistry community | Impacts on tropospheric chemistry due to tropospheric emissions and stratospheric changes |
| 4-6 | Climate model sensitivity to plume height | Injection height, smoke quantity and composition | Sensitivity studies of climate models to injection height, smoke quantity and composition | Earth system modelers | Improved understanding of climate sensitivity to plume height, smoke quantity and composition (Inform CMIP) |
| Recommendation | Process | Input | Research Needs | Research community | Outcome |
|---|---|---|---|---|---|
| 5-1* | Aerosol properties | Estimates and range from upstream models leading to aerosol forcing | Prescribe aerosol properties using new observations, and sensitivity studies of the Earth system to these properties | Plausible range of aerosol optical, physical, and microphysical properties for a given scenario (Inform CMIP) | |
| 5-2 | Cloud–aerosol interactions | Realistic aerosol quantities and properties | More simulations with cloud-resolving models with large aerosol loadings | ESM development | Better understanding of cloud–aerosol microphysical interactions following large aerosol loadings (Inform CMIP) |
| 5-2 B | Global cooling | Input from upstream models | Global climate models that can handle abrupt change | ESM development | Improve understanding of cooling intensity and duration across regions and seasons |
| 5-3 | Hydrologic cycle | Suite of scenarios to explore Earth system response to prescribed aerosol forcing varying in amount, injection height, composition, and seasonal timing and latitude of injection | Sensitivity studies; scenarios of water management in controlled waterways | A new ESM intercomparison for nuclear war scenarios similar to CMIP | Better understanding of possible changes in precipitation (location, timing, intensity, duration) |
| 5-4 | Ocean response | Sensitivity studies; range of representation of the ocean-impact drivers (acidity, thermal structure, currents, oxygen) | A new ESM intercomparison for nuclear war scenarios similar to CMIP | Better understanding of ocean state changes | |
| 5-5* | Stratospheric ozone | Systematic modeling as a function of model structure, resolution, treatment of aerosols, and forcing scenario | A new ESM intercomparison for nuclear war scenarios similar to CMIP | How compounds influence atmospheric circulation, chemistry, and ozone depletion rate | |
| 6-1 | Radiation effects on climate | Direct radiation effects and recovery (CMIP informed) | Interactive effects of aerosol on UV, PAR, and diffuse vs direct radiation | AgMIP, ISIMIP | Better understanding of ecosystem impacts as a result of radiation response to realistic scenarios |
| 6-2*, 6-5, 7-3 | Environmental conditions from sudden cooling (CMIP informed) | Sensitivity studies of organisms to the resulting environmental shocks, coupled to models with key natural resource systems that provide food, materials, medicines, and energy | AgMIP, ISIMIP | Sensitivity of organisms, ecosystems, and ecosystem services to potential sudden environmental shocks | |
| 6-3* | Differential response | Suite of realistic scenarios (radiation and temperature changes and duration) (CMIP informed) | Sensitivity studies of different regions with respect to scale of scenario | Regional sensitivity and differential response to induced variation in environmental factors | |
| 6-4* | Differential limitation of precipitation and temperature | Regional sensitivity and differential response to induced variation in environmental factors | Sensitivity studies of different ecosystems to simultaneous change in precipitation and evapotranspiration | CMIP, AgMIP, ISIMIP | Sensitivity of different terrestrial ecosystems to simultaneous changes in precipitation and evapotranspiration |
| 7-3* | Ecosystem response | New experiments and models that unravel the effects of sudden environmental change | Use of climate models that can handle rapid radiation changes | ISIMIP | Impacts of sudden and lasting cooling on drylands and forests of the world |
| 7-3* | Crop response | Suite of climate, environmental, societal, and economic scenarios (CMIP, AgMIP Informed) | Understanding of impacts on crops from surface ozone, ultraviolet radiation and freshwater availability and improved projections of agricultural productivity changes for major staple crops in key producing regions due to changing climatic conditions and trade implications in the short and long terms. | AgMIP | Identification of regions and agricultural systems that are most vulnerable or resilient to nuclear war-induced climate effects, as well as opportunities to enhance resilience. |
