Atmospheric Chemistry and Physics

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1 Atmos. Chem. Phys., 10, , doi: /acp Author(s) 010. CC Attriion.0 License. Atmospheric Chemistry Physics Geoengeerg by stratospheric SO jection: results from Met Office HadGEM climate model comparison with Goddard Institute for Space Studies ModelE A. Jones 1, J. Haywood 1, O. Boucher 1, B. Kravitz, A. Robock 1 Met Office Hadley Centre, Exeter, UK Department of Environmental Sciences, Rutgers University, New Brunswick, NJ, USA Received: 5 February 010 Published Atmos. Chem. Phys. Discuss.: March 010 Revised: 16 June 010 Accepted: 17 June 010 Published: 5 July 010 Abstract. We exame response of Met Office Hadley Centre s HadGEM-AO climate model to simulated geoengeerg by contuous jection of SO to lower stratosphere, compare results with those from Goddard Institute for Space Studies Despite differences between models, we fd a broadly similar geographic distriion of response to geoengeerg both models terms of near-surface air temperature mean June August. The also suggest that significant s regional climate would be experienced even if geoengeerg was successful matag globalmean temperature near current values, both models dicate rapid warmg if geoengeerg is not sustaed. 1 Introduction Over last, global warmg has been well documented both observational records with climate models (IPCC, 001, 007). Furrmore, scenarios of unmitigated ( busess as usual ) future climate with se models suggest an creasgly rapid global-mean warmg next century. The primary cause of global warmg is from creased atmospheric concentrations of greenhouse gases (GHG) such as carbon dioxide, methane nitrous oxide, as a result of anthropogenic activity. These gases exert a positive radiative forcg of climate hence duce a warmg. Increases concentrations of aerosols are thought to ameliorate effects of global warmg via ir impacts Correspondence to: A. Jones (y.jones@metoffice.gov.uk) on radiation (direct effects) on clouds (direct effects), whereby y exert a negative radiative forcg of climate hence duce a coolg (e.g., Haywood Schulz, 007). Recently, utilizg se coolg effects from aerosols has been suggested as emergency geoengeerg measures to counterbalance effects of global warmg. The impact of brighteng stratocumulus clouds via jection of cloud condensation nuclei to low-level stratocumulus clouds has been vestigated by Jones et al. (009) usg one of models used present study. They suggest that, although global-mean warmg from creased GHG concentrations can deed be reduced, re are significant geographical s temperature patterns which could have adverse effects on some regions of Earth such as Amazonia. The impact of jection of sulphur dioxide (SO ) to stratosphere has also received much attention, formerly through eruption of volcanoes with large stratospheric sulphate jections (e.g., Robock, 000) latterly through deliberate geoengeerg (e.g., Rasch et al., 008; Robock et al., 008). Once aga, potential non-uniformity of response to geoengeerg is highlighted. Here we exame response of two climate models to geoengeerg by jection of SO to lower stratosphere. The two models used are Met Office Hadley Centre s HadGEM-AO National Aeronautics Space Admistration Goddard Institute for Space Studies HadGEM-AO is fully-coupled atmosphere-ocean version of Hadley Centre Global Environment Model version (Colls et al., 008). The atmosphere has a horizontal resolution of 1.5 latitude by longitude, with 8 vertical levels up to about 40 km. This is coupled to a 40- level ocean/sea-ice model with a zonal resolution of 1 Published by Copernicus Publications on behalf of European Geosciences Union.

