Reduced Poleward Transport Due to Stratospheric Heating Under Stratospheric Aerosols Geoengineering

Abstract

By injecting SO2 into the stratosphere at four latitudes (30°, 15°N/S), it might be possible not only to reduce global mean surface temperature but also to minimize changes in the equator‐to‐pole and inter‐hemispheric gradients of temperature, further reducing some of the impacts arising from climate change relative to equatorial injection. This can happen only if the aerosols are transported to higher latitudes by the stratospheric circulation, ensuring that a greater part of the solar radiation is reflected back to space at higher latitudes, compensating for the reduced sunlight. However, the stratospheric heating produced by these aerosols modifies the circulation and strengthens the stratospheric polar vortex which acts as a barrier to the transport of air toward the poles. We show how the heating results in a feedback where increasing injection rates lead to stronger high‐latitudinal transport barriers. This implies a potential limitation in the high‐latitude aerosol burden and subsequent cooling.

Is Turning Down the Sun a Good Proxy for Stratospheric Sulfate Geoengineering?

“…For the 3 × 3 cases, this effect is less pronounced, since the injection locations are chosen so as to have a similar profile to the one actually achieved by the solar dimming (MacMartin et al, 2017). At very high latitudes in both hemispheres, however, some differences are present mostly due to the polar transport barriers (Visioni, MacMartin, Kravitz, Lee, et al, 2020) that reduce the high-latitude AOD. It is likely that a more uniform AOD distribution using more latitudes of injection (see for instance Dai et al, 2018) could produce results more closely resembling those from 1 × 1 SD: however, some differences would still remain due to the considerable variation across different months of the AOD (Figure 2) compared to the constant dimming produced by the SD cases: as shown by , seasonal variations in AOD can result in notably different surface climates.…”

Sensitivity of Total Column Ozone to Stratospheric Sulfur Injection Strategies

“…This results in increased radiative heating of the tropopause and with that in even larger increases in water vapor compared to GLENS and Low, and more pronounced changes in the HO x E -ozone destroying cycle. The equatorial injections further result in a stronger confinement of air masses in the tropical pipe (Niemeier & Schmidt, 2017;Visioni et al, 2020;Kravitz et al, 2019) and a stronger and colder polar vortex. Increases in ozone destroying catalytic cycles and reduced horizontal transport result in the slowest TCO recovery during SH spring towards the end of the century compared to the other experiments (Figures 3 and 4b) and also results in larger increases in NH mid-latitude TCO in January (Figure S9).…”

The Overlooked Role of the Stratosphere Under a Solar Constant Reduction

“…Since atmospheric temperatures and winds are closely related via the thermal wind balance, this strengthens the stratospheric jets in each hemisphere (Figures 2 and S3 in Supporting Information S1). Such strengthening of the stratospheric jets has now been recognized as an important part of the atmospheric response to SAI (e.g., Richter et al, 2017;Simpson et al, 2019;Visioni et al, 2020;Banerjee et al, 2021), which would otherwise not be reproduced by the reduction in the solar of 10 constant itself (Ferraro et al, 2015;. In late austral spring, the SH westerly response likely corresponds to delayed break-up of the polar vortex (Figure 2a.…”

“…However, multiple studies showed that in several respects reducing the solar constant is not representative of atmospheric impacts from geoengineering using SAI. Namely, solar dimming simulations do not include the aerosol-driven warming of the tropical lower stratosphere and its impacts on stratospheric and tropospheric circulation (Ferraro et al, 2015;McCusker et al, 2015;Simpson et al, 2019;Visioni et al, 2020), nor accelerated heterogeneous lower stratospheric ozone depletion (Tilmes et al, 2008), nor the increase in the ratio between the scattered to direct shortwave radiation in the troposphere (Kalidindi et al, 2015;Xia et al, 2017).…”