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Geophysical Research Letters RESEARCH LETTER 10.1002/2015GL064314 Key Points: • Sulfate aerosol injection allows warm Southern Ocean water to reach ice shelves • Risk of Antarctic ice sheet instability possible without surface warming • Quantitative assessment of sea level rise under aerosol injections needed Inability of stratospheric sulfate aerosol injections to preserve the West Antarctic Ice Sheet K. E. McCusker1,2 , D. S. Battisti1 , and C. M. Bitz1 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA, 2 Now at School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada Abstract Injection of sulfate aerosols into the stratosphere has the potential to reduce the climate Supporting Information: • Figures S1–S3 Correspondence to: K. E. McCusker, [email protected] Citation: McCusker, K. E., D. S. Battisti, and C. M. Bitz (2015), Inability of stratospheric sulfate aerosol injections to preserve the West Antarctic Ice Sheet, Geophys. Res. Lett., 42, doi:10.1002/2015GL064314. Received 21 APR 2015 Accepted 2 JUN 2015 Accepted article online 4 JUN 2015 impacts of global warming, including sea level rise (SLR). However, changes in atmospheric and oceanic circulation that can significantly influence the rate of basal melting of Antarctic marine ice shelves and the associated SLR have not previously been considered. Here we use a fully coupled global climate model to investigate whether rapidly increasing stratospheric sulfate aerosol concentrations after a period of global warming could preserve Antarctic ice sheets by cooling subsurface ocean temperatures. We contrast this climate engineering method with an alternative strategy in which all greenhouse gases (GHG) are returned to preindustrial levels. We find that the rapid addition of a stratospheric aerosol layer does not effectively counteract surface and upper level atmospheric circulation changes caused by increasing GHGs, resulting in continued upwelling of warm water in proximity of ice shelves, especially in the vicinity of the already unstable Pine Island Glacier in West Antarctica. By contrast, removal of GHGs restores the circulation, yielding relatively cooler subsurface ocean temperatures to better preserve West Antarctica. 1. Introduction The retreat of the Pine Island and Thwaites Glaciers has accelerated, with a rapid collapse of the West Antarctic Ice Sheet (WAIS) possible within as little as a few centuries [Joughin et al., 2014; Rignot et al., 2014; Favier et al., 2014]. Estimates of the current WAIS mass contribution to sea level are relatively small (about 4 mm for the period 1992–2011) [Shepherd et al., 2012]; however, a full collapse could yield sea level rise (SLR) as great as 4.3 m [Church et al., 2013], exceeding projected steric SLR for the next five centuries [Church et al., 2013]. Given that the retreat of Pine Island and Thwaites Glaciers may be slowed or reversed if basal melt rates are decreased [Favier et al., 2014], an important question is whether solar radiation management (SRM) may be able to delay or even prevent a WAIS collapse, as has been suggested [Blackstock et al., 2009]. A large amount of mass loss from ice sheets occurs due to basal melt of the ice shelves [Joughin and Alley, 2011], the floating extensions of land ice sheets. In West Antarctica, the faces of these ice shelves extend from the surface to several hundred meters depth, near the level of the Circumpolar Deep Water (CDW), a subsurface water mass that is slightly warmer (−1∘ C to 1∘ C) [Yin et al., 2011] than waters above and below it. An increase in the temperature of CDW or in the rate that CDW is brought onto the continental shelf due to regional wind variability can greatly increase basal melting of ice shelves [Thoma et al., 2008; Joughin and Alley, 2011], causing ice shelf thinning. While the thinning of floating ice shelves does not in itself affect the sea level, the subsequent reduction in buttressing stress leads to an increase in ice stream velocity and ice mass loss, indirectly leading to global average SLR [Oppenheimer, 1998; Pritchard et al., 2012]. Thus, ice shelf disintegration and the subsequent SLR can happen even in the absence of large atmospheric warming [Oppenheimer, 1998]. ©2015. American Geophysical Union. All Rights Reserved. MCCUSKER ET AL. SRM has been shown to reduce the rate of