Sebastian D. Eastham, Debra K. Weisenstein, David W. Keith, and Steven R. H. Barrett. 5/25/2018. “Quantifying the impact of sulfate geoengineering on mortality from air quality and UV-B exposure.” Atmospheric Environment. Publisher's VersionAbstract
    Sulfate geoengineering is a proposed method to partially counteract the global radiative forcing from accumulated greenhouse gases, potentially mitigating some impacts of climate change. While likely to be effective in slowing increases in average temperatures and extreme precipitation, there are known side-effects and potential unintended consequences which have not been quantified. One such consequence is the direct human health impact. Given the significant uncertainties, we take a sensitivity approach to explore the mechanisms and range of potential impacts. Using a chemistry-transport model, we quantify the steady-state response of three public health risks to 1 °C global mean surface cooling. We separate impacts into those which are “radiative forcing-driven”, associated with climate change “reversal” through modification of global radiative forcing, and those “direct impacts” associated uniquely with using sulfate geoengineering to achieve this. We find that the direct (non-radiative forcing driven) impact is a decrease in global mortality of ∼13,000 annually. Here the benefits of reduced ozone exposure exceed increases in mortality due to UV and particulate matter, as each unit of injected sulfur incurs 1/25th the particulate matter exposure of a unit of sulfur emitted from surface sources. This reduction is exceeded by radiative forcing-driven health impacts resulting from using sulfate geoengineering to offset 1 °C of surface temperature rise. Increased particulate matter formation at these lower temperatures results in ∼39,000 mortalities which would have been avoided at higher temperatures. As such we estimate that sulfate geoengineering in 2040 would cause ∼26,000 (95% interval: −30,000 to +79,000) early deaths annually relative to the same year without geoengineering, largely due to the loss of health benefits associated with CO2-induced warming. These results account only for impacts due to changes in air quality and UV-B flux. They do not account for non-mortality impacts or changes in atmospheric dynamics, and must be considered in the wider context of other climate change impacts such as heatwave frequency and sea level rise.
    Zhen Dai, Debra Weisenstein, and David Keith. 1/2018. “Tailoring Meridional and Seasonal Radiative Forcing by Sulfate Aerosol Solar Geoengineering.” Geophysical Research Letters, 45. Publisher's VersionAbstract
    We study the possibility of designing solar radiation management schemes to achieve a desired meridional radiative forcing (RF) profile using a two-dimensional chemistry-transport-aerosol model. Varying SO2 or H2SO4 injection latitude, altitude, and season, we compute RF response functions for a broad range of possible injection schemes, finding that linear combinations of these injection cases can roughly achieve RF profiles that have been proposed to accomplish various climate objectives. Globally averaged RF normalized by the sulfur injection rate (the radiative efficacy) is largest for injections at high altitudes, near the equator, and using emission of H2SO4 vapor into an aircraft wake to produce accumulation-mode particles. There is a trade-off between radiative efficacy and control as temporal and spatial control is best achieved with injections at lower altitudes and higher latitudes. These results may inform studies using more realistic models that couple aerosol microphysics, chemistry, and stratospheric dynamics.
    David Keith, Debra Weisenstein, John Dykema, and Frank Keutsch. 12/12/2016. “Stratospheric Solar Geoengineering without Ozone Loss.” Proceedings of the National Academy of Sciences. Publisher's VersionAbstract

    Injecting sulfate aerosol into the stratosphere, the most frequently analyzed proposal for solar geoengineering, may reduce some climate risks, but it would also entail new risks, including ozone loss and heating of the lower tropical stratosphere, which, in turn, would increase water vapor concentration causing additional ozone loss and surface warming. We propose a method for stratospheric aerosol climate modification that uses a solid aerosol composed of alkaline metal salts that will convert hydrogen halides and nitric and sulfuric acids into stable salts to enable stratospheric geoengineering while reducing or reversing ozone depletion. Rather than minimizing reactive effects by reducing surface area using high refractive index materials, this method tailors the chemical reactivity. Specifically, we calculate that injection of calcite (CaCO3) aerosol particles might reduce net radiative forcing while simultaneously increasing column ozone toward its preanthropogenic baseline. A radiative forcing of −1 W⋅m−2, for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y−1 of 275-nm radius calcite aerosol. Moreover, the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol. Although solar geoengineering cannot substitute for emissions cuts, it may supplement them by reducing some of the risks of climate change. Further research on this and similar methods could lead to reductions in risks and improved efficacy of solar geoengineering methods.

    Sebastian D. Eastham, Debra K. Weisenstein, and Steven R. H. Barrett. 2014. “Development and evaluation of the unified tropospheric–stratospheric chemistry extension (UCX) for the global chemistry-transport model GEOS-Chem.” Atmospheric Environment, 89, Pp. 52-63. Publisher's VersionAbstract

    Global chemistry-transport models (CTMs) typically use simplified parameterizations or relaxation to climatology to estimate the chemical behavior of the stratosphere only in the context of its impact on tropospheric chemistry. This limits investigation of stratospheric chemistry and interactions between tropospheric and stratospheric chemistry-transport processes. We incorporate stratospheric chemical and physical processes into the model GEOS-Chem in the form of a unified chemistry extension (UCX). The stratospheric chemistry framework from NASA’s Global Modeling Initiative (GMI) is updated in accordance with JPL 10-06 and combined with GEOS-Chem’s existing widely applied and validated tropospheric chemistry to form a single, unified gas-phase chemistry scheme. Aerosol calculations are extended to include heterogeneous halogen chemistry and the formation, sedimentation and evaporation of polar stratospheric clouds (PSCs) as well as background liquid binary sulfate (LBS) aerosols. The Fast-JX v7.0a photolysis scheme replaces a hybrid of Fast-J and Fast-JX v6.2, allowing photolytic destruction at frequencies relevant to the stratosphere and of species not previously modeled. Finally, new boundary conditions are implemented to cover both surface emissions of new species and mesospheric behavior. Results for four simulation years (2004-2007) are compared to those from the original, tropospheric model and to in situ and satellite-based measurements. We use these comparisons to show that the extended model is capable of modeling stratospheric chemistry efficiently without compromising the accuracy of the model at lower altitudes, perturbing mean OH below 250 hPa by less than 5% while successfully capturing stratospheric behavior not previously captured in GEOS-Chem such as formation and collapse of the Antarctic ozone hole. These extensions (with supporting validation and intercomparison) enable an existing and extensively validated tropospheric CTM to be used to investigate a broader set of atmospheric chemistry problems and leverages GEOS-Chem’s existing tropospheric treatment.