Climate Response to Pulse Versus Sustained Stratospheric Aerosol Forcing

Solar geoengineering has been suggested as a potential means to counteract anthropogenic warming. Major volcanic eruptions have been used as natural analogues to large-scale deployments of stratospheric aerosol geoengineering, yet difference in climate responses to these forcings remains unclear. Using the National Center for Atmospheric Research Community Earth System Model, we compare climate responses to two highly idealized stratospheric aerosol forcings that have different durations: a short-term pulse representative of volcanic eruptions and a long-term sustained forcing representative of geoengineering. For the same amount of global mean cooling, decreases in land temperature, precipitation, and runoff in the pulse case are much larger than that in the sustained case. The spatial pattern changes differ substantially between these two cases. Thus, direct extrapolations from volcanic eruption observations provide limited insight into impacts of potential stratospheric aerosol geoengineering. However, simulations of volcanic eruptions can be useful to test process representations in models that are used to simulate geoengineering deployments. Plain Language Summary Major volcanic eruptions are considered as natural analogues for stratospheric sulfate aerosol geoengineering that aims to cool the climate by increasing the burden of stratospheric sulfate aerosols. Volcanic eruptions produce a layer of sulfate aerosols that stays in the stratosphere for a couple of years, whereas geoengineering efforts would need to sustain the aerosol layer persistently to counteract CO2-induced warming. Here we use a climate model to compare climate changes in response to a volcano-like pulse aerosol forcing and a geoengineering-like sustained aerosol forcing. When producing similar amount of global mean cooling, the pulse aerosol forcing results in a much larger reduction in land temperature and land minus ocean temperature when compared to that induced by a sustained aerosol forcing. Also, both land precipitation and runoff decrease more in response to the pulse aerosol forcing. Spatial patterns of temperature and the hydrological cycle change also differ substantially between these two types of forcings. These differences in the climate response between the pulse forcing and sustained forcing clearly show that caution should be taken when using climate consequences of volcanic eruptions to directly infer climate responses to stratospheric aerosol geoengineering.