global mean sea level rise. By examining observed relationships between sea level and volcanic eruptions, which temporarily lower sea level by briefly reducing ocean heat content [Church et al., 2005; Gleckler et al., 2006], Moore et al. [2010] argued that sulfate aerosol injections as SRM would reduce the rate of global mean sea level rise. Irvine et al. [2012] show that steric height changes in an intermediate complexity climate model are reduced by insolation reductions as SRM, and additionally include the contribution to sea level from the mass input of glaciers, icecaps, and Greenland and Antarctic ice sheets with a scaling to the global mean surface temperature anomaly. However, changes in oceanic STRATOSPHERIC AEROSOLS AND WAIS 1 Geophysical Research Letters 10.1002/2015GL064314 circulation that can have significant impacts on basal melting of ice shelves [Steig et al., 2013; Joughin and Alley, 2011; Thoma et al., 2008] have not previously been considered. The subsurface Southern Ocean temperature structure is strongly influenced by the surface wind stress [Fyfe et al., 2007; Spence et al., 2014], which in turn is set by the circumpolar westerly jet. McCusker et al. [2012] showed in a set of idealized simulations that when the global average temperature is stabilized using a stratospheric sulfate layer that counteracts growing CO2 concentrations, an anomalous poleward intensified zonal wind stress on the Southern Ocean remains that causes upwelling of warmer subsurface waters to the level of ice sheet outlets. Warmed subsurface ocean waters that destabilize marine ice sheets may introduce threshold behavior in ice sheet mass loss [Notz, 2009] such that ice sheet loss, and hence SLR, becomes nonlinear with increased warming [Joughin et al., 2014]. To promptly avoid SLR in the face of rising greenhouse gases (GHGs), a large, rapid input of stratospheric aerosols would need to be deployed to reverse heat uptake by the ocean. Any delays in implementation would allow additional ocean heat storage that may linger at intermediate depths for centuries [Gillett et al., 2011]. SRM is most likely to be deployed only after a period of global warming that is deemed to be unacceptably large. Could a subsequent implementation of sulfate SRM preserve Antarctic ice sheets? Here we investigate with the Community Climate System Model, version 4 (CCSM4) the impact of a rapidly increased stratospheric sulfate aerosol burden on atmospheric and oceanic circulation in the proximity of Antarctica. We contrast these results with an alternative management scenario in which GHGs are abruptly returned to preindustrial levels—the ultimate, if not impractical, climate engineering. 2. Methods In order to capture the relevant range of climate system time scales and dynamical feedbacks, we use the CCSM4 [Gent et al., 2011], a general circulation model (GCM) with a finite volume 0.9∘ × 1.25∘ resolution atmosphere coupled to sea ice, land, and 1∘ full-depth ocean models. The ocean model is a constant volume model with reference salinity used to compute salinity changes from freshwater input. Glaciers and ice sheets are allowed up to 1 m of snow water equivalent to accumulate, and the rest is routed directly to the ocean. Given that there is very little melt on Antarctica presently, precipitation minus evaporation is approximately equal to runoff. In a warming scenario, however, the snowpack is allowed to melt if it becomes warm enough. Steric sea level change is computed as follows: 𝜂 n = H((𝜌0 ∕𝜌n ) − 1), where 𝜂 n is the change in surface elevation at time step n, H is the global mean ocean depth, 𝜌0 is the ‘baseline’ global mean ocean density from a twentieth century simulation (20thC) obtained from the National Center for Atmospheric Research (NCAR) averaged over 1970–1999, and 𝜌n is global mean density at time step n. Ocean density, 𝜌, is primarily modified by changes in ocean temperature but is also affected by changes in salinity. This calculation assumes no volume change due to freshwater input (e.g., precipitation, evaporation, river runoff, melting, and freezing of sea ice). Two simulations from NCAR are used as controls: the Coupled Model Intercomparison Project 5 “business-as-usual” scenario, called the Representative Concentration Pathway 8.5 simulation (RCP8.5), reaching a radiative forcing from greenhouse gas emissions of 8.5 W/m2 