Post on 13-Mar-2022
GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1002/,
1
2
Influence of West Antarctic Ice Sheet collapse on
Antarctic surface climate and ice core records
Eric J. Steig,1,2
Kathleen Huybers,3
Hansi A. Singh,2
Nathan J. Steiger,2
Qinghua Ding,4
Dargan M.W. Frierson,2, Trevor Popp
5, James W.C. White
6
Corresponding author: Eric J. Steig, Department of Earth and Space Sciences, University of
Washington, Seattle, Washington 98195, USA. (steig@uw.edu)
1Department of Earth and Space Sciences, University of Washington, Seattle, WA, USA.
2Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA.
3Department of Geosciences, Pacific Lutheran University, Tacoma, WA, USA.
4Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA, USA.
5Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.
6Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, USA.
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Abstract. Climate-model simulations are used to examine the impact of3
a collapse of the West Antarctic Ice Sheet (WAIS) on the surface climate of4
Antarctica. The lowered topography following WAIS collapse produces anomalous5
cyclonic circulation with increased flow of warm, maritime air towards the6
South Pole, and cold-air advection from the East Antarctic plateau towards7
the Ross Sea and Marie Byrd Land, West Antarctica. Relative to the background8
climate, some areas in the East Antarctic interior warm moderately, while9
substantial cooling (several ◦C) occurs over parts of West Antarctica. Anomalously10
low isotope-paleotemperature values at Mt. Moulton, West Antarctica, compared11
with ice core records in East Antarctica, are consistent with collapse of the12
WAIS during the last interglacial period, Marine Isotope Stage (MIS) 5e. More13
definitive evidence might be recoverable from an ice core record at Hercules14
Dome, East Antarctica, which would experience significant warming and oxygen-isotope15
anomalies if the WAIS collapsed.16
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1. Introduction
Sea level records provide indirect evidence that the West Antarctic Ice Sheet (WAIS)17
collapsed during the last interglacial period, Marine Isotope Stage (MIS) 5e, ∼130 to18
116 ka (thousands of years before present). Eustatic sea level was 4 to 6 meters higher19
than present, possibly more [Dutton and Lambeck , 2012]. Mass loss from the Greenland20
ice sheet is thought to have contributed about 2 m of that total [NEEM Community21
Members , 2013]. Full retreat (“collapse”) of the WAIS would produce about 3.3 m of sea22
level rise [Bamber et al., 2009], which could account for the balance of MIS 5e sea level23
rise. Sediment cores from beneath the WAIS [Scherer et al., 1998] and the present-day24
Ross Ice Shelf [Naish et al., 2009] provide more direct evidence of WAIS collapse, several25
times in the past few million years, but it remains equivocal whether WAIS collapsed26
during MIS 5e.27
In this paper, we evaluate whether collapse of the WAIS, if it occurred, would have28
caused regional climate changes sufficiently large to be detectable in ice core records,29
several of which extend to MIS 5e. The idea of using ice core records to determine ice sheet30
changes has been exploited previously, chiefly under the assumption of a fixed climate,31
with elevation change expressed through the lapse-rate effect on temperature and on the32
stable-isotope ratios in precipitation [Steig et al., 2001; Bradley et al., 2013]. Here, we33
consider changes in regional climate induced by a reduced WAIS topograpy. We evaluate34
the influence of WAIS collapse on atmospheric circulation and temperature using general35
circulation model (GCM) experiments at varying levels of complexity. We compare our36
results with existing ice core records that extend through MIS 5e.37
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2. Methods
2.1. Climate Model Experiments
We compare GCM control simulations using modern Antarctic topography, with38
“WAIS-collapse” simulations, in which the Antarctic topography is reduced. In all39
experiments, climate boundary conditions (greenhouse gases, orbital configuration) are40
set to preindustrial values and are not changed. The surface characteristics (e.g. albedo)41
are unchanged in all areas between the full modern topgraphy and the altered topography42
that simulates WAIS collapse. The ice shelves are unchanged.43
We conducted aquaplanet simulations with the Gray Radiation Moist (GRaM) GCM44
[Frierson et al., 2006], and the Geophyical Fluid Dynamics Laboratory Atmospheric Model45
2 (AM2) [GFDL, 2004], in which the only topography is a “water mountain” having the46
