Download Inability of stratospheric sulfate aerosol injections to preserve the

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
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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.
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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.
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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,
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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
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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
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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
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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. We
thank Judy Twedt for helpful
discussion on experimental design
and analysis and Colin Goldblatt,
Alan Robock, and an anonymous
reviewer for comments that improved
the manuscript.
The Editor thanks Alan Robock and
an anonymous reviewer for their
assistance in evaluating this paper.
MCCUSKER ET AL.
References
Ammann, C. M., W. M. Washington, G. A. Meehl, L. Buja, and H. Teng (2010), Climate engineering through artificial enhancement of natural
forcings: Magnitudes and implied consequences, J. Geophys. Res., 115, D22109, doi:10.1029/2009JD012878.
Bala, G., P. B. Duffy, and K. E. Taylor (2008), Impact of geoengineering schemes on the global hydrological cycle, Proc. Natl. Acad. Sci., 105(22),
7664–7669.
Blackstock, J. J., D. S. Battisti, K. Caldeira, D. M. Eardley, J. I. Katz, D. W. Keith, A. A. Patrinos, D. P. Schrag, R. H. Socolow, and S. E Koonin (2009),
Climate Engineering Responses to Climate Emergencies, Novim, Santa Barbara, Calif.
Chen, G., and I. M. Held (2007), Phase speed spectra and the recent poleward shift of Southern Hemisphere surface westerlies, Geophys. Res.
Lett., 34, L21805, doi:10.1029/2007GL031200.
Church, J., et al. (2013), Sea level change, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth
Assessment Report of the Intergovernmental Panel on Climate Change, edited by T. F. Stocker et al., Cambridge Univ. Press, Cambridge,
U. K., and New York.
Church, J. A., N. J. White, and J. M. Arblaster (2005), Significant decadal-scale impact of volcanic eruptions on sea level and ocean heat
content, Nature, 438(7064), 74–77, doi:10.1038/nature04237.
English, J. M., O. B. Toon, and M. J. Mills (2012), Microphysical simulations of sulfur burdens from stratospheric sulfur geoengineering, Atmos.
Chem. Phys., 12(10), 4775–4793, doi:10.5194/acp-12-4775-2012.
Favier, L., G. Durand, S. L. Cornford, G. H. Gudmundsson, O. Gagliardini, F. Gillet-Chaulet, T. Zwinger, A. J. Payne, and A. M. Le Brocq (2014),
Retreat of Pine Island Glacier controlled by marine ice-sheet instability, Nat. Clim. Change, 4, 117–121, doi:10.1038/nclimate2094.
Feely, R. A., C. L. Sabine, K. Lee, W. Berelson, J. Kleypas, V. J. Fabry, and F. J. Millero (2004), Impact of anthropogenic CO2 on the CaCO3 system
in the oceans, Science, 305(5682), 362–366, doi:10.1126/science.1097329.
Ferraro, A. J., E. J. Highwood, and A. J. Charlton-Perez (2011), Stratospheric heating by potential geoengineering aerosols, Geophys. Res. Lett.,
38, L24706, doi:10.1029/2011GL049761.
Ferreira, D., J. Marshall, C. M. Bitz, S. Solomon, and A. Plumb (2014), Antarctic ocean and sea ice response to ozone depletion: A
two-time-scale problem, J. Clim., 28, 1206–1226.
Fyfe, J. C., O. A. Saenko, K. Zickfeld, M. Eby, and A. J. Weaver (2007), The role of poleward-intensifying winds on Southern Ocean warming,
J. Clim., 20(21), 5391–5400.
Gent, P. R., et al. (2011), The community climate system model version 4, J. Clim., 24(19), 4973–4991.
Gillett, N. P., and D. W. J. Thompson (2003), Simulation of recent Southern Hemisphere climate change, Science, 302(5643), 273–275,
doi:10.1126/science.1087440.
