A New Ocean State After Nuclear War. Issue 4 (7th July 2022) - Harrison, Cheryl S.
Abstract: Nuclear war would produce dire global consequences for humans and our environment. We simulated climate impacts of US‐Russia and India‐Pakistan nuclear wars in an Earth System Model, here, we report on the ocean impacts. Like volcanic eruptions and large forest fires, firestorms from nuclear war would transport light‐blocking aerosols to the stratosphere, resulting in global cooling. The ocean responds over two timescales: a rapid cooling event and a long recovery, indicating a hysteresis response of the ocean to global cooling. Surface cooling drives sea ice expansion, enhanced meridional overturning, and intensified ocean vertical mixing that is expanded, deeper, and longer lasting. Phytoplankton production and community structure are highly modified by perturbations to light, temperature, and nutrients, resulting in initial decimation of production, especially at high latitudes. A new physical and biogeochemical ocean state results, characterized by shallower pycnoclines, thermoclines, and nutriclines, ventilated deep water masses, and thicker Arctic sea ice. Persistent changes in nutrient limitation drive a shift in phytoplankton community structure, resulting in increased diatom populations, which in turn increase iron scavenging and iron limitation, especially at high latitudes. In the largest US‐Russia scenario (150 Tg), ocean recovery is likely on the order of decades at the surface and hundreds of years at depth, while changes to Arctic sea‐ice will likely last thousands of years, effectively a "Nuclear Little Ice Age." Marine ecosystems would be highly disrupted by both the initial perturbation and in the new ocean state, resulting in long‐term, global impacts to ecosystem services such as fisheries. Plain Language Summary: If nuclear arsenals were used accidentally or intentionally, they would produce dire consequences for all life on Earth. We simulated climate impacts of nuclear wars in a global Earth system model, focusing on marine impacts. We simulated a US‐Russia war and several India‐Pakistan wars. In all scenarios, firestorms from nuclear war would deliver soot to the upper atmosphere, blocking out the sun and causing global cooling. Impacts of the nuclear cooling event include expansion of sea ice into populated coastal areas and decimation of ocean marine life. In all scenarios, the ocean cools rapidly but does not return to the pre‐war state when the smoke clears. Instead, the ocean takes many decades to return to normal, and some parts of the ocean would likely stay in the new state for hundreds of years or longer. When the cooling event ends, Arctic sea ice is left in a new state, a sort of "Nuclear Little Ice Age." Marine ecosystems would be highly disrupted by both the initial perturbation and the resulting new ocean state, resulting in impacts to ecosystem services worldwide, lasting for decades. This study underscores the danger of nuclear war and the long‐term impacts to humans and our environment. Key Points: Nuclear war driven global cooling perturbs the ocean physical, biogeochemical, ecological and sea ice states These perturbations last for many decades and likely hundreds of years or longer Increased macronutrient delivery drives changes in phytoplankton community structure and enhanced iron scavenging at high latitudes