2 6000 A. Jones et al.: Geoengeerg by stratospheric SO jection meridional resolution of 1 from poles to 0, reafter varyg smoothly to 1/ at equator. The sulphate aerosol scheme is described Jones et al. (001) Bellou et al. (007). Briefly, scheme cludes gaseous aqueous phase reactions of SO to sulphuric acid sulphate aerosol, with partitiong between Aitken, accumulation dissolved modes parameterisations for ter-modal transfers. The optically-active accumulation mode has a median radius of µm geometric stard deviation of 1.4, with hygroscopic growth based on d Almeida et al. (1991). In stratosphere gaseous phase oxidation pathway of SO to H SO 4 via reactions with hydroxyl radical domates, with concentrations of hydroxyl radical beg prescribed. An additional parameterisation for aerosol gravitational sedimentation was added to scheme as this process is important for stratosphere/troposphere aerosol transport. ModelE is also a coupled atmosphere-ocean model. The stratospheric version of model was used (Schmidt et al., 006), which has a horizontal resolution of 4 latitude by 5 longitude with vertical levels up to 80 km. This is coupled to a 1 level ocean model with same horizontal resolution (Russell et al., 1995). The sulphate aerosol scheme of ModelE is described Koch et al. (006). The model forms sulphate aerosols from SO by reaction with hydroxyl radical, which is prescribed. The dry aerosol effective radius is specified to be 0.5 µm with growth response to ambient humidity followg Tang (1996), resultg a gamma distriion with an effective radius of µm. Both models clude wet dry aerosol deposition (Bellou et al., 007, Koch et al., 006, for HadGEM ModelE, respectively), aerosol radiative forcg both models is fully teractive with atmospheric circulation. Experimental design The experimental designs were somewhat different for two models, are sufficiently similar for a comparison to be useful. The ModelE are a subset of those reported Robock et al. (008). They comprise (i) a -member ensemble followg IPCC scenario (Nakićenović et al., 000) run for 40 years from 1999; (ii) anor -member ensemble plus geoengeerg by SO jection at a pot [0 N, 10 E] to tropical lower stratosphere (ca. 16 km altitude) at a constant rate of 5 Tg[SO ] yr 1 for first 0 years, after which geoengeerg is termated simulation contued for a furr 0 years; (iii) a -member Control ensemble run perpetual 1999 conditions for 40 years. As variability between members is small (Robock et al., 008) only ensemble means are used. The presentation of results from ModelE follows that Robock et al. (008) showg difference from perpetual 1999 Control simulation, to mimise effect of any climate drift. Three HadGEM were performed, followg on from a 0th century simulation usg historical forcgs:, each of 60 years duration, also a third simulation where SO jection was suspended after 5 years. Given lower vertical extent of atmosphere component of HadGEM compared with ModelE (40 vs. 80 km), which has implications for simulation of stratospheric dynamics, a more idealized approach was taken to simulatg geoengeerg. A globally uniform jection of SO to lower stratosphere was used, at altitudes similar to those ModelE at same rate of 5 Tg[SO ] yr 1. The fact that poleward transport of stratospheric aerosol ModelE is a little too fast (Robock et al., 008) furrs general similarity of two approaches. Tests show that, for constant SO jection rate applied here, stratospheric aerosol burden stabilises after 4 years. As ModelE only cluded SO jection for first 0 years, comparison will generally focus on mean difference between (years 11 0 clusive) for each model. Results.1 Sulphate aerosol Figure 1 shows burden of sulphate aerosol from geoengeerg two models, calculated as difference between, meaned years The correspondg aerosol optical depths are 0.050± ±0.004 for HadGEM ModelE, respectively (decadal means±one stard deviation, at a wavelength of 550 nm). The global mean burden HadGEM is about 70% of that ModelE, resultg from numerous differences model formulation. However, both show a similar geographic distriion of geoengeerg aerosol, with higher values at higher latitudes due to poleward transport by large-scale circulation. Burdens are higher Arctic than Antarctic both models due to stronger wave-driven stratospheric meridional circulation Norrn Hemisphere (e.g., Rosenlof, 1995). The difference relative sulphate burden Antarctica is thought to be related to models simulation of Antarctic polar vortex. This acts as a barrier to aerosol transport from lower latitudes, simulation of vortex may be affected by difference resolution of two models. The small equatorial maximum ModelE distriion is due to pot-jection on equator used that model. The general similarity distriion of geoengeered aerosol two models dicates that different SO jection strategies employed makes little difference. Atmos. Chem. Phys., 10, , 010

