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

Abstract

Injection of SO2 into the stratosphere has been proposed as a method to, in part, counteract anthropogenic climate change. So far, most studies investigated injections at the equator or in a region in the tropics. Here we use Community Earth System Model version 1 Whole Atmosphere Community Climate Model (CESM1(WACCM)) to explore the impact of continuous single grid point SO2 injections at seven different latitudes and two altitudes in the stratosphere on aerosol distribution and climate. For each of the 14 locations, 3 different constant SO2 emission rates were tested to identify linearity in aerosol burden, aerosol optical depth, and climate effects. We found that injections at 15°N and 15°S and at 25 km altitude have equal or greater effect on radiation and surface temperature than injections at the equator. Nonequatorial injections transport SO2 and sulfate aerosols more efficiently into middle and high latitudes and result in particles of smaller effective radius and larger aerosol burden in middle and high latitudes. Injections at 15°S produce the largest increase in global average aerosol optical depth and increase the change in radiative forcing per Tg SO2/yr by about 15% compared to equatorial injections. High‐altitude injections at 15°N produce the largest reduction in global average temperature of 0.2° per Tg S/yr for the last 7 years of a 10 year experiment. Injections at higher altitude are generally more efficient at reducing surface temperature, with the exception of large equatorial injections of at least 12 Tg SO2/yr. These findings have important implications for designing a strategy to counteract global climate change.

The Climate Response to Stratospheric Aerosol Geoengineering Can Be Tailored Using Multiple Injection Locations

“…A comparison between the linear prediction and actual simulation is shown for several cases in Figure ; this figure also illustrates the zonal‐mean response patterns that result from some of the single‐latitude injection cases. As described by Tilmes et al (), the dominant mechanism of nonlinearity is due to increased aerosol size through condensation and coagulation close to the injection point in the region of new particle formation. Figure (left column) corresponds to simultaneous injection of 12 TgSO 2 per year each at 15°N and 15°S.…”

“…The spatial distribution of aerosols that results from small‐amplitude injection will depend primarily on the baseline stratospheric circulation. However, as the injection rate increases, the aerosol size, lifetime, and spatial distribution will shift due to nonlinearities (Tilmes et al, ). There are two main sources of nonlinearity: higher SO 2 injection rates lead to larger aerosols and hence a reduction in AOD per unit injection, and aerosol heating and other radiative and chemical interactions will change stratospheric circulation and transport, altering the spatial distribution as the injection rate increases.…”

“…Figure (left column) corresponds to simultaneous injection of 12 TgSO 2 per year each at 15°N and 15°S. When injecting only in one hemisphere, new particle formation occurs primarily in that hemisphere (Tilmes et al, ), and thus, there is very little interaction between the injections in each hemisphere in this case, and the total AOD is roughly the sum of the AODs from each injection separately. Figure (middle column) corresponds to simultaneous injection of 12 Tg SO 2 per year each at 15°N and 30°N.…”

“…One of the key questions is what the climate effects would be, including concern over differential regional effects (Kravitz et al, ; Ricke et al, ). However, one can choose not only how much aerosol to add but also where (e.g., latitude and altitude), and the choice of injection location influences the resulting climate (Tilmes et al, ). This motivates the need to explore the design space of stratospheric‐aerosol geoengineering, and more fundamentally, to explicitly treat geoengineering as a design problem, bringing an engineering perspective into research on solar geoengineering.…”

Upper tropospheric ice sensitivity to sulfate geoengineering

“…Different injection scenarios have been proposed and adopted in modelling experiments, the most used being the one with a constant sulfur injection rate at the equator for a certain number of years, in order to understand the climate response to such an atmospheric perturbation. Simulations have also been performed to identify the magnitude and location of the sulfur injection, in order to obtain the highest ratio between radiative forcing (RF) and injection magnitude (Niemeier and Schmidt (2017); Tilmes et al (2017) ;Kleinschmitt 10 et al (2017)). …”