Tarshish and Romps, Latent heating is required for firestorm plumes to reach the stratosphere, JGR Atmospheres, 2022

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For decades, it has been debated whether nuclear war is capable of triggering a "nuclear winter," i.e., a planetary cooling severe enough to cause global crop failure. The idea is that nuclear explosions would ignite massive urban fires whose smoke would reach up into the stratosphere, where it would take years to dissipate. In the meantime, that soot would block some of the sunlight that would ordinarily make its way to Earth's surface. This would have a cooling effect that could potentially push agriculture yields into dramatic declines, yielding a global famine. By some estimates, even a limited regional exchange of nuclear warheads (between, e.g., India and Pakistan) could lead to more deaths globally due to starvation than are killed by blasts and radiation in the warring countries.

In 2018, a team of nine scientists at Los Alamos National Laboratory, one of the nation's nuclear-weapons laboratories, published a simulation of an urban firestorm in which the soot fails to reach the stratosphere. Based on this simulation, they predicted "significantly lower global climatic impacts than assessed in prior studies" (Reisner et al. , 2018). It would seem, therefore, that nuclear winter is not a concern.

But that study contained a crucial flaw: the researchers omitted water vapor from their simulations. As we show here, water vapor is critical for getting the firestorm plume up into the stratosphere, where emplaced soot can reside for years. We demonstrate this in two ways: with theoretical calculations based on an earlier study using direct numerical simulation and using a large-eddy simulation designed to replicate the Reisner et al. (2018) simulation as closely as possible. Despite the enormous rate of sensible heat added to the atmosphere through combustion, water vapor makes essential contributions to the plume's dynamics through the release of latent heat. Indeed, in many locations and times around the world, water vapor powers storm clouds to the stratosphere without the benefit of any heating from combustion. Sadly, therefore, nuclear winter remains a very real threat.

Comparison of (top) moist and (bottom) dry simulations based on the dry simulation of Reisner et al. (2018). Soot cross sections are evaluated at the end of simulations and in the center of the domain. The moist simulation has 70% relative humidity. As in Reisner et al. (2018), the dry simulation does not get soot above the tropopause, while the moist simulation does.