First Simulations of Designing Stratospheric Sulfate Aerosol Geoengineering to Meet Multiple Simultaneous Climate Objectives

Abstract

We describe the first simulations of stratospheric sulfate aerosol geoengineering using multiple injection locations to meet multiple simultaneous surface temperature objectives. Simulations were performed using CESM1(WACCM), a coupled atmosphere‐ocean general circulation model with fully interactive stratospheric chemistry, dynamics (including an internally generated quasi‐biennial oscillation), and a sophisticated treatment of sulfate aerosol formation, microphysical growth, and deposition. The objectives are defined as maintaining three temperature features at their 2020 levels against a background of the RCP8.5 scenario over the period 2020–2099. These objectives are met using a feedback mechanism in which the rate of sulfur dioxide injection at each of the four locations is adjusted independently every year of simulation. Even in the presence of uncertainties, nonlinearities, and variability, the objectives are met, predominantly by SO2 injection at 30°N and 30°S. By the last year of simulation, the feedback algorithm calls for a total injection rate of 51 Tg SO2 per year. The injections are not in the tropics, which results in a greater degree of linearity of the surface climate response with injection amount than has been found in many previous studies using injection at the equator. Because the objectives are defined in terms of annual mean temperature, the required geongineering results in “overcooling” during summer and “undercooling” during winter. The hydrological cycle is also suppressed as compared to the reference values corresponding to the year 2020. The demonstration we describe in this study is an important step toward understanding what geoengineering can do and what it cannot do.

Sensitivity of Aerosol Distribution and Climate Response to Stratospheric SO2 Injection Locations

“…Newly formed particles coagulate with existing larger particles, which continuously increases the particle size with SO 2 emission amount. The resulting increase in sedimentation and reduced reflectivity of larger particles leads to reduced scattering of sunlight and efficiency to cool the climate per Tg S injection (e.g., Kravitz et al, ; Niemeier & Timmreck, ; Visioni et al, ).…”

Brief communication: Understanding solar geoengineering's potential to limit sea level rise requires attention from cryosphere experts

“…For example, whilst a uniform reduction in incoming sunlight would not offset all warming at high latitudes, stratospheric aerosol geoengineering could be deployed to produce a thicker aerosol cloud at high latitudes to reduce high-latitude temperatures in line with global mean temperatures or to cool them further (Dai et al, 2018;Kravitz et al, 2018). However, it is important to note that the effects of solar geoengineering cannot be limited to the area of application and there would be remote impacts even if strato- spheric aerosol geoengineering was limited just to polar regions .…”

Upper tropospheric ice sensitivity to sulfate geoengineering

“…This reduction is larger than the one in G4K relative to the Base case (∆τ =-0.020, -5%), pointing out to the dominant and controlling role of the reduced updraft injected, especially if such efficiency is used to determine the amount of SO 2 that needs to be injected in the stratosphere to achieve climate targets ; Kravitz et al (2017)). …”

“…(2017); Kravitz et al (2017)). However, to properly do so, a clear understanding is needed of how multiple side effects of this technique can modify the net RF (Visioni et al, 2017a).…”

“…Understanding the RF contribution of the UT ice perturbation in a SG scenario is particularly crucial if the scientific community wants to design experiments whose goal is to meet a given climate target, as proposed in Kravitz et al (2017) and MacMartin et al (2017).…”