| 7-1*, 7-2*, 7-3* | Food system resilience | Suite of climate, environmental, societal, and economic scenarios (CMIP, AgMIP Informed) | Evaluation adaptive capacities, technological limits, and vulnerabilities of regional agricultural systems (crops and livestock); analysis of changing requirements for land, water, energy, and financial resources; capture of complex connections and feedbacks between climate changes, pollution, and human systems and improvement in geographic data on global food stocks, agricultural inputs, production capacities, and supply chain infrastructures; identification of exacerbations of inequality and differential vulnerabilities. | IAMC, AgMIP, ISIMIP | Understanding how the system effects of food impacts can cascade through value chains and trade, impacting producers and consumers, and influencing competitive balances |
| 7-1*, 7-2*, 7-3*, 7-4, 7-5 | Community resilience | Suite of climate, environmental, societal, and economic scenarios (CMIP, AgMIP Informed) | Scenario modeling and economic analysis to guide policies that could mitigate food insecurity and ecological degradation if nuclear war occurs; advancing the assessment and periodic monitoring of social capital domains in at-risk | IAMC, AgMIP, ISIMIP | Solutions and opportunities to build and enhance resilience and reduce inequalities in vulnerability and health exposures and outcomes from nuclear war-induced climate responses, scaled to decisions and system exposure |
| Recommendation | Process | Input | Research Needs | Research community | Outcome |
|---|---|---|---|---|---|
| 7-1*, 7-2*, 7-3*, 7-4, 7-5 | communities; research on predictively modeling the integration of data characterizing the social capital domains and the pillars of community resilience in selected vulnerable communities; exploring dietary/nutritional effects of differential impacts on proteins, cereal crops, and nutrient-rich fruits and vegetables. |
NOTES: AgMIP (Agricultural Model Intercomparison and Improvement Project), BC (black carbon), CMIP (Coupled Model Intercomparison Project), ESM (Earth system model), IAMC (Integrated Assessment Modeling Community), ISIMIP (Inter-Sectoral Impact Model Intercomparison Project), OC (organic carbon), PAR (photosynthetically active radiation).
* Key Recommendations
For each scenario, the size of the exchange, and thus the number of weapons that could be deployed between two warring nations, acted as reasonable factors for the types of locations that could be impacted by potential nuclear war. Additionally, each scenario assumed that only 50% of a country’s stockpile would be deployed in any given scenario. This assumption was previously used in 1975 and 1985 studies looking at potential environmental effects of nuclear war and reasons that a country would consider geopolitical factors and reserve part of their stockpile for defense from other countries. A major difference between past studies and the current study relies on different weapon yields and more precisely targeted weapons.
Of these locations, 100% of scenarios are between certain degrees of latitude and all fall within the Northern Hemisphere. Even a one-off detonation event, as a demonstration, could still likely fall within the same latitude bounds as other scenarios. The scenarios have an equal probability of occurring at any point in the year and are not biased toward a particular season. Although past studies have found that there is little difference in the environmental effects based on seasonality, it should be noted that physics such as wet removal or fire ignition were not considered.
In general, nuclear exchanges are expected to occur over days to weeks of duration, and there would likely be pauses following the initial employment of weapons. The overall target types and locations, being in an urban setting versus a non-urban setting, would likely skew toward non-urban targets in a military use strategy. The target type is critical in understanding source terms and modeling impacts. This would then cascade into potential local and global effects resulting from the nuclear exchange. In general, global impacts act as lower bounds for potential local impacts.
Key Finding and Recommendation
FINDING 2-2: Some extant studies of weapon exchange scenarios have assumed that up to 100% of an adversaries’ nuclear weapons stockpile would be employed against the other country. Given historical military behaviors, employment of 100% of a large stockpile is unlikely; therefore, an employment assumption of closer to 50% of an adversary’s stockpile is more plausible from a military strategy perspectives and is consistent with past National Research Council studies.
RECOMMENDATION 2-2: EENW researchers should use a representative understanding, from military experts and policymakers, of a reasonable range in percentages of each adversary’s nuclear weapons stockpile that could be used in different exchange scenarios.
