Nuclear Niño response observed in simulations of nuclear war scenarios

The threat of nuclear war and its potential climatic consequences has been a concern since the 1980s. Nuclear detonations would ignite massive fires, injecting soot into the stratosphere, leading to global cooling and potential crop failures. This paper revisits this threat in light of current geopolitical tensions and existing nuclear arsenals. With increased reliance on ocean-based food sources, understanding the impact on the oceans is crucial. Previous studies have shown that large-scale injections of smoke into the atmosphere cause surface cooling and stratospheric heating, impacting agriculture and food security. Survivors would likely depend on oceans for food, making it essential to study the ocean's response to nuclear war. The El Niño-Southern Oscillation (ENSO) is a major driver of Pacific Ocean circulation and biogeochemistry, significantly impacting marine productivity and fisheries. Since ENSO is sensitive to climate influences, studying its response to nuclear war-induced aerosol cooling is crucial. Volcanic eruptions and proposed geoengineering schemes provide analogous scenarios, with past research suggesting an increased probability of El Niño events following such events. However, the response of ENSO to a persistent, near-decadal global perturbation like that caused by nuclear war remains unstudied. This study aims to fill this gap by simulating the response using a state-of-the-art Earth system model.

Early studies in the 1980s, such as the TTAPS report, explored the concept of “nuclear winter,” focusing on the climatic effects of atmospheric soot. Subsequent research, using increasingly complex climate models, has confirmed that large-scale smoke injections would lead to significant surface cooling. Studies have shown the potential for even smaller, regional nuclear conflicts to severely disrupt climate and compromise global food security by damaging agricultural yields. The impact of volcanic eruptions on ENSO has been extensively investigated. Studies using paleoclimate data and modeling experiments have shown that volcanic eruptions can increase the likelihood of El Niño events in subsequent years. Similarly, research on stratospheric aerosol geoengineering has indicated a potential increase in El Niño probability. However, the response of ENSO to a long-lasting, large-scale perturbation like that from a nuclear war has not been thoroughly investigated. This study builds upon this existing literature by explicitly examining the ENSO response to the prolonged radiative forcing of a nuclear winter scenario.

This study utilized the Community Earth System Model (CESM, version 1.3) coupled with the Whole Atmosphere Community Climate Model (WACCM, version 4) and the Parallel Ocean Program (POP, version 2) including the Biogeochemical Elemental Cycling model. This model configuration is known to replicate ENSO periodicity and the response to volcanic eruptions. Six nuclear war scenarios were simulated, including a large-scale conflict between the US and Russia (150 Tg soot injection) and five smaller India-Pakistan scenarios (5–46.8 Tg soot injection). The soot injections were modeled as occurring over seven days, starting on May 15th. Each simulation was run for 15 years, with three control run ensembles of 20 years each for comparison. Sensitivity experiments were conducted to isolate the contribution of various mechanisms to the Nuclear Niño, including cooling of the Maritime Continent, tropical Africa, and differential Northern Hemisphere cooling. Surface albedo was modified in these sensitivity tests to mimic the radiative forcing in the main simulations. In addition to analyzing physical climate variables, the study also examined changes in net primary productivity (NPP) in the equatorial Pacific, separating the effects of light reduction, nutrient availability, and temperature. The BEC model within CESM was used for this biogeochemical analysis, considering phytoplankton biomass, cell division rates, and limitations from light, nutrients, and temperature.

Simulations revealed a significant and prolonged “Nuclear Niño” event in all scenarios with soot injections exceeding 5 Tg. The largest simulation (150 Tg soot injection) exhibited a westerly wind stress anomaly lasting up to seven years, reversing the trade winds intermittently for three years. This resulted in warming SSTs in the Niño3.4 region, despite overall global cooling. The zonal sea surface height gradient shifted positive, indicating a disruption of normal equatorial Pacific circulation and a shutdown of upwelling. The strength and duration of the wind response were correlated with the amount of injected soot, although a saturation point was observed beyond 27.3 Tg. Sensitivity tests revealed that cooling of the Maritime Continent was the most effective mechanism in initiating the Nuclear Niño, with significant contributions from African continent cooling and a smaller contribution from the equatorward shift of the Intertropical Convergence Zone (ITCZ). The multi-year persistence of the Nuclear Niño was attributed to sustained trade wind forcing and internal Bjerknes feedback processes. The study also found substantial reductions in equatorial Pacific net primary productivity (NPP), primarily driven by reduced incident light reaching the ocean's surface due to atmospheric soot. Reductions of up to 68% in monthly averaged NPP were observed in the Peru EEZ, with reductions exceeding 30% in annual mean NPP over several years. Even though nutrient limitation was impacted, this effect was outweighed by light limitation and temperature changes.

The findings demonstrate an unprecedented, extreme mode of ENSO triggered by nuclear war-induced cooling, exceeding the magnitude and duration of historical and simulated El Niño events. The prolonged residence time of soot aerosols is a key factor contributing to the long-lasting Nuclear Niño. While the atmospheric teleconnections may differ from a typical El Niño, the overall impact of plummeting global temperatures will be significant. The termination of the Nuclear Niño coincides with the removal of aerosols, warming of the Maritime Continent and tropical Africa, and the return of the ITCZ to its normal pattern. A subsequent La Niña-like rebound occurs years later. The substantial reductions in NPP, primarily driven by reduced light, would have severe consequences for marine ecosystems and fisheries. The combined effects of reduced light, temperature changes, and altered upwelling lead to significant drops in phytoplankton production, which would impact the entire food web. This surpasses the impacts typically associated with El Niño events. The implications for global food security, coupled with predicted agricultural losses, are severe.

This study demonstrates the potential for catastrophic climate change following a nuclear war, including an unprecedented, prolonged El Niño-like response in the tropical Pacific Ocean (Nuclear Niño). The Nuclear Niño, mainly initiated by cooling of the Maritime Continent and tropical Africa, along with the equatorward ITCZ shift, persists for many years due to persistent aerosol forcing and internal climate feedbacks. Significantly reduced equatorial Pacific NPP, primarily driven by reduced solar radiation, further exacerbates the threat to global food security. These findings underscore the devastating and long-lasting global consequences of nuclear conflict. Further research should investigate the potential for mitigating these effects and exploring other aspects of the coupled climate-biogeochemical response in different climate models.

The study relies on a specific climate model (CESM-WACCM4) and a set of nuclear war scenarios. The results may be model-dependent, and other models could yield different results. The simulations assume a specific soot injection profile, and variations in the amount, altitude, and spatial distribution of smoke could alter the results. The assessment of biogeochemical impacts is limited by the resolution and complexity of the biogeochemical model. The study focuses on the equatorial Pacific; more research is needed to fully understand the global impacts on ocean circulation and productivity.