shape of Antarctica. Both are coupled to a shallow (2.4 m) slab ocean and have no seasonal47
cycle. In GRaM, radiative fluxes are functions of temperature alone; there is a convective48
scheme but no cloud parameterization. In AM2, there are more complete radiation and49
convection schemes and the model includes cloud and water-vapor feedbacks. GRaM was50
run at T85 (spatial resolution of ∼ 1.4◦), with 25 vertical levels, AM2 at 2◦ latitude x 2.5 ◦51
longitude with 24 vertical levels. In the WAIS-collapse simulations with these models, all52
elevations in West Antarctica were reduced to zero elevation; topographic features such53
as mountains are not retained. We use the last 5 years of 15-year simulations.54
55 We also conducted simulations with the ECHAM4.6 atmospheric model [Roeckner et al.,
1996] at T42 ( ∼2.5◦ ) with 19 vertical levels coupled to a slab ocean with thermodynamic56
sea ice. We included the ECHAM4.6 isotope module [Hoffmann et al., 1998] to obtain57
simulations of changes in the oxygen isotope ratios in precipitation. We use the final58
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30 years of 40-year-long simulations. In these experiments, we use global topography,59
and realistic topographic changes in Antarctica. The “WAIS-collapse” topography is60
given by the ice-sheet model simulations of Pollard and DeConto [2009], as shown in61
Fig. 1. The model topography accounts for isostatic adjustment to ice thickness changes.62
Major topographic features such as Hercules Dome and the mountain ranges of coastal63
Marie Byrd Land do not change elevation, though moderate (50–100 m) lowering occurs64
as an artifact of the smoothing of the high-resolution topography to match the GCM65
resolution. There are elevation changes in the major East Antarctic drainage basins66
with reductions of more than 100 m extending well inland.67
Finally, we used the fully coupled ocean-atmosphere GCM, Community Climate System68
Model version 4 [CCSM4, Gent et al., 2011], with the same topography as for the69
ECHAM4.6 experiments, at a land/atmosphere resolution of 1.9◦ x 2.5◦ with 26 vertical70
levels and an ocean resolution of 1◦; the sea ice is fully dynamic. The WAIS-collapse71
simulation was branched from the control simulation at year 700, and run for an additional72
230 years; the climatology of the final 30 years is compared to that of the control.73
2.2. Ice Core Data
We restrict our analysis of ice core data to the conventional stable isotope ratio of74
oxygen (δ18O) in the ice, a well-established proxy for temperature, facilitating comparison75
with the climate model output. Isotope-ratio records that include MIS 5e are currently76
available at six locations in East Antarctica (Fig. 1): at Dome C [EDC, Jouzel et al.,77
2007], Taylor Dome [Steig et al., 2000], Talos Dome [Stenni et al., 2011], Vostok [Jouzel78
et al., 1993], Dronning Maud Land [EDML, EPICA Community Members , 2006] and79
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Dome F [Watanabe et al., 2003] and one in West Antarctica, at Mount Moulton, Marie80
Byrd Land [Dunbar et al., 2008; Korotkikh et al., 2011]. We also discuss Hercules Dome in81
East Antarctica. Hercules Dome is of interest because it is close to West Antarctica, and82
owing to the bounding bedrock topography, would have remained at a similar elevation83
and ice thickness even if WAIS collapsed [Jacobel et al., 2005]. Only a shallow (72-m)84
ice core has been recovered from Hercules Dome but radar profiles indicate that ice from85
MIS 5e may be recoverable there [Jacobel et al., 2005]. Locations are shown in Fig. 1,86
and isotope records in Fig. S1 in the supporting information.87
The Mt. Moulton record is the only currently-available West Antarctic record that88
includes MIS 5e. It is not strictly an ice “core” record; it was developed from a transect89
along an exposed section on the southern shoulder of Mt. Moulton, in the Flood Range90
of Marie Byrd Land, where ice flow brings old ice to the surface. The exposed section is91
at ∼76◦S, 134.7◦W, at an elevation of 2820 m above sea level. The Mt. Moulton record92
was dated radiometrically by a series of volcanic tephra layers that span the ∼600 m long93
exposed section [Dunbar et al., 2008; Korotkikh et al., 2011]. The isotope data we use here94
are from the Ph.D. thesis of Popp [2008].95
We are specifically interested in the differences among the ice core δ18O records during96
MIS 5e. To minimize artifacts arising from errors in the ice core age vs. depth97
relationships, we use the common AICC2012 timescale [Veres et al., 2013; Bazin et al.,98