8.3 FIRE DYNAMICS AND EMISSIONS
The fuels and their density in the ground range of a nuclear blast are critical to the prediction of the behavior and total energy of a fire, emissions, and eventually, their lofting into the atmosphere. Fuels located in urban and wildland systems range from commercial, industrial, and residential structures, roadways and vehicles and to vegetated areas, and from grasslands to forests. The composition and density of the fuels vary widely, and may be a function of population densities. Features including topography and weather, the fuels to be burned based on target area (urban, wildland–urban interface, wildlands, special sites), the amount of fuel consumed in the initial blast, rubblization, and resulting fires can determine the emissions and their injection into the atmosphere. Additionally, scenario-dependent fire dynamics in both ignition and suppression as a result of potential subsequent blasts add to the complexity and are not well characterized.
Accurately evaluating fuel loading and composition is key to the prediction of pollutant emissions, including aerosols and gases, which determine downstream potential environmental effects of nuclear war. While certain classes of fuels are better characterized (e.g., vegetation), pollutant emissions are dependent on the type (vegetation, urban, mix) and are highly uncertain.
Key Findings and Recommendations
FINDING 3-1: Fuel composition and loading—which are important in the estimation of fire spread and fuel consumption, the assignment of emission factors, and the prediction of the resulting plume height—of urban, suburban, and industrial areas are not well-characterized and vary significantly across the world. These also differ significantly from those of rural and wildland regions, for which more is known. Accurately characterizing fuel loading for a given nuclear exchange scenario is important in determining downstream environmental effects.
RECOMMENDATION 3-1: The National Nuclear Security Administration should coordinate with other federal agencies (e.g., the U.S. Departments of Agriculture, Defense, and Justice, the Federal Emergency Management Agency, the National Institute of Standards and Technology, and the National Science Foundation) to fund research by national laboratories and independent researchers with requisite capabilities to develop an ecosystem of modeling of urban fuels and buildings that would reduce uncertainties in the estimate of fire spread and emissions. These efforts should include:
- Integration of vegetation with the determination and mapping of urban and wildland–urban interface fuels in different regions;
- Improving available data and consistency of information on construction materials and building contents for improved accuracy of fuel load modeling;
- Validation and verification of fire load data and fire load modeling used in existing studies, model input sensitivity studies, models that account for uncertainties; and
- Clearly defined assumptions and modeling approaches for improved model intercomparison.
FINDING 3-4: The quantities of emitted black carbon and organic carbon, as well as optical and physical properties of emitted particulate matter, are important for determining plume behavior and potential downstream environmental effects from fires resulting from a nuclear detonation, but highly uncertain.
RECOMMENDATION 3-3: When estimating the emissions from a nuclear explosion and resulting fires, appropriate ranges of all emissions—including black carbon and organic carbon, trace gases, and other pollutants—should be determined and sensitivity analyses performed.
RECOMMENDATION 3-4: EENW researchers should improve on estimates of the quantity and physical and optical properties of black carbon and organic carbon including size upon emission to evaluate the environmental impacts of a nuclear explosion and resulting fires.
8.4 PLUME RISE, FATE AND TRANSPORT OF AEROSOLS, AND GAS-PHASE CHEMISTRY
Nuclear weapon scenarios with a greater number of weapons detonated in areas of high fuel loading may produce global and longer-lasting distributions of particulates in the stratosphere, resulting in larger-scale environmental impacts related to changes in Earth’s radiation balance. At larger yields, optical physics dictates a saturation effect, and at lower yields, there is a robust understanding of the particulate loading that will lower the temperature by less than a tenth of a degree. Nuclear detonation scenarios using fewer weapons, or detonations in regions of less fuel, may produce regional atmospheric perturbations with associated environmental and societal and economic impacts.
The residence time and pathways of lofted emission particulates that disrupt Earth’s radiative balance depend on the physicochemical properties of the aerosols, atmospheric chemistry, and atmospheric circulation. To evaluate these, one needs the heat flux at the surface and the moisture content within the plume that can contribute to latent heating effects. Fuel loading is also a major driver in the resulting plume height, material, and chemistry, and the uncertainties in fuel loading discussed in Chapter 3 will propagate. Additionally, variability in overlying atmospheric conditions means that it is difficult to predict whether a fire will develop such that it injects soot and smoke into the stratosphere.