above preindustrial levels by 2100, and the twentieth century simulation (20thC) forced with historical greenhouse gas and aerosol emissions plus volcanic eruptions. The climate engineering simulations are branched from RCP8.5 at year 2035 when the global mean surface air temperature (SAT) is approximately 1∘ C greater than that of the end of 20thC average (computed for years 1970–1999). In year 2035, a prescribed stratospheric sulfate aerosol burden is commenced that is ramped up from zero at a rapid rate of increase. The scenario has 3 years of rapid ramping of total annual average sulfate (SO4 ) burden at 8 teragrams of SO4 per year (Tg/yr), followed by ramping of the burden at 0.67 Tg/yr for the remainder of the simulation (calculated to provide a roughly equal and opposite radiative forcing to RCP8.5). The prescribed SO4 burden, as in McCusker et al. [2012] and McCusker et al. [2014], is generated from output from simulations described in Rasch et al. [2008] wherein volcanically sized (0.05, 2.03, and 0.17 mm for dry mode radius, standard deviation, and effective radius, respectively) SO2 was injected into the stratosphere over the tropics and allowed to circulate and oxidize to SO4 . In the present study, we prescribe an SO4 burden rather than an SO2 injection rate, so the particle size distribution is fixed. Given evidence that increasing MCCUSKER ET AL. STRATOSPHERIC AEROSOLS AND WAIS 2 Geophysical Research Letters 10.1002/2015GL064314 Figure 1. Time series of SAT and SLR. Global- mean annual-mean (a) surface air temperature (SAT; ∘ C), and (b) steric sea level rise (SLR; cm) due to changes in ocean density, shown as anomalies from 20thC (1970–1999 average). The Sulf curve is an ensemble average. Light blue shading in Figure 1a indicates the spread of the Sulf ensemble of simulations. injection rates may cause increased particle size and faster sedimentation rates [e.g., English et al., 2012], the radiative forcing achieved for a given burden may be overestimated in our simulations. We conducted an ensemble of four sulfate engineering simulations, initialized from four RCP8.5 ensemble members. Throughout the paper, we refer to the ensemble average as “Sulf.” We contrast the sulfate engineering scenario with an idealized representation of complete mitigation and carbon sequestration by conducting one simulation, called GHGrem, in which anthropogenic greenhouse gases (CO2 , CH4 , N2 O, and CFCs 11 and 12) are abruptly set to year 1850 levels in the year 2035. Unless otherwise noted, anomalies are presented as the departures of the average response (Sulf, GHGrem, and RCP8.5) in years 2045–2054 from the 1970–1999 average from 20thC. 3. Sea Level Rise and Atmospheric Circulation Density changes in our scenarios constitute the “steric” contribution to sea level rise (section 2). As CCSM4 (and most models of its class) lacks dynamical ice sheet behavior, and ice shelves and ice shelf cavities are absent, the contribution of mass input from the WAIS and other Antarctic ice sheets cannot be explicitly calculated. We, instead, focus on the large-scale oceanic temperature anomalies that determine melt rates near the ice shelves. MCCUSKER ET AL. STRATOSPHERIC AEROSOLS AND WAIS 3 Geophysical Research Letters 10.1002/2015GL064314 The time evolution of global mean surface air temperature (SAT) for RCP8.5 and climate engineering scenarios is shown in Figure 1a. Sulf represents a scenario in which drastic climate engineering is required to stop sea levels from further rising, and hence entails a quick and large decrease in net radiative forcing via a rapid increase in stratospheric aerosol burden. This decrease in radiative forcing avoids a large amount of warming compared to business as usual (Figure 1a; RCP8.5). The global and annual mean SAT linear trend for the first decade following the start of SRM (years 2035–2044) is −1.2∘ C/decade for the ensemble mean, and the land-only trend is even greater at −1.5∘ C/decade. One decade later (2045–2054), Sulf returns global mean SAT to roughly the end of 20thC (1970–1999), with a SAT anomaly of −0.03∘ C in Sulf compared to 1.83∘ C for RCP8.5. A second climate engineering scenario consisting of an instantaneous removal of GHGs to preindustrial conditions (GHGrem) gives a change in SAT that is broadly equivalent to that of Sulf for the same time period (−0.36∘ C). Global average sea level due to changes in ocean density, in isolation of possible mass change from dynamical ice loss or exceptional melt (i.e., melt beyond 1 m of snow water equivalent; see section 2), reverses course and starts declining upon initiation of climate engineering (Figure 1b). Nearly 30 cm of steric sea level Figure 2. Zonal average anomalies with height. Annual average, zonal rise is avoided by century’s end when a average (a) temperature (∘ C), and (b) zonal wind (m/s) averaged from sulfate layer is implemented, consistent 2045–2054 in the Sulf ensemble minus the 1970–1999 average from with previous findings [Irvine et al., 2012]. 