The Choice of Baseline Period Influences the Assessments of the Outcomes of Stratospheric Aerosol Injection

Abstract

The specifics of the simulated injection choices in the case of stratospheric aerosol injections (SAI) are part of the crucial context necessary for meaningfully discussing the impacts that a deployment of SAI would have on the planet. One of the main choices is the desired amount of cooling that the injections are aiming to achieve. Previous SAI simulations have usually either simulated a fixed amount of injection, resulting in a fixed amount of warming being offset, or have specified one target temperature, so that the amount of cooling is only dependent on the underlying trajectory of greenhouse gases. Here, we use three sets of SAI simulations achieving different amounts of global mean surface cooling while following a middle‐of‐the‐road greenhouse gas emission trajectory: one SAI scenario maintains temperatures at 1.5°C above preindustrial levels (PI), and two other scenarios which achieve additional cooling to 1.0°C and 0.5°C above PI. We demonstrate that various surface impacts scale proportionally with respect to the amount of cooling, such as global mean precipitation changes, changes to the Atlantic Meridional Overturning Circulation and to the Walker Cell. We also highlight the importance of the choice of the baseline period when comparing the SAI responses to one another and to the greenhouse gas emission pathway. This analysis leads to policy‐relevant discussions around the concept of a reference period altogether, and to what constitutes a relevant, or significant, change produced by SAI.

Stratospheric Aerosol Injection Can Reduce Risks to Antarctic Ice Loss Depending on Injection Location and Amount

“…In summary, the surface ice accumulation anomalies shown in the NH single‐latitude or multi‐latitude SAI cases with respect to the Historical simulation are largely driven by precipitation increases. These precipitation increases are due to the enhanced hydrological cycle from the background greenhouse warming relative to the Historical that is not completely offset by these SAI cases (Visioni et al., 2023). Moreover, the regional ice accumulation variability is consistent with changes to the SAM index or longitudinal shifts of the ASL via the PSA teleconnection.…”

“…The first set of SAI cases, consisting of seven simulation setups, each inject SO 2 at every timestep at a single latitude into the lower stratosphere at a constant rate equivalent to 12 Tg-SO 2 per year (Table 1). The first five of these simulations, which inject SO 2 year-round at ∼21.5 km altitude and either at 30°N, 15°N, 0°N, 15°S, or 30°S, respectively, were introduced in Visioni et al (2023) and Bednarz et al (2023) and used to examine the response of the SAM to SAI in Bednarz et al (2022). Additionally, we introduce two other single-latitude injection cases that inject at 60°S during the austral spring (SON) or at 60°N during the boreal spring (MAM) at about 15 km in altitude.…”

“…The single-latitude injection cases, with a fixed amount injected per year, serve to more easily diagnose changes to the climate system by considering one location at a time. Their purpose is not to illustrate a desirable deployment strategy; rather, they serve as a "step-function" response which can inform more complex strategies that rely on a combination of multiple injection locations and use time-varying injection rates to maintain one or more climate targets, assuming a certain linearity of the system and additivity between different locations (Visioni et al, 2023). Bednarz et al (2022) showed the potential of these single-latitude injection simulations to explore physical mechanisms driving the SAI responses and to explain changes in another set of multi-latitude injection simulations for one particular climate driver, the SAM.…”

“…The first five of these simulations, which inject SO 2 year‐round at ∼21.5 km altitude and either at 30°N, 15°N, 0°N, 15°S, or 30°S, respectively, were introduced in Visioni et al. (2023) and Bednarz et al. (2023) and used to examine the response of the SAM to SAI in Bednarz et al.…”

“…These four SAI cases are abbreviated as Global+1.5, Global+1.0, Global+0.5, and Polar+1.0. For these cases, the total amount of SO 2 injected per year averaged for years 2050–2069 is about 8.6, 17.0, 25.6, and 20.4 Tg‐SO 2 yr −1 , respectively (Visioni et al., 2023; Zhang et al., 2023).…”