2013], and adjust the preliminary timescale [Popp, 2008] of the Mt. Moulton record by99
maximizing its correlation with EPICA Dome C record (Fig. S2). This adjustment is100
conservative with respect to the calculations we make when comparing the records.101
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3. Results
Fig. 2 shows the temperature and potential-temperature response of the climate models102
to the changed Antarctic topography, as compared to the climatology from the control103
runs. The largest temperature increases occur in areas with reduced elevation, but there is104
also substantial warming over areas of East Antarctica where the topography is unchanged.105
In all model experiments there is warming over parts of the East Antarctic ice sheet areas106
adjacent to West Antarctica, including Hercules Dome and coastal Dronning Maud Land107
(though not extending as far inland as the EDML ice core site). There is also cooling108
over parts of East Antarctica and significant cooling in West Antarctica along the Siple109
Coast (eastern Ross Sea), and either cooling or no warming in coastal Marie Byrd Land.110
In the experiments with ECHAM4.6 and CCSM4, there are also areas of greater warming111
in East Antarctica corresponding to, and upslope from, the areas of reduced topography112
in the major ice drainage basins there. Temperature changes off the Antarctic continent113
vary, but all models agree in showing warming over the Ronne-Filchner Ice Shelf and the114
South Atlantic sector of the Southern Ocean. The spatial pattern of the δ18O response is115
very similar to the temperature response (Fig. S3).116
Fig. 3 shows the response of the surface wind field to the changed topography. All117
the models show the same pattern, with increased cyclonic flow over areas where the118
topography is lower. There is commonality between the patterns of wind and temperature119
anomalies, with greater onshore flow towards areas of West Antarctica and East Antarctica120
that warm, and greater flow off of the East Antarctic continent towards the Ross Sea and121
Marie Byrd Land – both areas that cool. Changes in the large scale circumpolar winds122
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are less consistent between the models, and in all cases these changes are small compared123
with those over the West Antarctic region and adjacent areas of East Antarctica.124
Fig. 4 compares the δ18O record from Mt. Moulton with the average of the East125
Antarctic records (excluding the low-resolution Taylor Dome record), normalized to the126
peak warmth (maximum δ18O values) of the early Holocene. The Mt. Moulton record127
is distinctive in having a smaller maximum in δ18O during MIS 5e. This difference is128
significant (more than 2 standard deviations below the mean) for most of MIS 5e.129
4. Discussion
4.1. Climate Dynamics
The large scale wind and temperature changes over West Antarctica can be understood130
as consequences of relatively simple dynamics. The cyclonic circulation anomaly over131
West Antarctica that occurs when the topography is lowered would be expected from132
potential vorticity conservation, in that stretching of the air column requires that it spin133
up in the same direction as the planetary vorticity, i.e. cyclonically. James [1988] showed134
that linear, barotropic dynamics causes an anticyclonic anomaly over topography when135
an idealized Antarctic-like continent is added in a barotropic model; thus, removal of136
the topography causes a cyclonic anomaly. Watterson and James [1992] found further137
support for this purely dynamical mechanism in a primitive equation model. Parish and138
Cassano [2003] show that there is little sensitivity of the winds to strong longwave cooling139
of the continent, implying that the katabatic component of the observed large-scale winds140
is weak. These considerations explain why all four models show such similar responses to141
the removal of topography, despite quite different model details.142
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Adiabatic warming would be expected as a direct response to the thicker atmosphere143
where the topography is lowered. The changed atmospheric circulation then accounts144
for the warming observed over the Ronne-Filchner Ice Shelf and the Atlantic sector of145
the Southern Ocean, as well as areas of the ice sheet where the topography has not146
changed, including along coastal Dronning Maud Land, and in the vicinity of Hercules147
Dome towards the South Pole from West Antarctica. Cooling over the Ross Sea and148
towards coastal areas of Marie Byrd Land reflects advection of cold-air anomalies from149
