How Would Nuclear War Affect Climate? Scientists Are Divided

October 2025
By Andrew J. Ross

The National Academies of Sciences, Engineering, and Medicine published a consensus study report June 25 entitled “Potential Environmental Effects of Nuclear War.”¹ It was authorized by the U.S. Congress in 2021 to evaluate the “non-fallout atmospheric effects of plausible scenarios for nuclear war, ranging from low-quantity regional exchanges to large-scale exchanges between major powers.” Thereafter, the National Academies of Sciences, Engineering, and Medicine convened a committee of scientists with a wide variety of disciplinary expertise to compile and assess the relevant scientific literature on nuclear war’s potential environmental repercussions.

The release of this report presents an opportunity to reflect upon the institutional interests that have shaped the scientific literature on this subject. Initially concerned by the Cold War nuclear arms race, independent scientists at universities and civilian research agencies have been engaged in this form of climate research for more than four decades. Their research finds that nuclear war could drastically cool Earth’s climate and collapse the world’s agricultural systems. Many believe that their work weakens the viability of a nuclear first-strike capability as a credible national security posture. These scientists posit that the environmental aftereffects of even a successful first strike would reverberate to the attacking nation, placing its own survival in jeopardy. They call this nuclear dilemma “self-assured destruction.”

However, institutional interests embedded within the U.S. nuclear weapons complex wield substantial influence over nuclear policymaking. Many nuclear strategists within this complex promote an aggressive nuclear posture premised upon a potential first- strike capability. Scientists who work within these institutions have published studies in recent years downplaying the findings of independent scientists whose research questions the security imperatives of a first-strike capability.

Disagreements among scientists, based upon institutional affiliation, will likely grow more pronounced. Taking a historical view of debates on this subject, however, it is clear how structural incentives have shaped scientists’ judgment of technical uncertainties and societal risks pertaining to the environmental impacts of nuclear war.

Identifying Gaps in the Literature

Since 2023, reports by the National Academies have drawn increased attention to the U.S. Department of Defense’s lack of knowledge about the aftereffects of nuclear strikes. Existing assessments by the department have been limited to the “prompt military effects” of nuclear weapon detonations relevant to tactical operations, such as blast impact and localized fallout. As a result, one NAS report found that “there is a need to improve the understanding of the physical effects of nuclear weapons […] as well as the assessment and estimation of psychological, societal, and political consequences of nuclear weapons use.”² These assessments have heightened the anticipation for the National Academies’ latest report on nuclear war’s environmental effects.

The new report’s principal aims were to identify gaps in the existing literature and provide a basis of coordination for future research. The study committee broke down research into discrete and sequential steps. To conduct this research, scientists first investigate how and to what degree nuclear detonations and subsequent urban fires might inject particulate matter, especially heat-absorbing black carbon soot, into the upper troposphere and stratosphere, where it would remain above weather systems and take years before precipitating back down to Earth. Second, scientists examine how this highly lofted particulate matter would alter the planet’s biogeochemical cycles, ecologies, agricultural, and socioeconomic systems. The first set of variables determines how nuclear war could reshape the planet’s atmosphere, while the second set establishes how such an altered atmosphere could affect life on Earth.

From these two categories, the committee then deconstructed the study of nuclear war’s environmental effects into six basic subcategories and illustrated how each layer of causation related to the others. These subcategories included: how nuclear weapons might be used; what type of fires might result from nuclear detonations; how massive urban fires might transport aerosols into the atmosphere; how these aerosols might reshape climatic and hydrological patterns; how ecosystems might respond to changes in temperature, solar radiation, and rain levels; and how these new ecological regimes would impact human societies and infrastructures. Each element has complex and interwoven causal pathways, which make deciphering the ramifications of any given nuclear war scenario exceedingly challenging to ascertain with a high level of confidence, the committee asserted.

