Stratospheric Response in the First Geoengineering Simulation Meeting Multiple Surface Climate Objectives

Abstract

We describe here changes in stratospheric dynamics and chemistry in a first century‐long sulfate aerosol geoengineering simulation in which the mean surface temperature and the interhemispheric and equator‐to‐pole surface temperature gradients were kept near their 2020 levels despite the RCP8.5 emission scenario. Simulations were carried out with the Community Earth System Model, version 1 with the Whole Atmosphere Community Climate Model as its atmospheric component [CESM1(WACCM)] coupled to a feedback algorithm controlling the magnitude of sulfur dioxide (SO2) injections at four injection latitudes. We find that, throughout the entire geoengineering simulation, the lower stratospheric temperatures increase by ∼0.19 K per Tg SO2 injection per year or ∼10 K with ∼40 Tg SO2/year total SO2 injection. These temperature changes are associated with a strengthening of the polar jets in the stratosphere and weakening of the mean zonal wind in the lower stratosphere subtropics and throughout the troposphere, associated with weaker storm track activity. In the geoengineering simulation the quasi‐biennial oscillation of the tropical lower stratospheric winds remains close to the presently observed quasi‐biennial oscillation, even for large amounts of SO2 injection. Water vapor in the stratosphere increases substantially: by 25% with ∼20 Tg SO2/year annual injection and by up to 90% with a ∼40 Tg SO2/year injection. Stratospheric column ozone in the geoengineering simulation is predicted to recover to or supersede preozone hole conditions by the end of the century.

The Regional Hydroclimate Response to Stratospheric Sulfate Geoengineering and the Role of Stratospheric Heating

“…A complicating factor in GEOHEAT is a global warming that likely arises as a result of increased stratospheric water vapor. In GLENS, the stratospheric water vapor concentration is almost double that of BASE (Figures f and e) which is expected given the warming of the tropical lower stratosphere and the cold‐point tropopause (Richter et al, ). Indeed, in the GEOHEAT experiments, stratospheric water vapor also increases (Figure g), although not by quite as much as in GLENS, probably because the stratosphere has seen warmer cold point temperatures for a shorter time.…”

Comparing Surface and Stratospheric Impacts of Geoengineering With Different SO2 Injection Strategies

“…In both geoengineering simulations, lower stratospheric zonal wind decelerates around 30°N and 30°S, a reversal of the global warming pattern in RCP8.5 (Richter et al, ). In the equatorial simulation, this decrease spreads to the tropics, consistent with a change of the QBO (Figure ) to a persistent Westerly phase.…”

Timescale for Detecting the Climate Response to Stratospheric Aerosol Geoengineering

“…The model includes oxidation of SO 2 to H 2 SO 4 and includes a modal description of sulfate aerosol microphysics (Liu et al, ) to represent nucleation, condensation, coagulation, evaporation, and sedimentation processes; the modal description is a compromise choice between computation and fidelity and may limit the ability to capture the aerosol size distribution. The model also includes interactive stratospheric chemistry and dynamics, both of which can alter the stratospheric circulation (Richter et al, ; ) and thus influence aerosol spatial distributions as well as directly influencing the tropospheric response (e.g., Driscoll et al, ). The model is fully coupled to land, ocean, and sea ice components.…”

“…Second, the climate model used (Mills et al, ; Mills et al, , see section ) includes a number of important stratospheric processes that have not all been included in past geoengineering simulations (see e.g., Table 2 in Pitari et al, ) but are important for capturing the climate response to SO 2 injection. These include a model of aerosol microphysical processes, stratospheric chemistry including ozone, and sufficient spatial resolution to capture potential dynamic influences on stratospheric circulation including changes to the quasi‐biennial oscillation (Aquila et al, ; Richter et al, , ).…”