Assessing Responses and Impacts of Solar climate intervention on 1 the Earth system with stratospheric aerosol injection (ARISE-2 SAI): protocol and initial results from the first simulations

. Solar climate intervention using stratospheric aerosol injection is a proposed method of reducing global 13 mean temperatures to reduce the worst consequences of climate change. A detailed assessment of responses and 14 impacts of such an intervention is needed with multiple global models to support societal decisions regarding the use 15 of these approaches to help address climate change. We present here a new modeling protocol aimed at simulating a 16 plausible deployment of stratospheric aerosol injection and reproducibility of simulations using other Earth system 17 models, Assessing Responses and Impacts of Solar climate intervention on the Earth system with stratospheric aerosol 18 injection (ARISE-SAI). The protocol and simulations are aimed at enabling community assessment of responses of 19 the Earth system to solar climate intervention. ARISE-SAI simulations are designed to be more policy relevant than 20 existing large ensembles or multi-model simulation sets. We describe in detail the first set of ARISE-SAI simulations, 21 ARISE-SAI-1.5, which utilize a moderate emissions scenario, introduce stratospheric aerosol injection at ~ 21.5 km 22 in year 2035, and keep global mean surface air temperature near 1.5 o C above the pre-industrial value utilizing a 23 feedback or control algorithm. We present here the detailed set-up, aerosol injection strategy, and preliminary climate 24 analysis from a 10-member ensemble of these simulations carried out with the Community Earth System Model, 25 version 2 with the Whole Atmosphere Community Climate Model version 6 as its atmospheric component.

aims to mimic the effects of volcanic eruptions on climate, has been shown to be a promising method of global climate intervention in terms of restoring climate to present day conditions in global climate or Earth system models (e.g.: al., 2021) or prescription of an aerosol distribution (Tilmes et al., 2015) or a spatially uniform injection rate of SO₂ 48 (i.e. continuous injection from 10ºN to 10ºS in the most recent G6sulfur experiments (Visioni et al., 2021b). Visioni in 2020, adding another unrealistic element from a real-world standpoint. Furthermore, SO2 injections were at 23-25 km altitude, which is technologically more difficult to achieve than a lower altitude injection (Bingaman et al. 2020).
70 Tilmes et al. (2020) has carried out simulations with SO 2 injections with CESM2(WACCM6) and GLENS-71 like set-up for the Shared Socioeconomic Pathway SSP5-8.5 and SSP5-3.4-OS scenarios (O'Neill et al., 2016). Here 72 we propose a new SAI modeling protocol for a suite of simulations designed to simulate a more plausible 73 implementation scenario of SCI using SAI that can be replicated by other modeling centers. We denote the entire set 74 of current and future simulations conducted under this protocol as "Assessing Responses and Impacts of Solar climate 75 intervention on the Earth system," or "ARISE," with simulations of SAI denoted "ARISE-SAI". We anticipate that in

Reference Simulations
SAI simulations utilize a moderate emission scenario, SSP2-4.5 (O'Neill et al., 2016) and cool the Earth to a global mean temperature target (TT) above preindustrial levels denoted in the specific name of the simulations (e.g.: ARISE- The injection latitudes are the same as used in GLENS and in previous studies examining the model's responses to 138 single-point SO2 injections (Tilmes et al., 2017;Richter et al., 2017). These four injection locations are sufficient to 139 independently control the targets that we are trying to achieve (Kravitz et al., 2017). These four injection locations 140 have also been demonstrated to be sufficient to produce the optical depth patterns that independently control the targets 141 that we are trying to achieve in various versions of CESM(WACCM) (MacMartin et al., 2017;Zhang et al., 2022; technologies that could be adapted for climate intervention use (Bingaman et al., 2020). After each year of simulation, 144 the algorithm calculates the global mean temperature, T0, north-south temperature gradient, T1, and equator-to-pole

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Comprehensive monthly output as well as high-frequency output for analysis of high-impact events (described in 150 detail in the Data Records section) is needed for analysis of SCI impacts on the Earth System. Acknowledging 151 limitations of various modeling centers, we recommended a minimum set of monthly-mean output fields in Table A1 152 in the Data Records section and include the full comprehensive output list that was created with the CESM2(WACCM) 153 simulations based on input from the broader community. All model output for the simulations should be provided in  et al., 2002; 2004; 2013). CESM2 uses version 3.14 of 208 the NOAA WaveWatch-III ocean surface wave prediction model (Tolman, 2009

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The first five members of ARISE-SAI-1.5 simulations were initialized in 2035 from the first five members 237 (001 to 005) of the SSP2-4.5 simulations carried out with CESM2(WACCM6); hence, all had different initial ocean, 238 sea-ice, land, and atmospheric initial conditions on January 1, 2035. Similarly to the SSP2-4.5 simulations, subsequent

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One of the intents of ARISE-SAI simulations is to provide the broader community a data set for examining various 262 impacts of SCI on the multiple components of the Earth system. Below we present basic diagnostics that verify that

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The zonally    overall decrease in annual mean precipitation (Fig 5), however regionally both increases and decreases occur. In 363 ARISE-SAI-1.5, the increased precipitation across the Equatorial Pacific seen in SSP2-4.5 decreases in magnitude,

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but is still a persistent feature. ARISE-SAI-1.5 also shows drying north and south of that region as well as intensified 365 drying over Northern South America, South Africa, Indian Ocean south of the Equator and northernmost Australia.

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The Indian Ocean north of the Equator and India are projected to be wetter in ARISE-SAI-1.5 as compared to the

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SAI-1.5 is very similar to that during the reference period (2020 -2039), however residual changes still remain, in 394 particular in the North Atlantic, where surface temperature is cooler than in the reference period. The robustness of 395 these projected regional residuals in other climate models, or under different climate targets, would also be of extreme 396 interest. Consistent with prior studies, global mean precipitation in ARISE-SAI-1.5 is smaller than during the reference 397 period.

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The output for the ARISE-SAI-1.5 simulations is extensive and includes variables from