WACCM4, a state‐of‐the‐art climate model, and GISS Model E, an older climate model, were used more than a decade apart to simulate the environmental aftermath of a full nuclear conflict, a near worst case scenario.The models have significant differences in particle microphysics and spatial resolution, as well as different algorithms for radiative transfer, dynamics, and other modelling approaches. Despite this, the models agree that a nuclear winter would follow a large‐scale nuclear war between the United States and Russia, a result previously found by a large number of diverse but much less sophisticated models in the 1980s. Despite differences in sensitivity to short-wave radiative anomalies, both models exhibit a peak temperature drop of near 9 K below climatological values. The massive size of the forcing explains many of the similarities in globally averaged values initially, and differences emerge as the aerosols are removed at different rates.The new model agrees not just in global averages but in spatial patterns for temperature, and precipitation changes and other climate parameters. Both models highlight the risk of a crash in global surface temperatures, but WACCM4 points to a collapse in the summer monsoon, a dramatic shift in El Niño variability,drastic changes to the Northern Hemisphere winter time circulation, and a climate state that is 0.5 to 1 K below climatological temperatures from before the war with no sign of further warming. The WACCM4modelfinds that the lifetime of the smoke is greatly enhanced over 1980s models, because it extends to much higher altitudes where the smoke is more isolated from tropospheric rainfall, a result first found in Model Eby Robock, Oman, and Stenchikov (2007)
However, compared to GISS Model E, the lifetime of soot in the WACCM4 run is shorter due to the inclusion of particle coagulation and fractal optics, despite the higher vertical resolution and model top, alleviating the duration of the most extreme climate effects. Despite this, the cooling for the first few years is more extreme in WACCM4 and temperatures at the end of the simulation suggest a new colder climate state. The inclusion of additional particle removal processes addresses a long‐standing uncertainty about the black carbon-aerosols released following a nuclear war and allows us to further constrain theire‐folding lifetime. While we did not consider the effect of organic coatings on top of pure black carbon particles, future work should incorporate more direct calculations of smoke generation using high‐resolution fuel loading databases and high‐resolution fire modelling of urban landscapes to determine the distribution, type, and amount of material emitted from nuclear fires. Future work will build upon the results of Yu et al. (2019) to quantify the role of organic carbon in smoke from pyroCbs and the sensitivity tests of different ratios of organic carbon and black carbon by Pausata et al. (2016) for a regional nuclear war. Addressing the uncertainty of aero-sol composition would further quantify the lifetime of these aerosols and their effects on chemistry in the stratosphere. The research conducted here supports the results of Turco et al. (1983), Sagan (1984), Pittocket al. (1986), Robock, Oman, and Stenchikov (2007), Mills et al. (2008), Robock and Toon (2012), andMills et al. (2014) that a full‐scale nuclear attack would be suicidal for the country that decides to carryout such an attack. The use of nuclear weapons in this manner by the United States and Russia would have disastrous consequences globally. To completely remove the possibility of an environmental catastrophe as a result of a full‐scale nuclear war, decision makers must have a full understanding of the grave climatic con-sequences of nuclear war and act accordingly. Ultimately, the reduction of nuclear arsenals and the eventual disarmament of all nuclear capable parties are needed.