3 A. Jones et al.: Geoengeerg by stratospheric SO jection 6001 Mean = /- Wm Mean = /- mg[so4] m 10 Mean1= /14 mg[so 16 4] m = mg[so m 8 1 Mean /0 4 4] Mean =. +/- Wm Mean = /- 0.0 mg[so4] m 14 Mean -4 = /- Wm 4 8 Mean -4 =.0+/- Wm Fig.. Annual-mean cident surface shortwave radiation Fig. 1. Annual-mean burdens of sulphate aerosol from geoengi ±one stard deviation) for years 11 0 m ±one stard dueradiation to geoengeerg neerg (mg[so ] m 4 Fig..(mg[SO Annual-mean cident surfacedeviation) shortwave (Wm by ±SO one stard devi mean burdens of sulphate aerosol from geoengeerg ± one(w stard deviation) 4] m jection to lower stratosphere averaged HadGEM Distriions are calculated as geoengeerg by SO jection to surface lower stratosphere averaged (Wm ± devi (a stard HadGEM radiation burdens HadGEM aerosol Distriions are.calculated as between difference difference between Fig. Annual-mean cident shortwave one stard mean of sulphate from geoengeerg (mg[so m ± one deviation) 4 ]burdens each model. Note different scales. plus-geoengeerg burdens each Distriions model. Notegeoengeerg scales. by jection to lower HadGEM are different calculated asso difference between stratosphere averaged (a plus-geoengeerg burdens each model. Note different scales.. Solar radiation Figure shows distriion of downward all-sky surface shortwave radiation (SW ) caused by geoengeerg, averaged for both models. The distriions are broadly similar structure, aga dicatg that difference SO jection methods is not important. The SW does not follow geoengeered aerosol burden (Fig. 1) very closely, as it also depends on distriion of comg solar radiation. The global mean ModelE is more negative than HadGEM, consistent with 11 higher geoengeered aerosol burden The fact that re are some positive values distriions is 11 because SW is difference between parallel (with without geoengeerg) which evolve with different meteorology, cloud distriions, etc. The areas of positive SW generally correlate with areas where cloud fraction has decreased geoengeerg. The global mean s cloud amounts are 0.0 ± 0.05% HadGEM +0.5 ± 0.0% A measure of top-of-atmosphere (ToA) shortwave forcg HadGEM was estimated from difference between a furr pair of (with without geoengeerg), this time usg an atmosphere-only configuration of model usg prescribed sea-surface temperatures sea-ice extents to mimise surface temperature response. In ModelE, on or h, ToA forcg was estimated usg two calls to radiation scheme both coupled. In one of se calls radiation scheme is allowed to see 1 sulphate aerosol, while or it is not. The difference between se two calls1 gives forcg due to sulphate aerosol given simulation, furr difference between forcg from gives an estimate of forcg due to geoengeerg sulphate. Neir HadGEM nor ModelE value is a true forcg, as strictly defed, as meteorology Atmos. Chem. Phys., 10, , 010

4 600 A. Jones et al.: Geoengeerg by stratospheric SO jection Temperature anomaly (K) Temperature anomaly (K) With SO jection Injection termated Time (years) Time (years) differs between HadGEM s atmosphere-only, (obviously) between ModelE s -plusgeoengeerg coupled. The mean ToA shortwave is 1.57 ± W m HadGEM 1.91±0.01 W m. Surface air temperature Figure shows evolution of global annual-mean nearsurface air temperature anomaly HadGEM (Fig. a) ModelE (Fig. b). The full impact of stratospheric SO jection on temperature appears to be realised both models after about ten years of geoengeerg, with mean coolg rates of K 1 HadGEM ModelE, respectively, first. This is quite a dramatic rate of temperature, although it should be borne md that this is due to our idealised experimental design where geoengeerg is not phased- is stead stantaneously fully activated. When geoengeerg is termated sulphate aerosol burden returns to its unperturbed state after about 5 years HadGEM global mean temperature creases at an average rate of 0.77 K 1, returng to value after about 15 years. This rate of warmg is more than twice that (0.4 K 1 years 0 60). The behaviour of ModelE is somewhat different, warmg strongly at 1.01 K 1 for first 7 years or so, after which rate of warmg reduces to approximately 0.7 K 1 as it slowly approaches temperatures. These rates compare with a mean warmg of 6 K 1 whole. The results from HadGEM shown Fig. a suggest that a given amount of warmg under scenario may be delayed by some 0 5 years by SO jection rates considered here. Figure 4a b shows distriion of near-surface temperature averaged HadGEM ModelE, respectively. This shows coolg more or less globally both models, with strongest coolg at higher norrn latitudes. The coolg is generally stronger l than ocean both models, HadGEM also shows coolg Arctic which is much stronger than that However, a problem has sce been identified with sea-ice scheme ModelE of Robock et al. (008) analysed here,. Evolution Fig. of. annual Evolution global-mean of annual near-surface global-mean air temperature near-surface anomaly air (K) temperature mean anomaly a historical (K) HadGEM The withred respect le istofor scenario, solid ture blue s le than it should be. This explas differences HadGEM whichwith resulted sea-ice beg less responsive to tempera- ect to mean a historical The