This intricately constructed framework formed the foundation of the committee’s assessment. Each chapter considered one subcategory, describing its plausible outcomes and identifying “key uncertainties and data gaps” needing further research. Moving forward, the committee judged that the study of the environmental effects of nuclear war would require prolonged financial and technical investments as well as sustained collaboration among scientists operating with multiple, integrated computer modeling programs.

Report Findings and Context

When reviewing the existing literature, the committee concluded that too many uncertainties remained to ascertain how nuclear war scenarios might impact climatic, ecological, and socioeconomic conditions. The panel’s evaluation largely steered clear from directly assigning more credibility to some studies over others, despite disagreements among scientists modeling the environmental outcomes of nuclear war. The existing literature largely agreed that if large volumes of particulate matter from urban fires reached the upper troposphere and stratosphere, then substantial climatic cooling would occur globally. Studies conflicted, however, on whether substantial amounts of soot and smoke would be lofted so high. The committee judged that the ways in which climatic cooling might impact ecosystems, agriculture production, and supply chains required additional study.

The committee acknowledged that nuclear war would have severe, deleterious environmental outcomes and that many of these disruptions would be associated with a climatic cooling effect caused by highly lofted particulate matter. Yet, it found that unknowns persisted as to the degree of cooling and the severity of its impact on communities worldwide. On the whole, the report was a comprehensive but circumspect appraisal of the literature on nuclear war’s environmental aftereffects.

Historically, disagreements on this subject have fallen along the lines of institutional affiliation. Generally speaking, the work of scientists employed at universities or civilian federal laboratories has supported the claim that nuclear war likely would trigger severe climatic disruptions with devastating consequences for the environmental and agricultural condition of nations both targeted and not targeted by nuclear hostilities. Conversely, scientists working within the U.S. National Nuclear Security Administration laboratories, such as Lawrence Livermore and Los Alamos, which design U.S. nuclear weapons, have criticized these findings. They argue that nuclear war is not likely to result in severe climatic perturbations. Potentially at stake in this disagreement is the character of nuclear strategy in U.S. national security.³

The First Wave of Debates

In the early 1980s, at one of the lowest moments of U.S.-Soviet relations, several scientists began to model the atmospheric aftereffects of a general nuclear exchange between the two major nuclear powers. They found evidence that in the event of such a war, massive urban fires could loft into the atmosphere large plumes of soot and smoke that would disseminate into a hazy shroud spanning Earth. Climate models suggested that this massive influx of burned particulate matter into the atmosphere would absorb large amounts of solar energy, warming the upper atmosphere and keeping the soot and smoke aloft for years thereafter. This climatic perturbation would lead to dramatically darker and cooler conditions on Earth’s surface. Scientists foretold twilight conditions at noon and freezing temperatures in summer. They dubbed this phenomenon “nuclear winter.”⁴

Thereafter, U.S. and international agencies funded a series of large-scale studies on the nuclear winter hypothesis.⁵ Their findings attenuated the most extreme freezing predictions but ultimately reinforced the general understanding that a major nuclear exchange between the two superpowers would have catastrophic freezing effects on a planetary scale.

The nuclear winter theory, nevertheless, had its detractors. Its most critical opponents were military-affiliated scientists, particularly top scientists at Lawrence Livermore National Laboratory, who accused advocates of the nuclear winter hypothesis as playing politics and improperly evoking science to advance allegedly partisan goals.⁶ Ironically, many of these same scientists stood to benefit from increased public spending on nuclear weapons research and a hawkish approach to U.S.-Soviet relations. The strategic implications of nuclear winter could potentially upend the Reagan administration’s ongoing military buildup, which was particularly beneficial to the U.S. national nuclear laboratories.

A longstanding feud between astrophysicist Carl Sagan of Cornell University and nuclear physicist Edward Teller of Lawrence Livermore National Laboratory exemplified this split within the scientific community.⁷ Sagan, a member of the research group that first introduced the concept of nuclear winter, argued that the potential climatic consequences of a nuclear war would weaken the viability of a first-strike posture. Even if the United States prevailed in a nuclear war with little retaliatory damage to the U.S. homeland, the resultant climatic effects of many massive firestorms in Communist bloc cities would be so great as to threaten U.S. survivability, he reasoned. According to this logic, the United States would be reluctant to launch a nuclear strike at all, as a matter of self-preservation.