20thC. Hatching indicates statistical significance at the 90% level using The relatively long timescale of ocean the Student’s t test for difference of two means. heat uptake means that by 2095, the sea level in Sulf is still not fully equilibrated to the negative radiative forcing imposed by the increasing aerosols and thus, further sea level decline is in the pipeline. Nevertheless, a high rate of sulfate injection could promptly reduce or avoid a substantial amount of steric sea level rise, at the expense of potentially hazardous, large SAT trends. Removal of GHGs is equally effective at decreasing sea level. Next we consider whether changes in the distribution of ocean temperature might be poised to cause greater basal melt to marine ice shelves. Numerical simulations of SRM with stratospheric aerosols have shown residual SH poleward intensified winds induced by differences in the vertical temperature structure of a stratospheric sulfate layer versus well-mixed tropospheric carbon dioxide [Ammann et al., 2010; McCusker et al., 2012]. These temperature anomalies and corresponding upper atmosphere and surface wind changes share features with those induced by ozone depletion [Gillett and Thompson, 2003; Gillett et al., 2013; Sigmond et al., 2011; Thompson et al., 2011] and increasing greenhouse gases [Gillett et al., 2013; Sigmond et al., 2011; Polvani et al., 2011]. Although mechanisms and structural details differ somewhat depending on the type of upper atmospheric forcing, MCCUSKER ET AL. STRATOSPHERIC AEROSOLS AND WAIS 4 Geophysical Research Letters 10.1002/2015GL064314 Figure 3. SH SAT and zonal wind stress anomaly maps. Annual mean surface air temperature (∘ C) anomaly from 20thC (1970–1999 mean) for (a) RCP8.5, (b) Sulf, and (c) GHGrem, years 2045–2054. (d–f ) Same as Figures 3a–3c, respectively, but for zonal wind stress (N/m2 ). Positive indicates westerly stress on the ocean. Contours indicate sea ice extent (15% concentration contour) for the 20thC (black) and perturbed simulation (gray); note that for Figures 3e and 3f the contours are nearly colocated. Hatching indicates statistical significance at the 90% level using the Student’s t test for difference of two means. the fundamental cause is the same: the zonal wind shear in the upper troposphere/lower stratosphere is increased due to an increase in the stratospheric pole-to-equator temperature gradient. Strengthened zonal winds then shift the jet poleward; however, what processes maintain the poleward-shifted jet is an open research question [Lorenz, 2014]. One study found that strengthened SH lower stratospheric/upper tropospheric winds increased eastward eddy propagation in the troposphere, which in turn shifts the critical latitude for Rossby wave breaking poleward, leading to poleward shifted surface westerlies [Chen and Held, 2007]. More recent studies using idealized models indicate that the poleward shift of strengthened zonal winds is maintained by the increased equatorward reflection of poleward propagating waves at a turning latitude, causing greater poleward momentum flux across the jet, shifting it poleward [Kidston and Vallis, 2012; Lorenz, 2014]. When aerosol concentrations are quickly increased in the stratosphere, large temperature anomalies are generated in the upper troposphere and stratosphere (Figure 2a). The zonal mean temperature structure with height is a combination of strong tropical lower stratospheric heating due to absorption of tropospheric long-wave radiation by aerosols [Ferraro et al., 2011] and high-latitude stratospheric long-wave cooling due to GHGs, with little change in the lower tropospheric temperature. The resultant zonal winds show a strengthened SH polar vortex and slight weakening of