Abstract
Sulfate geoengineering (SG) methods based on lower stratospheric tropical injection of sulfur dioxide (SO2) have been widely discussed in recent years, focusing on the direct and indirect effects they would have on the climate system. Here a potential alternative method is discussed, where sulfur emissions are located at the surface in the form of carbonyl sulfide (COS) gas. A time-dependent chemistry-climate model experiment is designed from year 2021 to 2055, assuming a 40 Tg-S/yr artificial global flux of COS, geographically distributed following the present day anthropogenic COS surface emissions. The budget of COS and sulfur species is discussed, as well as the effects of SG-COS on the stratospheric sulfate aerosol optical depth (Δ τ = 0.080 in years 2046–2055), aerosol effective radius (0.46 μm), surface SOx deposition (+8.7 %) and tropopause radiative forcing (RF) (−2.0 W/m2 for clear sky conditions and −1.5 W/m2 including the cloud adjustment). Indirect effects on ozone, methane and stratospheric water vapor are also considered, along with the COS direct contribution (with an overall gas phase global radiative forcing of +0.23 W/m2). According to our model results, the resulting net RF of this SG-COS experiment is −1.3 W/m2 for the year 2050, and it is comparable to the corresponding RF of −1.7 W/m2 obtained with a sustained injection of 4 Tg-S/yr in the tropical lower stratosphere in the form of SO2 (SG-SO2, able to produce a comparable increase of the sulfate aerosol optical depth). Significant changes of the stratospheric ozone response are found in SG-COS with respect to SG-SO2 (+4.9 DU versus +1.5 DU, globally). According to the model results, the resulting UVB perturbation at the surface accounts to −4.3 % as a global-annual average (versus −2.4 % in the SG-SO2 case), with a springtime Antarctic decrease of −2.7 % (versus a +5.8 % increase in the SG-SO2 experiment). Overall, we find that an increase in COS surface emission may be feasible, and produce a more latitudinally-uniform forcing without the need for the deployment of stratospheric aircrafts.
Highlights
Reducing part of the incoming solar radiation has been proposed as a short-term 20 strategy for reducing surface temperatures and mitigating some of the worst side-effects of the greenhouse gases-induced global warming (Budyko, 1978; Crutzen, 2006)
According to our model results, the resulting net radiative forcing (RF) of this Sulfate geoengineering (SG)-carbonyl sulfide (COS) experiment is -1.3 W/m2 for the year 2050, and it is comparable to the corresponding RF of -1.7 W/m2 obtained with a sustained injection of 4 Tg-S/yr in the tropical lower stratosphere in the form of SO2 (SG-SO2, able to produce a comparable increase of the sulfate aerosol optical depth)
With the use of the University of L’Aquila-Climate-Chemistry Model 45 (ULAQ-CCM) we investigate the possible increase of COS surface fluxes to obtain a stratospheric aerosol optical depth (AOD) similar to that obtained with the injection of 8 Tg-SO2 in the stratosphere
Summary
Reducing part of the incoming solar radiation (known as Solar Radiation Modification, SRM) has been proposed as a short-term 20 strategy for reducing surface temperatures and mitigating some of the worst side-effects of the greenhouse gases-induced global warming (Budyko, 1978; Crutzen, 2006). COS has a long atmospheric lifetime (4 to 6 years; Khalil and Rasmussen, 1984; Ulshofer et al, 1996) due to its very low reactivity in the troposphere Because of this, it is uniformly mixed in the atmosphere, with an average concentration of 0.5 ppbv, and it reaches the stratosphere: in quiescent volcanic conditions, COS is the main contributor of sulfate aerosols in the Junge layer (Bruhl et al, 2012), where 35 after photodissociation by ultraviolet light and oxidation processes, it is turned into SO2 and subsequently oxidized into sulfuric acid, forming sulfate aerosols (Crutzen, 1976). Future experiments 90 with a more comprehensive Earth-system model will be necessary to determine the full extent of the climatic response
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