Teller, a fierce advocate of U.S. nuclear primacy, referred to Sagan as a “propagandizer” and suggested he was deploying environmental alarmism to advance his preferred political objectives. Teller did not directly contradict the findings of potential climatic cooling after nuclear war but instead argued that too many uncertainties existed to know definitively how nuclear war might alter the climate.

Ultimately, the end of the Cold War, the collapse of the Soviet Union, and the signing of nuclear arms reduction treaties significantly reduced the risk of a large-scale nuclear war. For nearly two decades, many scientists and policymakers shifted their attention from nuclear war’s climatic effects.

Nuclear-Climate Studies Revival

Beginning in 2007, atmospheric scientists at universities and civilian federal agencies began to take up the issue of nuclear war’s climatic impacts once again, citing nuclear proliferation and nuclear terrorism as potentially destabilizing threats to global security. Several independent scientists who had studied nuclear winter scenarios in the 1980s began to model the climatic effects of “smaller” nuclear wars between regional powers, such as India and Pakistan.⁸ Preliminary findings suggested that although the climatic effects of a less powerful nuclear exchange between such states would not constitute a nuclear winter per se, deleterious levels of global climate cooling could still occur.

At the same time, these scientists revisited the question of nuclear war between Russia and the United States, given the two nations’ substantially reduced stockpiles since the 1980s. They found that even with smaller nuclear arsenals, a U.S.-Russian war could “produce cooling as large or larger than that experienced 18,000 years ago during the coldest period of the last Ice Age.”⁹ This revived study into nuclear war’s climatic effects focused less on whether various nuclear scenarios met the amorphous threshold of “nuclear winter” and more on how nuclear wars of varying levels of severity might disrupt life-maintaining environmental, agricultural, and logistical systems around the world.

Over the next decade, the size of the scientific community interested in the subject steadily grew and improved insight into how regional and global nuclear wars might reconfigure Earth’s biogeochemical cycles. Some research focused on nuclear war’s disruption of the ozone layer, which would increase human exposure to carcinogenic ultraviolet radiation. Other work narrowed in on the implications for food production and distribution, with one study suggesting that an Indian-Pakistani nuclear war would reduce significantly rice cultivation in China for a decade thereafter. This revival of the subject, though, grew out of university and civilian federal agencies.¹⁰

Undergirding much of this research was the concept of self-assured destruction, harkening back to Sagan’s thinking on nuclear strategy in the 1980s.¹¹ The concept posited that the collateral environmental impacts of even a nominally successful nuclear attack would have existential consequences upon the attacking state, thus weakening the credibility of a first-strike posture. Whereas the science of nuclear winter in the 1980s had initiated a protracted debate among scientists, policymakers, and the public, the slower and steadier development of the science during the Obama administration—a time of relative geopolitical stasis among the world’s major powers—did not draw as much attention from policy circles in Washington or the U.S. population generally.

A New High-Stakes Environment

The year 2017 marked a decided change in research into nuclear war’s environmental consequences. This was because of external political conditions that again made this research particularly relevant to international affairs, not because of the methods or findings of the research itself. Four major developments influenced this change: a series of international crises that increased the specter of nuclear war; the organization of many non-nuclear nations into a visible anti-nuclear coalition; the implementation of nuclear modernization efforts among nuclear-armed states; and the reemergence of scientists from NNSA laboratories into the literature on nuclear war’s climatic aftereffects.

