Abstract
Abstract. Sustained injection of sulfur dioxide (SO2) in the tropical lower stratosphere has been proposed as a climate engineering technique for the coming decades. Among several possible environmental side effects, the increase in sulfur deposition deserves additional investigation. In this study we present results from a composition–climate coupled model (University of L'Aquila Composition-Chemistry Model, ULAQ-CCM) and a chemistry-transport model (Goddard Earth Observing System Chemistry-Transport Model, GEOS-Chem), assuming a sustained lower-stratospheric equatorial injection of 8 Tg SO2 yr−1. Total S deposition is found to globally increase by 5.2 % when sulfate geoengineering is deployed, with a clear interhemispheric asymmetry (+3.8 and +10.3 % in the Northern Hemisphere (NH) and the Southern Hemisphere (SH), due to +2.2 and +1.8 Tg S yr−1, respectively). The two models show good consistency, both globally and on a regional scale under background and geoengineering conditions, except for S-deposition changes over Africa and the Arctic. The consistency exists with regard to time-averaged values but also with regard to monthly and interannual deposition changes. The latter is driven essentially by the variability in stratospheric large-scale transport associated with the quasi-biennial oscillation (QBO). Using an externally nudged QBO, it is shown how a zonal wind E shear favors aerosol confinement in the tropical pipe and a significant increase in their effective radius (+13 % with respect to W shear conditions). The net result is an increase in the downward cross-tropopause S flux over the tropics with dominant E shear conditions with respect to W shear periods (+0.61 Tg S yr−1, +42 %, mostly due to enhanced aerosol gravitational settling) and a decrease over the extratropics (−0.86 Tg S yr−1, −35 %, mostly due to decreased large-scale stratosphere–troposphere exchange of geoengineering sulfate). This translates into S-deposition changes that are significantly different under opposite QBO wind shears, with an E–W anomaly of +0.32 in the tropics and −0.67 Tg S yr−1 in the extratropics. Most online QBO schemes predict a significant change in the zonal wind periodicity, up to a blocked E shear condition for large enough injections, so that our results indicate an upper limit for the tropical increase in S deposition of 16.5 % relative to average conditions of unperturbed QBO periodicity and a correspondent extratropical S deposition decrease of 16 %.
Highlights
The evidence of the increase in greenhouse gases (GHGs) due to anthropogenic emissions and the subsequent increase in surface temperatures has started discussions on the possibility of temporarily altering the climate to alleviate some of the consequences
In the case of sulfate geoengineering (SG), the main effect of cooling the planet could surely be achieved to some extent if the sulfate was injected into the tropical stratosphere, and we are assured of this both by looking at explosive volcanic eruptions and their effect on climate and by many results from the Geoengineering Model Intercomparison Project (GeoMIP) project, which come from a vast array of simulations from independent models (Kravitz et al, 2012; Visioni et al, 2017a)
In this study we focused on the SG impact on the surface deposition of sulfur in the case of an injection of 8 Tg SO2 yr−1 simulated in two globalscale models: ULAQ-chemistry–climate model (CCM) and Goddard Earth Observation System (GEOS)-Chem
Summary
The evidence of the increase in greenhouse gases (GHGs) due to anthropogenic emissions and the subsequent increase in surface temperatures has started discussions on the possibility of temporarily altering the climate to alleviate some of the consequences. H2SO4 formed through OH oxidation of the initial volcanic SO2 cloud injected into the stratosphere (McCormick and Veiga, 1992; Lambert et al, 1993; Long and Stowe, 1994) These gas-particle microphysical processes, coupled to additional aerosol growth due to coagulation, produce an optically active cloud which is highly reflective in the visible and UV, causing a substantial decrease in solar radiation reaching the Earth surface and, subsequently, a global surface cooling. In the second part we analyze how the lifetime of geoengineering sulfate aerosols can be correlated to stratospheric circulation changes and different QBO phases, in order to better understand the mechanisms regulating the sulfur deposition time variability. Tion in the case SG aerosol heating rates are allowed to feedback on the QBO itself
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