red le is for scenario, solid blue le plus geoengeerg, dashed blue plus geoengeerg, dashed blue le after geoengeerg has been termated. The 10-year between HadGEM ModelE at high latitudes, also contries to lower climate sensitivity of ModelE compared with HadGEM. This also suggests that similar- which mean lenear-surface after geoengeerg air temperature hasanomaly been termated. is zero is marked. The 10-year As for ModelE, with anomaly beg withwhich respect to mean constant-1999 near-surface control. air temperature The th les anomaly dicate is±one zerostard deviation e difference between is marked. annual As means of for ModelE, ModelE ensemble with members. anomaly beg with respect to constant-1999 control. The th les dicate ±one stard deviation of difference between annual means of ModelE ensemble members. ity of global-mean temperature two models ( 0.74 K HadGEM 0.69 K ModelE ) is cocidental. However, important pot is that, with exception of extreme norrn latitudes, distriion of temperature response two models is reasonable agreement, with HadGEM showg a more detailed geographic pattern due to higher resolution of model fact that it is a sgle model experiment rar than a small ensemble. One defition of goal of geoengeerg could be to avoid any furr global warmg due to contug creases GHG concentrations. Figure a shows that after about 0 years of geoengeerg simulation global-mean nearsurface air temperature HadGEM is about same as at start of simulation, i.e. same as mean It is refore structive to exame mean s for 10-year which mean temperature anomaly is approximately zero (mean of years 9 8 clusive), which one could consider as beg an analogue for geoengeerg counterbalancg global warmg. The s temperature are shown Fig. 4c for HadGEM. Although global-mean temperature Atmos. Chem. Phys., 10, , 010

5 Temperature Change A. Joneset al.: Geoengeerg by stratospheric SO jection Temperature Change Temperature Change JJA Precipitation Change 600 JJA Precipitation Change JJA Precipitation Change Mean = mm day-1 Mean = K Mean -0.5 = K = Mean -0.5 = K = (e) (e) (e) Mean = mm day-1 Mean = K Mean -0.5 = K = Mean -0.5 = K = Mean = mm day-1 Mean = K = Mean K = = Mean K = Fig. 4. annual-mean near-surface air temperature (K) between HadGEM, meaned As for -plus of As comparg of Fig. 4. annual-mean near-surface air temperature (K)between years geoengeerg simulation with a historical As for mean 1 ) HadGEM; areas where s are significant at 5% level are dicated by dots. (e) As for As rate (mm day (K) of between As for Fig. 4. HadGEM, annual-meanmeaned near-surface air temperature for mean June August rate. comparg years 98meaned of simulation (K) of between As for Fig.As 4. HadGEM, annual-mean near-surface air temperature with 1 aas historical As forfrom.4 mean rate (mm ) may be near (+0.01 regionally this far Precipitation zero comparg years 98 ofis simulation with K), HadGEM, meaned of As forday case. Some l areas such as central Africa Australia 1 HadGEM; areas where s are significant at 5% level are dicated by dots.with (e) As aas historical As forwhereas mean rate (mmforday ) than comparg years 98 of 1 simulation are cooler mean by up to K, The mean June August rate is shown Amazon region is warmer by a similar amount. Polar 1 areas for mean rate. HadGEM; where s at 5% level are dicated by dots. (e) As forday aashistorical Asare significant for mean rate (mm ) Fig. 4d e for HadGEM ModelE, respectively. amplification due to ice-albedo feedbacks are also apparent clearly some (e.g. As warmg at high latitudes, dicatg that coolg areas for mean rate. HadGEM; where s are significant at 5%While level aredistriions dicated by dots.differ (e) As areas for ModelE shows a reduction of eastern effect of geoengeerg at se latitudes (Fig. 4a) has by USA,rate. whereas HadGEM suggests an crease), neverless this Astime been whelmed for bymean warmg due to GHGs. results from both models aga share certa broad features. Tropical maxima Atlantic Atmos. Chem. Phys., 10, , 010

6 6004 A. Jones et al.: Geoengeerg by stratospheric SO jection meant virtually no global-mean temperature. The maximum associated with ITCZ has generally moved northwards response to asymmetric warmg, although geoengeerg has somewhat ameliorated this (as shown Fig. 4d, dicatg tendency of geoengeerg to move ITCZ southwards), s duced by creasg GHG concentrations clearly domate..5 NH mean = NH mean = SH mean = SH mean = NH mean = NH mean = SH mean = SH mean = NH mean = NH mean = NH mean = NH mean = Sea-ice We only show sea-ice s from HadGEM due to problems with sea-ice scheme ModelE results noted above. Comparg from geo engeerg simulation with correspondg simulation, Arctic sea-ice area creased by km, Antarctic sea-ice area by km, as shown Fig. 5. This compares with decreases of km for Arctic Antarctic sea-ice area, respectively, when comparg of with mean of historical The larger Fig. 5a b compared with 5c d is directly related to tempersh mean = SH mean = ature differences between two ir secsh mean = SH mean = ond compared with that control (Fig. a) HadGEM sea-ice fractions sea-ice between: (a & b)sea-ice -plusfig. 