Reasonable assumptions when modeling the particulate behavior can therefore result in vastly different outcomes for the same scenario, particularly given the complexity associated with other factors such as seasonality, latitude, weather, and topography.
Key Finding and Recommendation
FINDING 4-1: Accurate predictions of plume height are critical for understanding the environmental effects of smoke. While atmospheric models for predicting plume height are largely capable, the existing models of fire ignition and spread are often inadequate (see finding 3-3) for correctly modeling fire fluxes important for predicting plume height, especially in intense wildland or urban fires as may occur upon the detonation of a nuclear weapon.
RECOMMENDATION 4-1: EENW researchers should further develop fire ignition and spread models, coupled with atmospheric models capable of resolving plume rise, and should validate predictions of plume-rise and stabilization height, especially for the unique fuel loadings in nuclear detonation scenarios.
8.5 PHYSICAL EARTH SYSTEM IMPACTS
A nuclear weapons exchange will cause the Earth system to respond to potential perturbations, with various components and processes responding at different timescales and recovering at different rates. The particulate loading into the upper atmosphere, detailed in Chapter 4, will impact Earth’s radiative balance, largely by decreasing solar radiation nearly instantaneously. Global temperature will respond with sudden cooling linearly to these radiative changes, and these changes will be felt throughout the various Earth system components, with lagged and extended response times in the ocean system and cryosphere. The hydrologic cycle response to these radiative changes will include both a fast component directly driven by the radiative change, and a slow component responding to temperature changes. Hydrologic impacts will be felt at watershed scales, which have implications for water availability and human habitability. There will be an immediate need for access to clean water, and the potential need to rotate between water banks, such as snowbanks (short term) and aquifers and groundwater (long-term water storage), resulting from possible contamination.
Over a range of timescales, the particulate loading in the atmosphere will affect the ocean state and circulation directly through its two main physical drivers, wind and buoyancy, and indirectly through its impact to modes of internal climate variability. The cryospheric response will be driven by the radiative effects of the atmospheric particulate loading, as well as short-term albedo effects from deposition of soot on Earth’s surface. The injection of NO and smoke from nuclear blast and fires will deplete stratospheric ozone. Organic and brown carbon will play an important role in chemical and radiative processes, which may increase tropospheric radiative effects and result in the loss of ozone and other chemistry. The impact of organic and brown carbon on photosynthetically active radiation (PAR) and UV remains unclear.
Altogether, different physical Earth system components will respond to perturbations from a nuclear weapons exchange and subsequently return to equilibrium at different rates. Land–ocean coupling may prolong the land response to perturbations due to the high heat capacity and memory of the ocean.
These impacted systems will directly influence ecosystems through changes in important parameters including temperature, PAR, and UV, described in Chapter 6.
Key Findings and Recommendations
FINDING 5-1: Current Earth system models must highly oversimplify the aerosol microphysics governing optical properties of smoke, and more detailed process models require an amount of data that is generally unavailable except from brief, site-specific field campaigns or laboratory experiments and these are generally directed at wildfires rather than urban and wildland–urban interface fires, which are more likely to be of concern following a nuclear detonation.
RECOMMENDATION 5-1: The Earth system modeling community should develop “rule-of-thumb” estimates of the plausible range of aerosol optical, chemical, physical, and microphysical properties, as well as
- The mass of aerosol generated per unit mass of source material;
- The altitude of its injection into Earth’s atmosphere and subsequent lofting;
- Its extinction cross section;
- Its single-scattering albedo;
- Its size distribution; and
- Its black carbon and organic carbon content.
Also, Earth system models should be run in a “perturbed parameter ensemble” across this full range of uncertainties for each of these parameters to determine whether the findings regarding environmental effects are robust to these unavoidable uncertainties.
FINDING 5-6: Many of the published modeling studies describe the response of stratospheric ozone to the pure black carbon (BC) scenarios. While this could overestimate upper stratospheric ozone losses due to reduced heating, these studies lack treatments of organics and BC/organic carbon (OC) ratios, which would act to increase lower stratospheric ozone loss. Injection of nitric oxide has also been considered as a factor in ozone loss and should be revisited using a range of up-to-date scenarios that could map to nuclear exchange scenarios and models with full BC/OC chemistry.