STRATOSPHERIC AEROSOLS AND WAIS
8
Geophysical Research Letters
10.1002/2015GL064314
Gillett, N. P., V. K. Arora, K. Zickfeld, S. J. Marshall, and W. J. Merryfield (2011), Ongoing climate change following a complete cessation of
carbon dioxide emissions, Nat. Geosci., 4(2), 83–87, doi:10.1038/ngeo1047.
Gillett, N. P., J. C. Fyfe, and D. E. Parker (2013), Attribution of observed sea level pressure trends to greenhouse gas, aerosol, and ozone
changes, Geophys. Res. Lett., 40, 2302–2306, doi:10.1002/grl.50500.
Gleckler, P. J., K. AchutaRao, J. M. Gregory, B. D. Santer, K. E. Taylor, and T. M. L. Wigley (2006), Krakatoa lives: The effect of volcanic eruptions
on ocean heat content and thermal expansion, Geophys. Res. Lett., 33, L17702, doi:10.1029/2006GL026771.
Heckendorn, P., D. Weisenstein, S. Fueglistaler, B. Luo, E. Rozanov, M. Schraner, L. Thomason, and T. Peter (2009), The impact of geoengineering aerosols on stratospheric temperature and ozone, Environ. Res. Lett., 4(4), 045108.
Irvine, P. J., R. L. Sriver, and K. Keller (2012), Tension between reducing sea-level rise and global warming through solar-radiation management, Nat. Clim. Change, 2(2), 97–100.
Jacobs, S. S., A. Jenkins, C. F. Giulivi, and P. Dutrieux (2011), Stronger ocean circulation and increased melting under Pine Island Glacier Ice
Shelf, Nat. Geosci., 4(8), 519–523, doi:10.1038/ngeo1188.
Jones, A., et al. (2013), The impact of abrupt suspension of solar radiation management (termination effect) in experiment G2 of the
Geoengineering Model Intercomparison Project (GeoMIP), J. Geophys. Res. Atmos., 118, 9743–9752, doi:10.1002/jgrd.50762.
Joughin, I., and R. B. Alley (2011), Stability of the West Antarctic Ice Sheet in a warming world, Nat. Geosci., 4(8), 506–513.
Joughin, I., B. E. Smith, and B. Medley (2014), Marine ice sheet collapse potentially under way for the Thwaites Glacier basin, West Antarctica,
Science, 344(6185), 735–738, doi:10.1126/science.1249055.
Kidston, J., and G. Vallis (2012), The relationship between the speed and the latitude of an eddy-driven jet in a stirred barotropic model,
J. Atmos. Sci., 69(11), 3251–3263.
Lorenz, D. J. (2014), Understanding midlatitude jet variability and change using Rossby wave chromatography: Poleward-shifted jets in
response to external forcing, J. Atmos. Sci., 71(7), 2370–2389.
McCusker, K. E., D. S. Battisti, and C. M. Bitz (2012), The climate response to stratospheric sulfate injections and implications for addressing
climate emergencies, J. Clim., 25(9), 3096–3116, doi:10.1175/JCLI-D-11-00183.1.
McCusker, K. E., K. C. Armour, C. M. Bitz, and D. S. Battisti (2014), Rapid and extensive warming following cessation of solar radiation
management, Environ. Res. Lett., 9(2), 024005.
Moore, J. C., S. Jevrejeva, and A. Grinsted (2010), Efficacy of geoengineering to limit 21st century sea-level rise, Proc. Natl. Acad. Sci., 107,
15,699–15,703.
Notz, D. (2009), The future of ice sheets and sea ice: Between reversible retreat and unstoppable loss, Proc. Natl. Acad. Sci., 106(49),
20,590–20,595.
Oppenheimer, M. (1998), Global warming and the stability of the West Antarctic Ice Sheet, Nature, 393(6683), 325–332, doi:10.1038/30661.
Paolo, F. S., H. A. Fricker, and L. Padman (2015), Volume loss from Antarctic ice shelves is accelerating, Science, 348(6232), 327–331.