the East Antarctic plateau. The importance of advection in explaining the temperature150
pattern is particularly evident in the potential temperature (Fig. 2); if the warming were151
strictly adiabatic, there would be no changes in potential temperature.152
The more variable response of the models over the Southern Ocean and most of East153
Antarctica suggests that the direct effects of WAIS collapse on climate over most of East154
Antarctica and the Southern Ocean are small, relative to other processes and feedbacks.155
In the aquaplanet experiments, large areas over the Southern Ocean cool, while in the156
fully coupled-results with CCSM4, warming occurs over nearly the entire Southern Ocean.157
The latter can be attributed to dynamical changes in the ocean due to wind forcing, as has158
also been observed in a recent WAIS-collapse experiment with a coarse-resolution fully159
coupled model [Justino et al., 2015], though we note that there is also warming over much160
of the Southern Ocean in the slab-ocean experiment with ECHAM4.6. Meltwater fluxes161
– not considered in our experiments – may play a role in the ocean circulation response162
to a real WAIS collapse [Holden et al., 2010], but this would not alter the fundamental163
atmospheric response.164
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Other changes in climate boundary conditions that might be associated with WAIS165
collapse could potentially influence our results. In particular, we assumed that the166
albedo does not change where the WAIS has been removed; in reality the land ice167
and ice shelves might be replaced by ocean water that could be partly free of sea ice168
in summer. The difference, however, is evidently not large: Otto-Bliesner et al. [2013]169
conducted a similar fully-coupled experiment to ours (though with simpler topography)170
using CCSM3, in which they replaced WAIS land ice and ice shelves with ocean, and used171
130 ka boundary conditions. Their results show the same pattern of potential temperature172
and winds changes (Fig. S4). We conclude that cooling along the mountains of Marie173
Byrd Land, relative to the background climate state, as well as the advection of warm174
anomalies towards adjacent areas of East Antarctica, are fundamental features of the175
climate response to a collapse of the WAIS.176
4.2. Ice Core Interpretation
Our results have important implications for the interpretation of ice-core paleotemperature177
records. Popp [2008] presaged our results by suggesting that the comparatively low δ18O178
values for MIS 5e in the Mt. Moulton record from coastal Marie Byrd Land could179
180
181
182
183
be explained by altered atmospheric circulation owing to lowered elevations over the
WAIS. Our results clearly support this idea. Given a characteristic scaling of δ18O with
temperature of ∼ 0.8h/◦C [e.g., Steig et al., 2013], the relatively small δ18O anomaly at
Mt. Moulton, about 1–2 h less than in the East Antarctic ice cores, is in good agreement
with the temperature and δ18O anomalies from the GCM experiments.184
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The distinctive character of the Mt. Moulton isotope record supports other, independent185
evidence of WAIS collapse during MIS 5e, but there are some caveats to this interpretation.186
The Mt. Moulton record has not only relatively low δ18O values during MIS 5e; it also has187
relatively high δ18O during the coldest part of the glacial period (∼20–40ka) compared188
with other records (Fig. S1). From our model results, elevated δ18O at Mt. Moulton189
might be expected if the WAIS were thicker than present, but evidence for a substantially190
thicker WAIS is equivocal [e.g., Steig et al., 2001]. Furthermore other changes associated191
with MIS 5e climate that are not considered here, including more modest changes in192
WAIS – short of a full “collapse” – may perhaps be sufficient to give rise to the anomalies193
observed.194
These caveats notwithstanding, δ18O records from the East Antarctic ice cores may195
provide indirect supporting evidence for the interpretation of the Mt. Moulton data in196
terms of WAIS collapse. Comparison of the model results for surface temperature with197
ice core δ18O implicitly assumes a fixed linear relationship between the two, whereas the198
significant circulation changes associated with WAIS-collapse may lead to non-linearities199
in the isotope-temperature relationship. Sime et al. [2009] suggested that varying200
magnitude of the MIS 5e δ18O peak could be explained by different δ18O/temperature201