the subtropical jet (Figure 2b). The upper atmospheric deviations result in surface wind and wind stress anomalies in the SH that are comparable to those in RCP8.5: they are poleward shifted and intensified and have similar magnitudes (Figures 3d, 3e, and 4a). In contrast, the surface wind stress is very nearly returned to 20thC values in GHGrem (Figures 3f and 4a). Additionally, while both climate engineering strategies cause the sea ice edge to recover to the 20thC position (20thC and climate engineering simulations show colocated 15% concentration contours in Figures 3e–3f ), annual average sea ice concentration near sea ice margins remains lower by up to 15% in Sulf, but are increased beyond 20thC concentrations by almost 10% in GHGrem (Figure S1 in the supporting information). The residual sea ice concentration anomalies in Sulf and GHGrem are echoed in the residual temperature anomalies shown in Figure 3: Figure 3b shows a weak local warming accompanying the reduced sea ice concentration in Sulf, while Figure 3c shows weak local cooling where there are small increases in sea ice concentration in GHGrem. 4. Implications for Antarctic Ice Sheets Both climate engineering scenarios presented here in Sulf and GHGrem greatly reduce the surface warming over Antarctica that is seen in RCP8.5—greater than 100% in the case of GHGrem (compare Figures 3b and MCCUSKER ET AL. STRATOSPHERIC AEROSOLS AND WAIS 5 Geophysical Research Letters 10.1002/2015GL064314 Figure 4. Southern Ocean wind stress, wind stress curl, and potential temperature. Annual mean, zonal mean (a) zonal wind stress (N/m2 ), and (b) curl of the wind stress (10−8 N/m3 ) over ocean only, as anomalies from 20thC (1970–1999 mean). Positive indicates westerlies in Figure 4a and downwelling in Figure 4b. Annual mean, zonal mean ocean potential temperature (∘ C) anomalies from 20thC with depth for (c) Sulf and (d) GHGrem, years 2045–2054. Thick lines in Figures 4a and 4b indicate regions of statistical significance at the 90% level using the Student’s t test for difference of two means. Regions encompassed by black contours in Figures 4c and 4d, which are values greater than about 0.05∘ C, are statistically significant at the 90% level using the Student’s t test for difference of two means. 3c with Figure 3a)—but the ocean heat uptake that occurred over the first third of the 21st century before climate engineering was initiated also poses a threat to Antarctic ice sheets, particularly the WAIS, whose grounding line is below sea level [Joughin and Alley, 2011] and retreating [Rignot et al., 2014]. Small ocean temperature perturbations can have a significant impact on basal melting of ice shelves; a warming of just 0.1∘ C can thin 1 m of ice shelf in a year [Rignot and Jacobs, 2002]. Zonally averaged Southern Ocean temperature anomalies in Sulf indicate that temperatures encroaching on the ice shelves average 0.15–0.20∘ C warmer than 20thC (Figure 4c). GHGrem, however, has zonal mean temperature near the level of ice shelves up to 0.25∘ C cooler than in Sulf (Figure 4d). The ice sheet region in Antarctica most at-risk from basal melt is the Amundsen Sea Embayment sector including Pine Island Glacier (PIG). Critically, this region reveals an even greater potential for melting under sulfate climate engineering than in the zonal average: Sulf features an anomalous residual warming of 0.5∘ C in the 200–500 m layer (Figure 5a). The enhanced residual warming in the PIG region compared to the zonal average is due to a steeper vertical temperature gradient in the 100–400 m layer. This, combined with slightly larger vertical velocity anomalies, which will be discussed next, is the primary reason for the larger temperature anomaly in the PIG region than in the zonal average. Extrapolating from Rignot and Jacobs [2002], this temperature anomaly represents a basal melt rate of 5 m/yr above 20thC. In contrast, GHGrem has a cooling of the subsurface ocean up to 0.5∘ C. As a point for comparison, a recent estimate of thickness rate of change from Pine Island over the period 1994–2012 is −23 ± 3.8 m/decade, which corresponds to a volume loss of −14 ± 2 km3 /yr [Paolo et al., 2015]. Hence, our estimate of 5 m/yr (50 m/decade) from Sulf suggests that volume loss from PIG over the decade 2045–2054 would be approximately twice that seen over the past two decades (however, this assumes