Polvani, L. M., M. Previdi, and C. Deser (2011), Large cancellation, due to ozone recovery, of future Southern Hemisphere atmospheric
circulation trends, Geophys. Res. Lett., 38, L04707, doi:10.1029/2011GL046712.
Pritchard, H. D., S. R. M. Ligtenberg, H. A. Fricker, D. G. Vaughan, M. R. van den Broeke, and L. Padman (2012), Antarctic ice-sheet loss driven
by basal melting of ice shelves, Nature, 484, 502–505, doi:10.1038/nature10968.
Rasch, P. J., P. J. Crutzen, and D. B. Coleman (2008), Exploring the geoengineering of climate using stratospheric sulfate aerosols: The role of
particle size, Geophys. Res. Lett., 35, L02809, doi:10.1029/2007GL032179.
Rignot, E., and S. S. Jacobs (2002), Rapid bottom melting widespread near Antarctic ice sheet grounding lines, Science, 296(5575),
2020–2023.
Rignot, E., J. Mouginot, M. Morlighem, H. Seroussi, and B. Scheuchl (2014), Widespread, rapid grounding line retreat of Pine Island, Thwaites,
Smith and Kohler glaciers, West Antarctica from 1992 to 2011, Geophys. Res. Lett., 41, 3502–3509, doi:0.1002/2014GL060140.
Robock, A. (2008), 20 reasons why geoengineering may be a bad idea, Bull. At. Sci., 64, 14–18.
Robock, A. (2014), Stratospheric aerosol geoengineering, in Geoengineering of the Climate System, vol. 38, edited by R. M. Harrison and
R. E. Hester, pp. 162–185, Royal Soc. of Chem., London.
Shepherd, A., et al. (2012), A reconciled estimate of ice-sheet mass balance, Science, 338(6111), 1183–1189, doi:10.1126/science.1228102.
Sigmond, M., M. Reader, J. Fyfe, and N. Gillett (2011), Drivers of past and future Southern Ocean change: Stratospheric ozone versus
greenhouse gas impacts, Geophys. Res. Lett., 38, L12601, doi:10.1029/2011GL047120.
Spence, P., S. M. Griffies, M. H. England, A. M. Hogg, O. A. Saenko, and N. C. Jourdain (2014), Rapid subsurface warming and circulation
changes of Antarctic coastal waters by poleward shifting winds, Geophys. Res. Lett., 41, 4601–4610, doi:10.1002/2014GL060613.
Steig, E. J., et al. (2013), Recent climate and ice-sheet changes in West Antarctica compared with the past 2,000 years, Nat. Geosci., 6(5),
372–375, doi:10.1038/ngeo1778. [Available at http://www.nature.com/ngeo/journal/v6/n5/abs/ngeo1778.html.]
Thoma, M., A. Jenkins, D. Holland, and S. Jacobs (2008), Modelling circumpolar deep water intrusions on the Amundsen Sea continental
shelf, Antarctica, Geophys. Res. Lett., 35, L18602, doi:10.1029/2008GL034939.
Thompson, D. W., S. Solomon, P. J. Kushner, M. H. England, K. M. Grise, and D. J. Karoly (2011), Signatures of the Antarctic ozone hole in
Southern Hemisphere surface climate change, Nat. Geosci., 4(11), 741–749.
Tilmes, S., R. Muller, and R. Salawitch (2008), The sensitivity of polar ozone depletion to proposed geoengineering schemes, Science,
320(5880), 1201–1204.
Yin, J., J. T. Overpeck, S. M. Griffies, A. Hu, J. L. Russell, and R. J. Stouffer (2011), Different magnitudes of projected subsurface
ocean warming around Greenland and Antarctica, Nat. Geosci., 4(8), 524–528, doi:10.1038/ngeo1189.
MCCUSKER ET AL.
STRATOSPHERIC AEROSOLS AND WAIS
9