New Nuclear Winter Simulations with the Whole Atmosphere Community Climate Model 4 (WACCM4)

A new simulation of a 150 Tg global soot layer (similar to a global nuclear conflict) using the WACCM4 model produces a similar climate response as that from the older NASA GISS ModelE general circulation model, but with slightly less cooling. In both cases, the simulations show that the current U.S. and Russian nuclear arsenals are capable of producing a nuclear winter, with summer temperatures in mid-continental agricultural regions staying below the freezing point of water, preventing food production and global famine. In both cases, 150 Tg of soot (black carbon) is injected into the upper troposphere/lower stratosphere, an amount that could be produced from fires in cities and industrial areas from a global nuclear war. The injection of soot into the stratosphere differs slightly in spatial distribution between both models. WACCM4 is an earth system model, with coupled atmosphere, ocean, land and sea ice at 2 degree spatial resolution with 66 vertical layers and a model top of 140km. ModelE is also, but with the atmosphere at a resolution of 4 by 5 degrees with 23 vertical layers and a model top of 80km. Both models simulate the transport and removal of aerosols, which is vital to calculating the radiative impact of soot in the stratosphere on global climate, however WACCM4 includes a full stratospheric chemistry, which simulates ozone impacts, and CARMA (Community Aerosol and Radiation Model for Atmospheres), a sectional aerosol model, which treats soot as fractal particles. As expected, global mean surface temperatures and precipitation drop rapidly in both cases while stratospheric temperatures rise. This has a dramatic impact on the circulation of the stratosphere as well as the concentration of stratospheric ozone. The differences between the model simulations are explained based on different resolutions, the inclusion of different processes, as well as slight differences in the injection of soot.