1H: Advancements in geoengineering research using WACCM

The Whole Atmospheric Community Climate Model (WACCM) has been updated to investigate advanced methods of solar geoengineering using stratospheric sulfur injections. Researchers from ACOM and CGD, in partnership with scientists at Cornell University and Pacific Northwest National Laboratory, have demonstrated that even under an extreme emissions scenario, global temperatures and North-to-South and interhemispheric temperature gradients could be kept to 2020 conditions by employing a feedback algorithm in WACCM. This new geoengineering approach offset the greenhouse warming of the planet more evenly, potentially reducing side effects and risks of geoengineering compared to earlier studies (Kravitz et al., 2017). This effort has produced a series of studies, including research on system identification of WACCM, to investigate the climate response of single point stratospheric sulfur injections (Tilmes et al., 2017), research on designing a feedback algorithm using information of multiple sulfur injection simulations (MacMartin et al., 2017), and research on dynamical changes due to stratospheric sulfur injections (Richter et al., 2017).

The studies use a newly developed WACCM version, updated to run at a 0.95°x1.25° horizontal resolution (Mills et al., 2017). WACCM now also includes a prognostic stratospheric aerosol based on emissions, which required the addition of sulfur chemistry and modification of modal aerosol microphysics (Mills et al., 2016). These prognostic aerosols interact with gas-phase species in WACCM via heterogeneous chemistry, which is important to assessing the impacts of geoengineering on stratospheric ozone. Additionally, the model includes now an internally generated quasi-biennial oscillation (QBO), which can be altered with geoengineering applications. WACCM includes complex interactions of chemistry, aerosol-microphysics, radiation and dynamics, and is coupled to other Earth System components, including land, sea-ice, and ocean. WACCM is currently the most complex model to simulate the impact of stratospheric aerosol geoengineering. Simulations of the 1991 eruption of Mt. Pinatubo with the prognostic stratospheric aerosol derived from emissions show good agreement with observations of aerosol properties and radiative impacts. The model further reproduces observed stratospheric chemistry well, including water vapor and column ozone in comparison to satellite and ground based observations.

Figure 1: Top-of-model all-sky radiative fluxes from our SDVOLC (solid red) and SDVC (solid blue) simulations are compared to top-of-atmosphere ERBS observations (black) merged with additional data to provide a global dataset [Allan et al., 2014; Liu et al., 2015]. Monthly mean net fluxes are shown for January 1991 to December 1995, normalized and de-seasonalized by subtracting the corresponding flux for each month from 1999, a volcanically quiescent year. Fluxes from our FRVOLC ensemble average (dashed orange line) and range (yellow shading) are also shown. (a) Absorbed solar radiation (positive for downward fluxes); (b) outgoing longwave radiation (positive for upward fluxes); (c) net radiative flux (positive for downward fluxes).

References:

Mills, M. J. et al. (2016), Global volcanic aerosol properties derived from emissions, 1990-2014, using CESM1(WACCM), J Geophys Res-Atmos, 121(5), 2332–2348, doi:10.1002/2015jd024290.

Mills, M. J., Richter, J. H., Tilmes, S., Kravitz, B., MacMartin, D. G., Glanville, A. A., … Kinnison, D. E. (2017). Radiative and chemical response to interactive stratospheric sulfate aerosols in fully coupled CESM1(WACCM). Journal of Geophysical Research: Atmospheres, 122. https://doi.org/10.1002/2017JD027006

Tilmes S., Richter J. H., Mills M. J., Kravitz B., MacMartin D. G., Vitt F., ... Lamarque J.-F. (2017). Sensitivity of aerosol dis- tribution and climate response to stratospheric SO2 injection locations. Journal of Geophysical Research: Atmospheres, 122, 1–25. https://doi.org/10.1002/2017JD026888

MacMartin, D. G., Kravitz, B., Tilmes, S., Richter, J. H., Mills, M. J., Lamarque, J.-F., Tribbia, J. J., & Vitt, F. (2017). The climate response to stratospheric aerosol geoengineering can be tailored using multiple-injection locations. Journal of Geophysical Research: Atmospheres, 122, 1–17. https://doi.org/10.1002/2017JD026868

Richter J. H., Tilmes S., Mills M. J., Kravitz B., MacMartin D. G., Vitt F., ... Lamarque J.-F. (2017). Stratospheric Dynamical Response and Ozone Feedbacks in the Presence of SO2 Injections, DOI: 10.1002/2017JD026912

Kravitz, B., MacMartin, D. G., Mills, M. J., Richter, J. H., Tilmes, S., Lamarque, J.-F., ... Vitt, F. (2017). First simulations of designing stratospheric sulfate aerosol geoengineering to meet multiple simultaneous climate objectives. Journal of Geophysical Research: Atmospheres, 122, 1–19. https://doi.org/10.1002/2017JD026874

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