Regarding the first development, the precipitous rise in the likelihood of nuclear warfare began during the U.S.-North Korean diplomatic crisis of 2017 but has since deepened to become a normative condition of modern geopolitics. Contributing factors to this new state of affairs include the erosion of relations between China and the United States, Russia’s invasion of Ukraine, Israel’s protracted military operations in the Gaza Strip, Iran, Lebanon, Qatar, Syria, and Yemen, and recurrent Indian-Pakistani border crises.¹²

Second, the passage of the Treaty on the Prohibition of Nuclear Weapons by the UN General Assembly in 2017 drew a stark contrast between nuclear states and non-nuclear states.¹³ The TPNW’s passing was the culmination of a process dating back to 2010, when many non-nuclear states began to take a more confrontational approach to nuclear disarmament, directly criticizing the logic of nuclear deterrence and focusing on “the unacceptable humanitarian consequences of nuclear violence” to delegitimize the nuclear status quo.¹⁴ This treaty cohered many non-nuclear states into an integrated and vocal bloc explicitly rejecting the logic of nuclear deterrence and calling for full denuclearization across all nuclear states. An important rationale for the treaty’s signatories was the prospect of severe climatic disruptions after nuclear war that could lead to catastrophic humanitarian harm globally.¹⁵

Third, nuclear modernization initiatives, which preceded 2017 but gained discernible momentum since then, have locked in the operational viability of existing nuclear arsenals for most of the 21st century. For example, although behind schedule, the new Sentinel intercontinental ballistic missile is expected to be operational until around 2075, extending the U.S. first-strike capability for generations. Essentially all nuclear states are now modernizing or expanding their nuclear stockpiles.¹⁶

Finally, scientists at NNSA laboratories resumed research into nuclear war’s environmental impacts.¹⁷ Much like their progenitors in the 1980s, they have called into question the severity of the climatic risks posed by nuclear war and emphasized the uncertainties within current modeling efforts.

Combined, these factors created a new terrain on which civilian scientists have had to contest the credibility of research on nuclear war’s environmental impacts. Their work has regained relevance with policymakers and strategists, yet they are facing additional scrutiny from scientific and political institutions that stand to benefit from an enhanced nuclear strike capability.

Adapting to a New Normal

Public confrontations on the environmental consequences of nuclear war have not been widespread since the 1980s. However, a new period of direct political action appears imminent. Scientists are organizing. For instance, the Physicists Coalition for Nuclear Threat Reduction is an emerging organization of practicing physicists that advocates for substantive changes to current nuclear stockpiles and policies with the eventual goal of nuclear disarmament.¹⁸ The coalition is actively recruiting within university physics departments across the country to grow its political strength.

Additionally, networks of scientists across national boundaries have become essential to research on nuclear war’s climatic aftereffects. Ag-GRID is an international crop modeling initiative that collaborates on modeling global agricultural productivity amid various climate change scenarios. One project within this initiative—the ANFOS^a Project at the International Institute for Applied Systems Analysis in Austria—is specifically focusing on modeling food insecurity after nuclear war.¹⁹

Perhaps the most significant upcoming study is the United Nations Scientific Panel on the Effects of Nuclear War, expected to be published in 2027, which will examine the “physical effects and societal consequences of a nuclear war on a local, regional and planetary scale.”²⁰ This will be the first time since 1988 that the UN has undertaken a study into nuclear war’s environmental aftereffects.²¹

a. Advanced ensemble projections for indirect impacts of nuclear conflict in global food systems

The Historical Role of Science in Society

Viewed historically, one can recognize how social and political contexts shape the present state of study into nuclear war’s climatic consequences. Scientists first broached the topic because nuclear war between the Soviet Union and the United States seemed plausible. They wanted to uncover the potential adverse outcomes of nuclear war and educate the public on the consequences of such a conflagration. Likewise, much of the scientific pushback against the nuclear winter hypothesis originated from institutions that would benefit materially from an aggressive nuclear posture. Neither of these approaches were politically neutral.

Even so, science is always a political process, in some form. To fully separate science from human subjectivity and social context is to not do science at all. One should not ask whether but how a particular scientific debate is political. Who gets to claim scientific authority? On what basis is this authority derived? Whom might a given research program help or harm? What public outcomes might result from the scientific research under consideration? Asking these questions can illuminate the power discrepancies and divergent institutional interests between civilian scientists and those at NNSA federal laboratories.

