Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer

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. In this study, we isolate and assess the sensitivity of stratospheric aerosol radiative forcing and the resulting climate change to the altitude of the aerosol layer. 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 the 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 the altitude of the aerosol layer both because aerosols higher in the stratosphere have larger effective radiative forcing and because they have higher stratospheric residence time; these two effects are likely to be of comparable importance.

The Climatic Effects of Hygroscopic Growth of Sulfate Aerosols in the Stratosphere

“…In CAM4, the background aerosols absorb only near-infrared (near-IR) solar radiation whereas the volcanic aerosols absorb both near-IR solar and terrestrial radiation. Thus, volcanic aerosols absorb more radiative energy and cause more radiative heating in the stratosphere, and the related fast adjustments are larger in Krishnamohan et al (2019) than our nHG experiments. However, large differences in radiative forcing are simulated among the HG experiments: radiative forcing is more negative when sulfate aerosols are prescribed in the lower levels of the stratosphere.…”

“…When aerosols are prescribed at lower levels, this additional negative radiative forcing leads to more surface cooling and more reduction in precipitation. Other fast adjustment processes such as an increase in water vapor in the stratosphere and cloud changes add to the altitude sensitivity of radiative forcing as identified in Krishnamohan et al (2019), but these fast adjustment effects are much smaller for background aerosols. The hygroscopic growth is also found to alter the direct and diffuse radiation availability at the surface and thus the primary productivity.…”

“…When aerosols are prescribed at 37 hPa, the temperature change is almost the same in both HG and nHG cases. The radiative warming due to background sulfate aerosols is less compared to the volcanic aerosols as simulated in Krishnamohan et al (2019) where the maximum warming was about 10 K. The difference is primarily associated with the aerosol absorption characteristics: in CAM4, the volcanic aerosols absorb in both SW near-IR and LW bands, whereas background sulfate aerosols absorb only SW near-IR radiation. In the slab ocean experiments,…”

“…We follow the "two-point method" as described in Modak et al (2018), Duan et al (2018), and Krishnamohan et al (2019) for the estimation of ERF. The "two-point method" is consistent with the Intergovernmental Panel on Climate Change (Myhre et al, 2013) general definition of "the radiative forcing with rapid adjustments accounted for."…”

“…However, the effects of changes in climate and partition between direct and diffuse radiation are included. In the nHG cases, changes in direct and diffuse component are larger when aerosols are prescribed at higher levels (Table 2) as in Krishnamohan et al (2019) but the sensitivity is too small as we consider background sulfate aerosols in this study. The carbon 10.1029/2019EF001326 cycle variables also show sensitivity similar to but weaker than in Krishnamohan et al (2019).…”