that volume loss has a linear relation to subsurface ocean temperature, which is unlikely [Joughin et al., 2014]). We have not taken into account the basal meltwater changes from ice shelves, because they are not resolved in our model and the ocean response to such meltwater is uncertain. Underneath an ice shelf, rising meltwater may drive a sub-ice shelf circulation that increases the rate CDW reaches the ice shelf base [Jacobs et al., 2011]. Once meltwater exits the base of the ice shelf, it would likely rise and drive mixing near the shelf front. When the meltwater reaches the surface, it may also act to strengthen the halocline and inhibit the upwelling of heat at some distance away from the ice shelf. This would lead to the warming of waters at the depth that may circulate to the ice shelf base, potentially further enhancing basal melt. GHG removal is a more effective means for cooling subsurface ocean temperatures than stratospheric aerosol injection fundamentally because of changes in ocean circulation that are brought about by changes in atmospheric circulation. The anomalous middepth zonal average warmth evident south of about 65∘ S and MCCUSKER ET AL. STRATOSPHERIC AEROSOLS AND WAIS 6 Geophysical Research Letters 10.1002/2015GL064314 Figure 5. PIG region ocean potential temperature and vertical advection. Annual mean, zonal mean ocean potential temperature (∘ C) with depth averaged for longitudes 80∘ W to 120∘ W in the Amundsen Sea Embayment/Pine Island Glacier (PIG) region as anomalies from 20thC for (a) Sulf and (b) GHGrem. Regions encompassed by black contours, which are values greater than about 0.05∘ C, are statistically significant at the 90% level using the Student’s t test for difference of two means. The vertical dashed line shows the equatorward limit of the region used to calculate the averages shown in Figures 5c–5e. (c) Vertical velocity averaged between 80∘ W–120∘ W and 65∘ S–74∘ S at each depth due to Eulerian (dashed), eddy-induced (thin solid), and total motion (thick solid) as anomalies from 20thC for RCP8.5 (red), Sulf (blue), and GHGrem (green) (m/yr). (d) The 20thC mean temperature gradient with depth (∘ C/m). Negative indicates increased temperature with increasing depth (z is defined positive up). (e) Same as Figure 5c but showing −Δw × dT∕dz —the largest component to the change in vertical advection (∘ C/yr). Not shown are the w × dΔT∕dz component, which is smaller than the Δw × dT∕dz component shown and has a pattern that does not explain the anomalous warmth in Sulf, and the Δw × dΔT∕dz component, which has a negligible magnitude compared to other terms. Gray shading in Figures 5c and 5e indicates anomalies that are greater than 1 standard deviation of the values in the 1970–1999 time series of the 20thC simulation. between 200 and 400 m depth in Sulf (Figure 4c) can be understood as anomalous upwelling of relatively warm CDW. This upwelling is caused by increased Ekman pumping that originates from the previously discussed poleward intensified zonal winds (Figures 3e) caused by stratospheric aerosols, greenhouse gases [Fyfe et al., 2007], and their combination here (Figure 4a). Engineering by sulfate aerosols features anomalous upwelling poleward of 60∘ S and downwelling equatorward of 60∘ S—just as in the RCP8.5 scenario (albeit with slightly smaller upwelling than in RCP8.5). In contrast, the removal of GHGs causes a reversal of sign in the zonal wind stress and wind stress curl anomalies compared to the anomalies associated with RCP8.5, resulting in weak anomalous downwelling south of 60∘ S (Figure 4b), which results in the cooling of the 200–400 m layer past the twentieth century mean (Figure 4d). In the PIG region, the primary driver of the contrasting behavior between climate engineering methods is again due to differences in circulation. Vertical velocity responses of opposing signs (Figure 5c) act on a climatological temperature gradient that features an increase in temperature with increasing depth in these layers (Figure 5d). The result is that anomalous vertical advection warms the water column under sulfate engineering, although it is smaller than the business-as-usual scenario, whereas removal of GHGs causes a cooling, which is primarily due to the Eulerian component of vertical advection (Figure 5e). The middepth residual warming resembles the “slow timescale” response