UC Berkeley

Which human activities have caused global warming? Using a climate model driven by historical emissions, we demonstrate that global warming is not primarily due to fossil fuels. Instead, agriculture and land use have likely driven historical warming. In fact, fossil fuels probably had a cooling effect throughout most of the 20th century due to their emissions of sulfur aerosols, which increase Earth's reflectivity. This aerosol cooling likely offset the warming induced by greenhouse gases from fossil fuels. However, with sulfur emissions decreasing in recent decades and greenhouse gases continuing to accumulate in the atmosphere due to fossil fuel combustion, the warming attributable to fossil fuels has rapidly risen and is expected to surpass that from other human activities this decade. Beyond this decade, what long-term temperature is locked in by past emissions? We explore the extent of committed warming by probing the temperature response following the cessation of all current emissions. Using a theoretical model constrained by the best available evidence, we find that ceasing emissions leads to a multi-century temperature decrease of approximately $-0.4^\circ\,$C. Thus, future temperature increases beyond present-day are due only to future emissions. Lastly, we shift our focus to investigating the potential for catastrophic cooling following a limited-scale nuclear exchange. This ``nuclear winter'' scenario results from a decrease in sunlight caused by soot propelled into the stratosphere by plumes above burning cities. Using a combination of idealized theory and numerical simulations, we demonstrate that these plumes must rely on the latent heat of condensing water to reach climate-altering heights. Consequently, firestorms in geographical regions prone to deep convection pose the highest risk of initiating a nuclear winter.