RECOMMENDATION 5-5: The Earth system modeling community should study how compounds injected into the stratosphere and troposphere (i.e., water, carbon monoxide, nitrogen compounds, aerosols containing varying amounts of black carbon and organic carbon, etc.) influence atmospheric circulation, chemistry, and stratospheric ozone depletion rate, and coordinate systematic modeling studies of the ozone response to nuclear war as a function of model structure, resolution, treatment of aerosols, smoke lofting, and forcing scenario.
8.6 ECOSYSTEM IMPACTS
Current understanding of the potential ecosystem impacts of a nuclear war are informed by Earth system model simulations and by comparisons to past events such as massive volcanic eruptions. The available evidence indicates that the environmental effects of even regional-scale nuclear exchanges would cause substantial drops in temperatures and levels of PAR reaching ecosystems, while also allowing more harmful UV rays to penetrate due to ozone depletion in the upper atmosphere. The severity and duration of the environmental disruptions would scale with the size of the nuclear exchange.
Some existing studies of larger nuclear war scenarios point to rapid regional or even global cooling in the aftermath. Earth system models indicate that this cooling would be accompanied by
significant reductions in the sunlight that plants depend on for photosynthesis. Research also suggests that increased UV radiation levels could persist for years due to ozone losses. In the oceans, the cooling would likely increase vertical mixing, providing a temporary nutrient boost to surface waters. However, simulations also show surface ocean acidification developing over time.
For ecosystems on land, the overall impacts on plant productivity remain uncertain. Cooling could benefit some water-limited regions by reducing evaporative demand more than precipitation declines. But shortened growing seasons in cooler high-latitude forests may decrease productivity there. In freshwater systems such as lakes and rivers, severe stresses such as algal blooms, species losses, and disrupted ecosystem services could arise through complex chains of events initiated by ash deposition, hydrologic shifts, and water quality changes. Marine ecosystems could see regional productivity declines from light limitation, despite temporary blooms elsewhere due to enhanced nutrient supplies.
Biodiversity impacts carry high uncertainty but potentially include local extinctions, habitat range shifts, and even new colonization as agricultural areas are abandoned because they have become unsuitable for cultivation. The need to offset the loss in production at higher latitudes and the reduction in the global food supply may trigger deforestation in lower latitudes. The resulting enhanced deforestation rate may have enormous negative consequences for global biodiversity since lower-latitude forests host the largest concentrations of biodiversity in the world. In freshwater systems, biodiversity would be threatened by blast effects, fires, altered water flows, and contamination. Ocean modeling suggests potential for major restructuring of phytoplankton communities, with some evidence that larger, nutrient-hungry diatom species could outcompete smaller ones under the low-light but nutrient-rich conditions.
Major uncertainties persist around fully projecting ecosystem disruptions. Models calibrated on modern conditions may poorly represent responses to the severe, abrupt climate changes involved. Insufficient experimentation and observational data hamper the validation of models on important processes such as adaptation, food web unraveling, and synergistic effects of the concurrent environmental stressors.
These ecosystem impacts would directly undermine human health, food security, and societal stability, as detailed further in Chapter 7. Degradation of ecosystems and their services would be a pivotal determinant of human impacts across individuals, communities, and societies in the aftermath of nuclear wars.
Key Findings and Recommendations
FINDING 6-2: Most of our studies on climate-induced environmental change are focused on studying implications of slow (century-scale) warming and its consequences on environmental conditions. However, the modeling research groups are short of studies of sudden cooling, as could result from a nuclear exchange scenario, and the associated effects on ecosystems and their components.
RECOMMENDATION 6-2: EENW researchers should develop models that can capture key ecosystem responses to abrupt changes in environmental conditions including sudden decreases in temperature, photosynthetically active radiation, and acidity, and sudden increases in ultraviolet radiation. Key ecosystem processes in this context include net primary production and organismal mortality with the potential to affect community composition. This new generation of models should be able to capture the ecosystem responses to volcanic eruptions and rare climatic extremes including cooling scenarios.