to an increase in ozone forcing wherein the Southern Ocean circulation adjusts to changes in atmospheric winds and causes ocean warming from below that overcomes initial SST cooling and sea ice growth on the time scale of decades [Ferreira et al., 2014]. Finally, although not germane to the stability of Antarctic ice sheets, it is interesting to note the disparate ocean temperature profiles in the full zonal average equatorward of 60∘ S between Sulf and GHGrem as well (Figures 4c and 4d). The warming evident equatorward of 60∘ S at all depths in Sulf is a combination of residual warmth from before climate engineering began, a poleward shift (fundamentally due to the atmospheric jet shift) of the nearly vertical climatological isotherms there, and increased downwelling of warmer surface MCCUSKER ET AL. STRATOSPHERIC AEROSOLS AND WAIS 7 Geophysical Research Letters 10.1002/2015GL064314 waters. This feature is noticeably absent when GHGs are instead removed from the atmosphere because the jet rapidly returns to the 20thC position, resulting in weak upwelling anomalies equatorward of 60∘ S. Thus, the warming equatorward of 60∘ S is also due to the changes in the atmospheric winds due to the net forcing of increased GHGs and sulfate aerosols. 5. Persistent and Problematic Circulation Circulation anomalies presented here result in a large-scale oceanic environment that would be favorable for further thinning of Antarctic ice shelves through basal melt, particularly in the region encompassing Antarctica’s largest contributor to sea level rise [Shepherd et al., 2012], the already unstable Pine Island Glacier outlet [Rignot et al., 2014]. Fundamentally, these ocean circulation changes are due to the modified meridional temperature gradient in the stratosphere and upper troposphere induced by GHGs and sulfate aerosols. As a result, the ocean circulation changes responsible for warming of the water adjacent to the ice shelves persist for as long as GHGs and sulfates are in the atmosphere (Figures S2 and S3). Arguments that SRM with stratospheric aerosols is a “backstop” measure that could be rapidly deployed to avoid so-called climate emergencies, such as destabilization of marine ice sheets [Blackstock et al., 2009], are therefore not supported in this study. Given that the PIG grounding line is currently retreating along a retrograde bed, its reversibility may only be achievable with substantial reductions in basal melt rate beyond present-day rates [Favier et al., 2014]—a task only removal of GHGs can accomplish based on our results. We emphasize that surface and subsurface ocean temperatures are undoubtedly cooler when stratospheric aerosols are in use than under business-as-usual warming. However, while general circulation models such as the one we employ do not resolve ice shelves and subscale processes that influence their thickness (e.g., small-scale mixing, tidal currents [Joughin and Alley, 2011]), we have determined that the subsurface Southern Ocean warmth, in proximity to ice shelves that remain after sulfate aerosol deployment, is fundamentally due to residual large-scale circulation changes. We suggest that quantitative calculations of changes in basal melt rates, changes in ice sheet dynamics, and associated sea level rise under various climate engineering techniques become a high priority once coupled GCM-ice-sheet models become available. The inability to stabilize the West Antarctic ice sheet must be added to the list of weaknesses of stratospheric sulfate aerosol injections [Robock, 2008, 2014], which already includes ozone depletion [Tilmes et al., 2008; Heckendorn et al., 2009], risk of extreme regional temperature trends upon cessation [Jones et al., 2013; McCusker et al., 2014], continued ocean acidification [Feely et al., 2004], and reduced precipitation [Bala et al., 2008]. Acknowledgments The global climate model can be obtained from the National Center for Atmospheric Research (ncar.ucar.edu). Model simulation output data used in the production of Figures 1–5 are available free of charge by emailing the corresponding author. This research was funded by the Tamaki Foundation and supported in part by the National Science Foundation through PLR-1341497 and TeraGrid resources provided by the Texas Advanced Computing Center under grant TG-ATM090059. 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