Quantification of sulfur deposition changes under sulfate geoengineering conditions

Abstract

Abstract.Sustained injection of sulfur dioxide (SO 2 ) in the tropical lower stratosphere has been proposed as a climate engineering technique with the purpose of temporarily mitigating the surface warming predicted for the coming decades. Among several possible environmental side effects, the increase of sulfur deposition at the ground surface still needs to be thoroughly investigated. In this study we present results from a composition-climate coupled model (ULAQ-CCM) and a chemistry-transport 5 model (GEOS-Chem), assuming a sustained lower stratospheric equatorial injection of 8 Tg-SO 2 /yr. 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 NH and SH, respectively). The latter is mostly due to the combination of a quasi-homogeneous tropospheric influx of sulfate from the stratosphere, and the highly inhomogeneous amount of anthropogenic sulfur emissions in the boundary layer (mostly located in the Northern Hemisphere). The two models show good consistency in their sulfur species behavior under 10 background and geoengineering conditions, not only for global and hemispheric budgets but also for regional S-deposition values (except over Arctic and Africa). The consistency between models is not limited to time averaged values, but it extends to monthly and inter-annual deposition changes. The latter is driven essentially by the variability of stratospheric large-scale transport associated to the quasi-biennial oscillation (QBO). According to model-mean values, geoengineering S-deposition percent changes on polar regions range between 7.7±0.7% over Antarctica and 8.5±1.3% over the Arctic, where the uncer-15 tainty reflects the model-averaged interannual variability. Similar S-deposition changes are found over quasi-clean continental regions of the Southern Hemisphere, and smaller values are calculated over polluted continental regions of the Northern Hemisphere (2÷4%). The largest difference between the two models is found over Africa and the Arctic (11% and 2%, respectively, for GEOS-Chem, against 2% and 15%, respectively, for ULAQ-CCM).

Upper tropospheric ice sensitivity to sulfate geoengineering

“…This is consistent with the findings of Visioni et al (2017b): the aerosol-driven surface cooling in G4 (contrary to G4K) favors a decreased wave activity and a consequent decrease in poleward mass fluxes from the tropical reservoir, for both gas and aerosol species. On the other Niemeier and Schmidt (2017); Visioni et al (2017c)). On light of this, smaller stratospheric heating rate anomalies are calculated in G4 with respect to G4K (Fig.…”

“…In particular, the lower stratosphere (LS) warming, caused by increasing heating rates in the optically thick sulfate cloud, tends to decrease the tropospheric lapse rate. A subsequent decrease in the available turbulent kinetic energy (TKE) would follow and translate 5 in slowing down updraft of water vapor to the UT, thus reducing the probability for sufficiently high supersaturation values capable to produce ice crystals formation via homogeneous freezing. Their study found a resulting large net RF reduction in magnitude with respect to clear sky conditions, where only the direct aerosol forcing is considered (-0.93 The aforementioned study, however, lacked an important part of the possible dynamical feedback of SG, that is changes in sea surface temperatures (SSTs) that would result from the decreased incoming solar radiation.…”

“…Another very important aspect to be taken into account for large-scale species transport in the lower stratosphere is the role of the quasi-biennial oscillation (QBO) in SG studies. It has been discussed under different points of view in some recent studies ; Niemeier and Schmidt (2017); Visioni et al, 2017c). A nudging procedure for the QBO is used in the ULAQ-CCM, based on an observed historical data series of equatorial mean zonal winds (Morgenstern et al (2017)).…”

“…OH oxidation of SO 2 starts the production of supercooled H 2 O-H 2 SO 4 particles, whose size distribution is calculated in an aerosol microphysics module with sectional approach, starting from gas-particle interaction processes (nucleation, H 2 SO 4 condensation and H 2 O growth) and then including aerosol particle coagulation. Removal processes from are included via gravitational settling across the tropopause and evaporation in the upper stratosphere 15 (Visioni et al (2017c)). …”