5.Fig. HadGEM hemispheric fractions between: (a & b) -plus5. hemispheric HadGEM hemispheric Fig. 5.fractions HadGEM hemispheric sea-ice fractions (a & -plus HadGEM hemispheric fractions between: (aof& b) -plusdiscussion between: sea-ice eerg meaned meaned simulation, (c &between: d) geoengeerg of simulation, (c4b) & d) geoengeerg meaned of simulation, (c & d) eerg meaned of simulation, (c & d) of simulation, of meaned historical of historical of of historical of historical historical much of Pacific oceans are displaced southwards both models, resultg reductions sub-saharan Africa l areas around Bay of Bengal. This is response to hemispheric asymmetry temperature (Fig. 4a), such that max15 imum associated with 15 ter-tropical convergence zone (ITCZ) moves southwards towards warmer hemisphere (e.g., Williams et al., 001; Rotstayn Lohmann, 00). It must be remembered that s described above are with respect to correspondg (years 11 0) of, not with respect to approximately current conditions. Furr, geoengeerg durg this are considerably cooler than current conditions due to idealised manner which SO jection is applied. The mean HadGEM between mean years 9 8, when global-mean temperature is about same as , is shown Fig. 4f. As well as a reduction global-mean, consistent with results of Bala et al. (008) Robock et al. (008), re are also significant s regional, despite fact that employg geoengeerg has Atmos. Chem. Phys., 10, , 010 conclusions We have compared impact of geoengeerg by stratospheric SO jection two fully coupled climate models, HadGEM These models differ numerous ways, havg different resolutions, usg different SO jection methods, producg different magnitudes of geoengeered sulphate aerosol burdens. Despite se differences, however, jectg same amount of SO to lower stratosphere duces climate responses which show considerable agreement between two models. Both suggest a reduction near-surface air temperature which is global extent distried a similar fashion to warmg caused by GHGs (e.g. Fig. 6a Jones et al., 009). Both models also dicate that this form of geoengeerg causes a southward displacement of tropical maximum. This may counteract to some degree northward shift caused by creases GHG concentrations, latter still domate. The HadGEM suggest that SO jection rates considered here could defer a given amount of globalmean warmg under scenario by 0 5 years. However, both models also dicate a rapid warmg if geoengeerg is not mataed, which raises serious issues when considerg amount of time which geoengeerg would need to be sustaed. The patterns of temperature responses to geoengeerg via stratospheric SO jection differ from

7 A. Jones et al.: Geoengeerg by stratospheric SO jection 6005 those via modification of mare stratocumulus cloud sheets HadGEM (Jones et al., 009). The stratospheric SO jection geoengeerg produce geographic responses which, beg more homogeneous, more closely counteract responses due to creasg concentrations of GHGs than do responses from stratocumulus modification. However, results from HadGEM suggest that creases GHG concentrations can still have a profound impact on regional climate even if geoengeerg is successful counteractg any global-mean temperature. Matag global-mean temperature near its current level might be considered a necessary goal for any geoengeerg proposals, it is by no means sufficient. It should also be borne md that, common with or geoengeerg proposals to modify Earth s radiation balance, stratospheric SO jection does nothg to offset or impacts of creasg GHG concentrations, such as ocean acidification. Furrmore, neir model addresses potential damage to ozone layer caused by deliberate troduction of stratospheric aerosols (e.g. Crutzen, 006). The similarity of temperature responses two models hardly constitutes a consensus on impacts of geoengeerg via stratospheric SO jection across scientific community. It is refore important for many different climate models to assess impact of such geoengeerg, ideally usg a common experimental design as suggested for GeoMIP (Kravitz et al., 010). This should be done before any consideration is given to practical implementation of such proposals. Acknowledgements. We would like to thank Luke Oman for performg ModelE providg helpful formation, Georgiy Stenchikov for useful comments. The work of BK AR was supported by National Science Foundation grant ATM Model development computer time at Goddard Institute for Space Studies are supported by National Aeronautics Space Admistration climate modelg grants. 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8 6006 A. Jones et al.: Geoengeerg by stratospheric SO jection Tang, I. N.: Chemical size effects of hygroscopic aerosols on light scatterg coefficients, J. Geophys. Res., 101(D14), , Williams, K. D., Jones, A., Roberts, D. L., Senior, C. A., Woodage, M. J.: The response of climate system to direct effects of anthropogenic sulfate aerosols, Clim. Dynam., 17, , 001. Atmos. Chem. Phys., 10, , 010