CESM1(WACCM) Stratospheric Aerosol Geoengineering Large Ensemble Project

Abstract

An overview is presented of the GLENS project, a community-wide effort enabling analyses of global and regional changes from stratospheric aerosol geoengineering in the presence of internal climate variability. CESM1(WACCM) STRATOSPHERIC AEROSOL GEOENGINEERING LARGE ENSEMBLE PROJECTSimone TilmeS, Jadwiga H. RicHTeR, Ben KRaviTz, douglaS g. macmaRTin, micHael J. millS, iSla R. SimpSon, anne S. glanville, JoHn T. FaSullo, adam S. pHillipS, Jean-FRancoiS lamaRque, JoSepH TRiBBia, Jim edwaRdS, SHeRi micKelSon, and SiddHaRTHa gHoSH S olar geoengineering using stratospheric sulfate aerosols has been discussed as a potential means of deliberately offsetting some of the effects of climate change (Crutzen 2006). Various model studies have demonstrated that reducing incoming solar radiation globally can offset the increase in global average surface temperature associated with increasing greenhouse gases (e.g., Kravitz et al. 2013). Despite the stabilization of global surface temperature, these simulations show significant changes in atmospheric conditions with global solar reductions or stratospheric sulfur or aerosol injections. Side effects in these simulations include "overcooling" of the tropics and "undercooling" of the poles, leading to continued Arctic summer sea ice loss (e.g., Moore et al. 2014;Tilmes et al. 2016). Additionally, the slowing of the hydrological cycle (e.g., Schmidt et al. 2012) and the potentially uneven cooling between the two hemispheres resulting from solar geoengineering can lead to shifts in precipitation patterns (Haywood et al. 2013; Jones et al. 2017) and reductions in monsoon precipitation (Tilmes et al. 2013). Many available model results to date are based on an artificial design intended to explore the impact of large forcing effects through global solar dimming. For other experiments, only a few ensemble members are performed, making it difficult to identify the robustness of regional climate effects.Simulations of stratospheric sulfate aerosol geoengineering inject sulfur dioxide (SO 2 ) into the stratosphere that oxidizes to form sulfate aerosols or they use direct injections of sulfate aerosols. These experiments require model capabilities beyond those in solar reduction simulations. The stratospheric aerosol distribution resulting from such injections depends on the model's aerosol microphysical scheme, as well as interactions with chemical, dynamical, and radiative processes (Pitari et al. 2014;Mills et al. 2017). Aerosol size and sedimentation are increased with the injection amount and the efficiency of the sulfates to affect the top of the atmosphere radiative imbalance is reduced (Niemeier and Timmreck 2015;Kleinschmitt et al. 2017). The warming of the tropical stratosphere in response to the enhanced aerosol burden results in circulation changes in the stratosphere with potential effects 2361NOVEMBER 2018 AMERICAN METEOROLOGICAL SOCIETY | on the quasi-biennial oscillation (QBO; Aquila et al. 2014), as well as impacts on the tropospheric circulation (Richter et al. 2018). Chan...

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

“…The analyses in this study were conducted on three ensembles of simulations, all generated with the 1° horizontal resolution version of CESM1(WACCM) as described by Mills et al (), with the exception of an updated land model, as described by Tilmes et al (). The atmospheric component of this model has 70 vertical layers extending up to 140 km.…”

“…Three of those ensemble members were extended out past 2095. In the second ensemble, hereafter called the Geoengineering Large Ensemble (GLENS; Tilmes et al, ), SO 2 was injected at four independent locations: 30°N, 15°N, 15°S, and 30°S (each into a single grid box at 180°E, at the model level ≈5 km above the annual mean tropopause). These locations were chosen because MacMartin et al () found that they are sufficient to meet the chosen objectives of GLENS.…”

“…This results in excellent success in maintaining T 0 at baseline levels (Figure ). T 1 has a slight positive bias, but the ensemble mean is still within the ensemble spread of the reference period (Tilmes et al, ). GLENS shows some bias in T 2 due to saturation in meeting T 0 and T 1 (MacMartin et al, ): meeting T 2 while also meeting T 0 and T 1 would have called for negative SO 2 injection at 15°S, which is impossible.…”

Timescale for Detecting the Climate Response to Stratospheric Aerosol Geoengineering

“…The GLENS simulations are described in detail by Tilmes, Richter, Kravitz, et al (). Simulations were conducted with the Community Earth System Model version 1, with the Whole Atmosphere Community Climate Model as its atmospheric component, CESM1(WACCM).…”

“…We estimate the climate response to stratospheric aerosol geoengineering and the resulting detection timescales using simulations from the Geoengineering Large Ensemble Project (GLENS), described by Tilmes, Richter, Kravitz, et al (). These simulations include three features that are important here.…”