Climate System Response to Stratospheric Sulfate Aerosols: Sensitivity to Altitude of Aerosol Layer

Abstract

Abstract. Reduction of surface temperatures of the planet by injecting sulfate aerosols in the stratosphere has been suggested as an option to reduce the amount of human-induced climate warming. Several previous studies have shown that for a specified amount of injection, aerosols injected at a higher altitude in the stratosphere would produce more cooling because aerosol sedimentation would take longer time. In this study, we isolate and assess the sensitivity to the altitude of the aerosol layer of stratospheric aerosol radiative forcing and the resulting climate change. We study this by prescribing a specified amount of sulfate aerosols, of a size typical of what is produced by volcanoes, distributed uniformly at different levels in the stratosphere. We find that stratospheric sulfate aerosols are more effective in cooling climate when they reside higher in the stratosphere. We explain this sensitivity in terms of effective radiative forcing: volcanic aerosols heat the stratospheric layers where they reside, altering stratospheric water vapor content, tropospheric stability and clouds, and consequently the effective radiative forcing. We show that the magnitude of the effective radiative forcing is larger when aerosols are prescribed at higher altitudes and the differences in radiative forcing due to fast adjustment processes can account for a substantial part of the dependence of amount of cooling on aerosol altitude. These altitude effects would be additional to dependences on aerosol microphysics, transport, and sedimentation, which are outside the scope of this study. The cooling effectiveness of stratospheric sulfate aerosols likely increases with altitude of the aerosol layer both because aerosols higher in the stratosphere have larger effective radiative forcing and because they have a longer stratospheric residence time; these two effects are likely to be of comparable importance.

A Model‐Based Investigation of Terrestrial Plant Carbon Uptake Response to Four Radiation Modification Approaches

“…Studies have shown that multiple mitigation targets could be achieved by adjusting the injection latitude and amount of sulfate aerosol injections (e.g., Kravitz et al, 2017; MacMartin et al, 2017; Tilmes et al, 2017). Changes in the aerosol distributions could alter the regional climate response and affect the land carbon response (e.g., Krishna‐Pillai Sukumara‐Pillai et al, 2019; Tilmes et al, 2017). Changes in the aerosol size may also affect the scattering effect of aerosol particles (Krishnamohan et al, 2020) and influence partitioning of solar radiation to direct and diffuse components (Eck et al, ).…”

Climate Response to Pulse Versus Sustained Stratospheric Aerosol Forcing

“…In addition, the climate response will be different for volcanic eruptions at different seasons or latitudes as high‐latitude eruptions would cause large hemispheric energy imbalance (e.g., Haywood et al, ; Pausata et al, ; Stevenson et al, ). Different deployment strategies of stratospheric aerosol geoengineering would also cause different climate changes (e.g., Kleinschmitt et al, ; Krishna‐Pillai Sukumara‐Pillai et al, ; Simone Tilmes et al, ). Robock et al () emphasized that the observed climate change from volcanic eruptions cannot be directly used to infer the climate response of continuous stratospheric sulfate aerosol injection geoengineering.…”

“…Studies have shown that the addition of sulfate aerosols into the stratosphere could offset the CO 2 -induced global mean warming but usually at the cost of a reduced intensity of global hydrological cycle mainly associated with the fast climate adjustments (e.g., Duan et al, 2018;Ferraro & Griffiths, 2016;Kalidindi et al, 2015;Myhre et al, 2018). Also, stratospheric aerosol geoengineering can induce changes in stratospheric ozone concentration and large-scale stratospheric circulation due to the change in stratospheric temperature, water vapor content, and heating rate (Ferraro et al, 2015;Kalidindi et al, 2015;Krishna-Pillai Sukumara-Pillai et al, 2019;Madronich et al, 2018;Nalam et al, 2017;Pitari et al, 2014;Richter et al, 2017;Simone Tilmes et al, 2009). A number of recent studies have shown that multiple temperature goals could be achieved by interactively adjusting the location and rate of sulfate aerosol injections, indicating the possibility to achieve multiple regional climate mitigation goals (e.g., Kravitz the climate response, SAI geoengineering has been simulated with multiple climate models using the same experimental protocol under the framework of the Geoengineering Model Intercomparison Project (e.g., Kravitz et al, 2011Kravitz et al, , 2015.…”

Uncertainty and the basis for confidence in solar geoengineering research

“…Process-level studies can reveal mechanistic understanding; for example, simulations that prescribe or turn off longwave radiative heating 47,82 can explore the effects of stratospheric heating on surface climate, revealing and isolating mechanisms of change. Idealized simulations investigating how the altitude of injection influences stratospheric heating may further inform the importance of these effects 83 . Model intercomparisons (for example, the Geoengineering Model Intercomparison Project (GeoMIP) 84 ) also offer useful insight, particularly with understanding the similarities and differences between responses to standardized modelling experiments.…”