Giovanni Pitari V. Aquila, S. Tilmes , I. Cionni , N. De Luca, Di Genova , and D. Iachetti

1. Sensitivity of Methane Lifetime to Sulfate Geoengineering: Results from the Geoengineering Model Intercomparison Project (GeoMIP) • Giovanni Pitari • V. Aquila, S. Tilmes, I. Cionni, N. De Luca, Di Genova, and D. Iachetti

2. Participating Models Source of geoengineeringstratosphericaerosols: SO2injection/oxidation Effectiveradius of background aerosolsat 75 hPa in the tropics: 0.2 µm 5 Tg/year SO2 from 1 Jan 2020 to 31 Dec 2069 (G4). CCSM-CAM4: 8 Tg/year SO2 Injection on the equator between 16 km and 25 km altitude Emission scenario: RCP4.5 ULAQ-CCM RCP4.5 SSTs: prescribed from CCSM-CAM4. G4 = RCP4.5 - 0.5K GEOSCCM RCP4.5 SSTs: prescribed from CESM4. G4 = RCP4.5

3. Effects of sulfate geoengineering on CH4 • Increase in radiation scattering and ozone depletion by the aerosols • Increase in planetary albedo • Increase in aerosol SADand O3 depletion and tropical stratospheric heating rates • Enhanced heterogeneous chemistry in the mid-upper troposphere • Decrease in tropospheric UV in the tropics • Surface cooling • More CH4poor stratospheric air is transported to the extra-tropical upper troposphere • Less tropospheric water vapor Less NOx and tropospheric O3production • Less tropospheric O(1D) production • Less UT CH4 • Less OH -> Longer CH4 lifetime

4. Effect of the albedo increase on CH4 Surface cooling  tropospheric water vapor decreaseless OH productionlonger CH4 lifetime

5. Effect of the UV-B changes on CH4 less OH formation and longer CH4 lifetime UV-B decrease in the tropics Aerosol scattering Geoengineering aerosol Ozone depletion UV-B increase at high latitudes

6. Effects of heterogeneous chemistry on CH4 ULAQ-CCM TroposphericNOxdepletioninduces OH loss  longer CH4lifetime GEOSCCM

7. Temperature anomaly Aerosol warming Effect of stratosphericwarming on CH4 Residual vertical velocity GEOSCCM The aerosol warmingstrengthens the tropicalupwelling and the extratropicaldownwelling in the UTLS  increaseddownwardflux of CH4poorstratospheric air atthe extratropical tropopause  impact on tropospheric OH and CH4lifetime depends on the net result of superimposed species perturbations in the UTLS: CH4 (negative), NOy and O3 (positive)

8. ULAQ-CCM GEOSCCM CH4 and N2O changes in the UTLS The signinversion of the UTLS long-livedspeciesperturbationafter 2070 is due to their long stratosphericlifetime (about 130 years for N2O). Higher mixing ratio valuespersist for severalyears in the stratospherictropical pipe aftersulfategeoengineeringhasceased, forcing an increase of stratosphericNOx and HOx production, with increased O3loss with respect to RCP4.5. Thisends up in a signchange of 50-100 hPatropicalheatingrates, sincegeoengineeringsulfateaerosolshavedisappeared slightweakening of the BD circulation  longerstratosphericlifetimes. Results of ULAQ-CCM and GEOSCCM are similar for N2O  consistent with changes of lowerstratosphericheatingrates and BD circulation (due to aerosols and O3). ULAQ shows about a factor of twolargerchanges for CH4 betterrepresentation of chemical feedback processes in the uppertroposphere.

9. CH4 and N2O lifetimechanges ULAQ-CCM The tropospheric CH4lifetimeperturbationisdriven by coupling of tropospheric OH changes and integrated CH4 mass  longerlifetime in G4 with respect to RCP4.5. The stratospheric N2O lifetimeperturbationisdriven by the intensifyingstrength of the Brewer-Dobsoncirculation shorterlifetime in G4 with respect to RCP4.5 longlivedspecies more abundant in the stratospherictropical pipe. Terminationeffectdiscussedbefore

10. Summary of direct and indirect radiative forcings due to sulfate geo-engineering (2040-2049) Direct aerosol RF (obviously) dominates; atmosphericstabilizationlead to lessuppertroposphericiceformation, with net negative RF. Amonggas species, in addition tocontributions from O3 (negative) and stratospheric H2O (positive), CH4producesthe largest indirect RF.

11. Summary Sulfate geoengineering, made by sustained injection of SO2 in the tropical lower stratosphere, may impact the abundance of tropospheric methane through several photochemical mechanisms affecting the tropospheric OH abundance and hence the methane lifetime. Stratosphere-troposphere exchange of CH4 poorer air outside the tropics is also affected by an intensified Brewer-Dobson circulation, as a consequence of the aerosol heating rates. Uncertainties: absence of various effects that could impact the RF, like changes in clouds due to increased aerosols; aerosol distribution; prescribed SSTs. Three models are used here to explore the above radiative, chemical and dynamical mechanisms affecting the methane lifetime (ULAQ-CCM, GEOSCCM, CCSM-CAM4). Our results show that the CH4 lifetime may become significantly longer with a sustained injection of 2.5 Tg-S/yr started in year 2020 (exp. G4), which implies an increase of tropospheric CH4 and a positive indirect RF of sulfate geoengineering due to CH4 changes, of the order of 10% the aerosols direct forcing, but with opposite sign. The indirect RF from CH4 is calculated as the largest RF among gas species (i.e. strat-trop O3 and strat H2O). Robust features: CH4 and other long-lived species perturbations are found to be consistent both in ULAQ-CCM and GEOSCCM with changes of lowerstratosphericheatingrates and BD circulation (due to aerosols and O3). Future developments: ULAQ-CCM will repeat the G4 simulation using SSTs from the G4 atmosphere-ocean coupled run of CCSM-CAM4.

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