This is not to say that criticism of the findings of civilian scientists is always dubious—far from it. These scientists acknowledge the informed assumptions that go into their modeling work as well as the unknowns that require further investigation. As the National Academies report states, many uncertainties do, in fact, remain as to how nuclear war may reorder Earth’s environmental systems, and additional resources are needed to develop a greater level of confidence on the subject.

The Current State of Research and Future Prospects

What is at issue, instead, is the ways in which NNSA scientists interact with their civilian peers. NNSA scientists do not collaborate with those at universities or civilian research centers who have been working on this topic for years, even decades. Civilian scientists find out what NNSA laboratories are working on through conference presentations and publications. These laboratories have far greater resources at their disposal than university scientists. They could combine the experience of their civilian colleagues with their own institutional resources to develop experiments in which the informed assumptions, parameters, and methods in their modeling are more likely to be accepted communitywide. But they do not do this. Cultivating consensus does not seem to be their aim.

Instead, NNSA scientists describe their civilian colleagues as “the nuclear winter community” from which they are presumably separate.²² They imply political motivation on the part of civilian scientists and fail to acknowledge the structural interests of their own institutional affiliations. They claim the mantle of objectivity and question that of their civilian peers.

Take, for example, the perspective of one Los Alamos National Laboratory climate scientist, Manvendra Dubey, who presented before the National Academies consensus study committee in September 2023.²³ One committee member asked Dubey about the sedimentation of urban fire particulate matter back to Earth’s surface. He elaborated a bit before shifting his focus to U.S. national security:

“There is another side [to] this problem, which is it has implications to national security … which is not my domain, but it’s out there. So, we need to be very objective. You know this issue of fear of not using [nuclear weapons] for whatever reason is, again, … skewed. [The United States] is very liberal and open-minded and science-driven, but, you know, other nations aren’t. So, I’m sure you’ve heard about the policy imperatives. So, we all don’t want impacts [from] nuclear weapons, but I think we [scientists] need to be very even handed when we talk about it.”

Dubey’s brief tangent reveals quite a bit about the logic at NNSA laboratories. His claim that the U.S. political class takes these issues seriously, whereas U.S. adversaries do not, is verifiably untrue. In the 1980s, Soviet Premier Mikhail Gorbachev took the prospect of nuclear winter far more seriously than did the Reagan administration. Recently, China was the only nuclear-armed nation to vote in favor of the UN study into the environmental effects of nuclear war. The United States abstained. Given that the Trump administration is currently burying congressionally mandated reports on climate change, Dubey’s understanding of the state of climate science in the United States and its adversaries was misguided.²⁴

Additionally, Dubey’s conception of evenhandedness wrongfully conflates analytical disinterest with analytical conservatism. To claim that nuclear war potentially could have severe climatic cooling effects worldwide is not to lack impartiality. In fact, claiming that moderate estimates should be preferred over more severe estimates, regardless of what one finds from observation, is evidence of one’s own partiality.

When combined, these two assumptions—that the United States should not take seriously the concept of self-assured destruction because its adversaries will not, and that claims of less severe climatic effects should be favored over those of greater severity—align with the interests of those who favor a first-strike nuclear posture. The implementation of this military posture, which Los Alamos will materially benefit from, seems to be an abiding objective of NNSA scientists.

Ultimately, how societies choose to manage societal risks are political, rather than empirical, questions. Politicians and the public, rather than scientists, will decide how the potential environmental consequences of nuclear war shape nuclear policy. When scientists disagree on issues of such consequence, it is incumbent upon civil society to scrutinize the underlying institutional incentives that shape the debate. This scrutiny is crucial because the outcome of these debates may well shape whether a first-strike nuclear posture remains an abiding objective of U.S. nuclear strategy, or whether alternative forms of military and diplomatic engagement are more aligned with the interests of the nation—and the planet.