History of Nuclear Winter Theory

The direct effects of nuclear war would be horrific, with blast, fires, and radiation killing and injuring many people. But in 1983, United States and Soviet Union scientists showed that a nuclear war could also produce a nuclear winter, with catastrophic consequences for global food supplies for people far removed from the conflict. Smoke from fires ignited by nuclear weapons exploded on cities and industrial targets would block out sunlight, causing dark, cold, and dry surface conditions, producing a nuclear winter, with surface temperatures below freezing even in summer for years. Nuclear winter theory helped to end the nuclear arms race in the 1980s and helped to produce the Treaty on the Prohibition of Nuclear Weapons in 2017, for which the International Campaign to Abolish Nuclear Weapons (ICAN) received the 2017 Nobel Peace Prize. The latest research (Xia et al., 2022), using the NCAR WACCM climate model with its included crop model and a fishery model, has shown that a nuclear war between India and Pakistan could result in the deaths by starvation of 1 to 2 billion people worldwide, and a war between Russia and the US could kill over 6 billion people.

The first climate model simulations in the 1980s used one-dimensional single-column models, energy-balance models, or low-resolution (horizontally and vertically) atmospheric general circulation models (GCMs) without oceans. They all showed that smoke from fires ignited by nuclear weapons attacks on cities and industrial areas would make it cold and dark at Earth’s surface. These simulations were limited by the climate models and computing power available for the calculations. But the basic science seemed settled in 1990. Another decade and a half passed before nuclear winter research got going again. Progress in computing and climate modeling allowed investigations that previously were impossible. In the 1980s the fastest “supercomputers” were orders of magnitude slower and had orders of magnitude less storage than the smartphones most of us carry around in our pockets today. Thus, simulations had to ignore much of the physics and chemistry of the atmosphere and they could not represent the full depth of the atmosphere or be run long enough to study the interannual response to a smoke injection. More recent simulations (Robock et al., 2023) used GCMs with modules to calculate the transport, growth, and removal of aerosol particles, ozone and UV impacts, and full oceans with biogeochemical cycles. Simulations were run for more than a decade, not just a few weeks. For the first time, we learned that smoke would stay in the stratosphere for multiple years because we could simulate the heating and lofting of the smoke, preventing it from quickly falling out of the air. The basic conclusion that a large-scale nuclear conflict would have devastating climatic consequences was not only supported but strengthened.

New results also found that nuclear conflict has the potential to increase surface ocean pH and decrease aragonite saturation state, that the decrease in saturation state would exacerbate shell dissolution from anthropogenic ocean acidification, and that a regional nuclear conflict may have far-reaching effects on global ocean carbonate chemistry. Agricultural losses could not be offset by the world’s fisheries, especially given widespread overfishing. Cold temperatures and reduced sunlight would decrease the growth of fish biomass, possibly as much as under unmitigated climate change. Although intensified postwar fishing could yield a small catch increase, dramatic declines would ensue due to overharvesting. Surface cooling caused by the nuclear war would also include expansion of sea ice in the first years after the war when food shortages would be highest, affecting shipping in regions into crucial ports where sea ice is not currently experienced, such as the Yellow Sea. The ocean would cool rapidly but would not return to the pre-war state when the smoke cleared. Instead, the ocean would take 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. Arctic sea ice would be 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.

The biggest unknown in this research, given a particular scenario, is the amount of smoke that would be injected into the stratosphere from fires, and its chemical and microphysical properties. New nuclear winter research has been difficult, because of the lack of U.S. government support. Recent progress has only been made possible because of generous grants from the Open Philanthropy Project and the Future of Life Institute.

Because awareness of nuclear winter is now widespread, nuclear nations have so far not used nuclear weapons in warfare. But the mere existence of nuclear weapons means that they can be launched by unstable leaders, accidently from technical malfunctions in computers and sensors, due to human error, or by terrorists. Because they cannot be used without the danger of escalation resulting in a global humanitarian catastrophe, because of recent threats to use them by Russia, and because nuclear deterrence doctrines of all nuclear-armed states are based on the capability and readiness to use nuclear weapons, it is even more urgent for scientists to study these issues, broadly communicate their results, and work for the elimination of nuclear weapons.

References:

Robock, Alan, Lili Xia, Cheryl S. Harrison, Joshua Coupe, Owen B. Toon, and Charles G. Bardeen, 2023: Opinion: How nuclear winter has saved the world, so far. Atm. Chem. Phys., 23, 6691-6701, doi:10.5194/acp-23-6691-2023.

Xia, Lili, Alan Robock, Kim Scherrer, Cheryl S. Harrison, Benjamin Leon Bodirsky, Isabelle Weindl, Jonas Jägermeyr, Charles G. Bardeen, Owen B. Toon, and Ryan Heneghan, 2022: Global food insecurity and famine from reduced crop, marine fishery and livestock production due to climate disruption from nuclear war soot injection. Nature Food, 3, 586-596, doi:10.1038/s43016-022-00573-0.