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 geoengineering stratospheric aerosols: SO 2 injection/oxidation Effective radius of background aerosols at 75 hPa in the tropics: 0.2 µm 5 Tg/year SO 2 from 1 Jan 2020 to 31 Dec 2069 (G4). CCSM-CAM4: 8 Tg/year SO 2 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 ResolutionOceanStratospheric Aerosol Aerosol effect on ozone Source Eff. radius at 75 hPa tropics [µm] Het. chemistry Photo- chemistry ULAQ-CCM Univ. L’Aquila 5˚x 6˚ L126 Prescribed SSTs From SO 2 0.6 microphysics ✓✓ GEOSCCM NASA GSFC 2˚x 2.5˚ L72 Prescribed SSTs From SO 2 0.6 prescribed ✓ CCSM-CAM4 NCAR 1.9˚x 2.5˚ L40 CoupledFrom SO 2 0.35 microphysics ✓✓

3. Effects of sulfate geoengineering on CH 4 Increase in planetary albedo Increase in radiation scattering and ozone depletion by the aerosols Increase in aerosol SAD and O 3 depletion and tropical stratospheric heating rates Surface cooling Less tropospheric water vapor Less OH -> Longer CH 4 lifetime Decrease in tropospheric UV in the tropics Less tropospheric O( 1 D) production Enhanced heterogeneous chemistry in the mid- upper troposphere Less NO x and tropospheric O 3 production More CH 4 poor stratospheric air is transported to the extra- tropical upper troposphere Less UT CH 4

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

5. Effect of the UV-B changes on CH 4 Geoengineering aerosol Aerosol scattering Ozone depletion UV-B decrease in the tropics UV-B increase at high latitudes less OH formation and longer CH 4 lifetime

6. Effects of heterogeneous chemistry on CH 4 Tropospheric NOx depletion induces OH loss  longer CH 4 lifetime ULAQ-CCM GEOSCCM

7. Residual vertical velocity Temperature anomaly Aerosol warming The aerosol warming strengthens the tropical upwelling and the extratropical downwelling in the UTLS Effect of stratospheric warming on CH 4  impact on tropospheric OH and CH 4 lifetime depends on the net result of superimposed species perturbations in the UTLS: CH 4 (negative), NO y and O 3 (positive)  increased downward flux of CH 4 poor stratospheric air at the extratropical tropopause

8. CH 4 and N 2 O changes in the UTLS ULAQ-CCMGEOSCCM Results of ULAQ-CCM and GEOSCCM are similar for N 2 O  consistent with changes of lower stratospheric heating rates and BD circulation (due to aerosols and O 3 ). ULAQ shows about a factor of two larger changes for CH 4  better representation of chemical feedback processes in the upper troposphere. The sign inversion of the UTLS long-lived species perturbation after 2070 is due to their long stratospheric lifetime (about 130 years for N 2 O). Higher mixing ratio values persist for several years in the stratospheric tropical pipe after sulfate geoengineering has ceased, forcing an increase of stratospheric NOx and HOx production, with increased O 3 loss with respect to RCP4.5. This ends up in a sign change of 50-100 hPa tropical heating rates, since geoengineering sulfate aerosols have disappeared  slight weakening of the BD circulation  longer stratospheric lifetimes.

9. CH 4 and N 2 O lifetime changes The tropospheric CH 4 lifetime perturbation is driven by coupling of tropospheric OH changes and integrated CH 4 mass  longer lifetime in G4 with respect to RCP4.5. ULAQ-CCM The stratospheric N 2 O lifetime perturbation is driven by the intensifying strength of the Brewer-Dobson circulation  shorter lifetime in G4 with respect to RCP4.5  longlived species more abundant in the stratospheric tropical pipe. Termination effect discussed before

10. Summary of direct and indirect radiative forcings due to sulfate geo- engineering (2040-2049) Among gas species, in addition to contributions from O 3 (negative) and stratospheric H 2 O (positive), CH 4 produces the largest indirect RF. G4 – RCP4.5 Units CH4 197 ppbV (trop change) Trop O3 -0.72 DU Strat O3 -0.44 DU Strat H2O 0.22 ppmV (TTL change) Trop SO4 0.0055 Optical thickness Strat SO4 0.067 Optical thickness Cirrus ice -0.011 Optical thickness PSCs 0.0004 Optical thickness Direct aerosol RF (obviously) dominates; atmospheric stabilization lead to less upper tropospheric ice formation, with net negative RF.

11. Summary 5.Uncertainties: absence of various effects that could impact the RF, like changes in clouds due to increased aerosols; aerosol distribution; prescribed SSTs. 1.Sulfate geoengineering, made by sustained injection of SO 2 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 CH 4 poorer air outside the tropics is also affected by an intensified Brewer-Dobson circulation, as a consequence of the aerosol heating rates. 2.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 CH 4 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 CH 4 and a positive indirect RF of sulfate geoengineering due to CH 4 changes, of the order of 10% the aerosols direct forcing, but with opposite sign. The indirect RF from CH 4 is calculated as the largest RF among gas species (i.e. strat-trop O 3 and strat H 2 O). 3.Robust features: CH 4 and other long-lived species perturbations are found to be consistent both in ULAQ-CCM and GEOSCCM with changes of lower stratospheric heating rates and BD circulation (due to aerosols and O 3 ). 4.Future developments: ULAQ-CCM will repeat the G4 simulation using SSTs from the G4 atmosphere-ocean coupled run of CCSM-CAM4.

12. Thank you for your attention