FINDING 6-3: As the intensity and number of nuclear weapon detonations increase, the differential sensitivity across regions is expected to decrease. Similarly, there are differences in irreversibility of ecosystem characteristics after a nuclear event. The impacts on ocean ecosystems are relatively insensitive to the location of the conflict whereas terrestrial and aquatic responses are relatively more sensitive.
RECOMMENDATION 6-3: EENW researchers should focus on the differential response across regions based on their relative sensitivity to detonation-induced variation in environmental factors including water, temperature, precipitation, and solar radiation.
FINDING 6-4: Terrestrial ecosystems responses to nuclear-war climate change will be modulated by current geographical location. Higher-latitude and cold ecosystems will be negatively affected by the cooling and shortening of their growing season expected under nuclear-war climate. However, a large fraction of terrestrial ecosystems that are currently limited by soil-water availability may exhibit an increase in net primary production as a result of regional cooling.
RECOMMENDATION 6-4: EENW researchers should explore the sensitivity of different terrestrial ecosystems to simultaneous changes in precipitation and evapotranspiration, including their location in environmental space.
8.7 SOCIETAL AND ECONOMIC IMPACTS
Quantification of the societal and economic impacts from nuclear war is hampered by our limited understanding of the immediate consequences of different nuclear exchange scenarios as well by the complex and varied interactions between human and natural systems. Two principal disruption pathways have been identified: global climatic impacts from large-scale exchanges between major powers and regional environmental degradation resulting from smaller-scale conflicts. These pathways are further complicated by deep societal and economic interdependencies at local, regional, and global scales.
Potential impacts on agriculture and food production could be extensive, with widespread cooling and precipitation reductions leading to diminished crop yields, reduced livestock productivity, and disruptions to planting and harvesting cycles. Impacts would likely vary regionally, with temperate areas above 30°N latitude facing major effects. Marine and freshwater fisheries could also experience significant upheaval. Environmental quality would deteriorate through air, water, and soil pollution from damaged infrastructure, fires, and mismanaged waste. However, substantial uncertainties exist in quantifying specific outcomes such as yield reductions and determining the thresholds for ecosystem disruption.
Direct health impacts include blast injuries and radiation exposure, reduced continued care, and food and water insecurity. Complex indirect impacts over longer timescales could involve increased disease transmission, malnutrition, mental illness, and broader public health crises arising from compromised health systems. Persistent exposure to environmental contamination and psychosocial stressors may have intergenerational consequences. Gaps remain in understanding the effects of secondary environmental drivers on health.
Mass displacement would severely strain the provision of shelter, healthcare, food, water and protection for refugees, potentially enabling exploitation and violence. Psychological impacts on the public and responders could be profound, while broader societal effects may include panic, hoarding, and compromised civil liberties. Moreover, substantial uncertainties around predicting complex human behavioral responses, detecting societal tipping points, quantifying cascading failures across critical systems, and assessing long-term psychological harm will hamper efforts to improve preparedness. Building community resilience would help mitigate these risks.
Key Findings and Recommendations
FINDING 7-1: The global supply chain, financial markets, and communication networks are all interconnected. A shock to these complex systems (e.g., nuclear war) could result in cascading risks beyond the directly impacted populations. Globally, the interconnected impacts on ecosystems especially food security, the economy, society, and the environment are influenced by the magnitude of the nuclear event and the directly and indirectly impacted communities’
resilience, health status, and well-being. Hence, there is a critical need for developing and investing in multidisciplinary frameworks and advanced research to anticipate and, where possible, mitigate the potential risks to and vulnerabilities of these interconnected systems.
RECOMMENDATION 7-1: Agencies such as the U.S. Departments of Health and Human Services, Defense, Energy, and Agriculture should collaborate in the development and implementation of an enterprise-wide approach to assess the potential societal and economic impacts of a nuclear detonation on food, water, and health, taking into account disruption to global trade, financial markets, supply chains, and communication networks.
FINDING 7-2: The health and well-being of people and society are deeply coupled with, and inextricably linked to, the health of ecosystems. A comprehensive characterization of the societal and economic and environmental impacts of a nuclear war is substantially hampered by limitations in observational data, process understanding, and modeling capabilities. For example, there is a critical need to invest in advanced research and modeling capabilities to assess the potential impacts on agriculture, food security, and food safety.
RECOMMENDATION 7-2: Agencies such as the U.S. Departments of Health and Human Services, Defense, Energy, and Agriculture, the Environmental Protection Agency, and the National Science Foundation should advance predictive modeling research to bolster preparedness strategies during the interdisaster period. Specifically, a multidisciplinary approach that integrates different predictive models simulating climate, crop growth, livestock dynamics, and economic factors at different scales through model intercomparison and ensembles can provide considerable insight into potential nuclear war scenarios and their consequences. These efforts can help policymakers reach informed decisions, develop contingency plans, and design strategies to mitigate the worst outcomes and ensure food resilience in the face of such events.
FINDING 7-3: Impacts of a sudden decline in mean temperature and precipitation on crop yields and fisheries production are little understood and poorly constrained. There is extensive research on effects of gradual global warming on agricultural productivity, for example, but limited information on impacts of sudden global cooling. Furthermore, existing agricultural models and freshwater ecosystem models are not well suited to capture the impacts of a sudden shock. Furthermore, an assessment of impacts of a nuclear detonation on environmental systems requires understanding the cascading effects from climate, ecosystem, and hydrologic alterations, and subsequent effects on food production, trade, and human health.
RECOMMENDATION 7-3: Model Intercomparison and Improvement: Agencies such as the U.S. Departments of Health and Human Services, Defense, Energy, and Agriculture, the National Institutes of Health, the National Science Foundation, the Centers for Disease Control and Prevention, the U.S. Agency for International Development, and the Environmental Protection Agency should advance transdisciplinary research to develop and interpret intercomparison models as well as promote information exchange by the following key stakeholders: Integrated Assessment Modeling Community, the Coupled Model Intercomparison Project, the Earth system model community; the Agricultural Model Intercomparison and Improvement Project, the Inter-Sectoral Impact Model Intercomparison Project, and population health predictive modeling experts. The purpose of these recommended actions is to provide key U.S. federal, state, and local authorities and the U.S. global partners with protocols and tools to prepare for, respond to, and manage nuclear events of any magnitude. Research should invest in the following key areas:
- Analysis of climate impacts (sudden cooling) on livestock, fisheries, forests, and other natural resource systems that provide food, materials, medicines, and energy.
- Atmospheric transport models to estimate the fate of pollutants from potential infrastructure damage and fires in order to quantify pollution risks to ecosystems.
- Examination of ecosystem impacts and recovery timeframes for relevant air, water, and soil contaminants through toxicity testing and field studies.
- Crop models to address impacts from surface ozone, ultraviolet radiation, and freshwater availability.
- Simulated climatic changes from nuclear war scenarios into crop models to strengthen projections of agricultural productivity losses for major staple crops in key producing regions.
- Evaluation of adaptive capacities and vulnerabilities of agricultural and natural resource systems to inform targeted resilience strategies such asland-use changes, policy responses (e.g. trade protection).
- Integrated modeling to capture complex connections and feedbacks between climate changes, pollution, and human systems including food trade, reserves, and consumer behaviors.
- Improvement of geographic data on global food stocks, agricultural inputs, production capacities, and supply chain infrastructures to identify buffer capacities along with vulnerabilities.
- Scenario modeling and economic analysis to guide policies that could mitigate food insecurity and ecological degradation if nuclear war occurs.
- Assessment and periodic monitoring of social capital domains in at-risk communities. These domains include natural, infrastructure, financial, human, and cultural, social, and political capital.
- Predictive modeling of the integration of data characterizing the social capital domains and the pillars of community resilience in selected vulnerable communities.
- Use of tabletop exercises, game theory and related tools to address aspects that cannot be modeled.
8.8 REFERENCES
Lindsey, R., and L. Dahlman. 2024. Climate Change: Global Temperature. Retrieved July 25, 2024, 2024, from https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature.
Ritchie, H., L. Rodés-Guirao, E. Mathieu, M. Gerber, E. Ortiz-Ospina, J. Hasell, and M. Roser. 2023. Population Growth. https://ourworldindata.org/population-growth (accessed July 25, 2024).