sensititives, owing to much higher MIS 5e temperatures than previously thought. However,202
they did not examine the possibility that atmospheric circulation changes associated with203
WAIS collapse could explain those differences. In our results with ECHAM4.6, the δ18O204
response is stronger at Dome C than at Vostok, even though the elevation at neither205
site changes. This is consistent with the observations. Such results are sensitive to the206
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assumed ice sheet configuration, but it is clear that topography-induced atmospheric207
circulation changes need to be accounted for in the interpretation of ice core records.208
Whether differences among the East Antarctic records can be used to constrain past209
topographic changes should be examined further with a fully-coupled GCM that includes210
isotope tracers.211
Finally, our experiments may be used to guide the selection of new ice core records212
that could be used to obtain more definitive evidence for or against WAIS collapse.213
Comparison between the Mt. Moulton record and East Antarctic sites is complicated214
by the uncertainty associated with East Antarctic elevation change, and the problem of215
ice flow for some locations (EDML, Vostok, Talos). Furthermore, Mt. Moulton may be216
less than ideally situated as an indicator of WAIS elevation changes as it lies (at least217
in our experiments) near the boundary between positive and negative anomalies in δ18O.218
Other blue-ice areas that retain ice from MIS 5e, if they can be identified further westward219
along the Marie Byrd Land coast, would be useful. Another interesting site is Fletcher220
Promontory, which extends from the WAIS into the Ronne-Filchner Ice Shelf; an ice core221
record there was recently obtained by the British Antarctic Survey and may contain MIS222
5e ice. However, local elevation changes at Fletcher Promontory, and the possibilty of223
ice flow from higher elevations, are likely to complicate interpretation of that record. An224
especially promising location is Hercules Dome, located between the WAIS and the South225
Pole. Hercules Dome is not likely to have significantly changed in elevation in response226
to WAIS collapse [Jacobel et al., 2005; Pollard and DeConto, 2009], but its proximity to227
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the WAIS means that warmer temperatures and elevated δ18O anomalies at that location228
may be diagnostic of the changed atmospheric circulation associated with WAIS collapse.229
5. Conclusion
Collapse of the West Antarctic Ice Sheet (WAIS) would cause regional changes in230
atmospheric circulation, with significant temperature anomalies over adjacent land and231
ocean areas. Robust responses to WAIS collapse include cooling over the Ross Ice Shelf232
and coastal Marie Byrd Land, and warming over the Ronne-Filchner Ice Shelf, coastal233
Dronning Maud Land, and at Hercules Dome. Ice core paleotemperature data show a234
significantly smaller difference between last-interglacial and Holocene temperature at Mt.235
Moulton, West Antarctica, than at all locations in East Antarctica. This observation is236
consistent with the climate model response, and thus provides supporting evidence for237
WAIS collapse during the last interglacial period.238
Acknowledgments. T. Sowers was the first to point out the evident correspondence239
between our model results and the Mt. Moulton record. We thank C.M. Bitz and E.240
Maroon for modeling support, L. Sime, E. Wolff, D.S. Battisti, T.J. Fudge and M. Whipple241
for helpful feedback, J. Pedro and F. Parrenin for assistance with the AICC2012 timescale,242
and R. DeConto for providing ice sheet model output data. This work was supported by243
U.S. National Science Foundation (grants 1043092 to EJS and 0230316 to JWCW) and244
the Deparment of Energy (Computational Science Graduate Fellowship to HAS). We also245
thank the Leverhulme Trust and the University of Edinburgh for support to EJS. This246
paper is dedicated to the memory of Stephen C. Porter. All data and model results from247
D R A F T March 25, 2015, 10:32am D R A F T
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this paper will be archived at the National Snow and Ice Data Center (nsidc.org), in248
accordance with the AGU Data Policy.249
250
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C
A
Amundsen
SeaBell.
RF
Ross Ice ShelfMarie Byrd Land
West Antarctic Ice SheetEAIS
AP
B EDML
Moulton Taylor
Dome F
EDCTalosVostok
HerculesDome
FP
Figure 1. Prescribed Antarctic topography used in the GCM simulations: (A) Modern
Antarctic topography, with geographic features. WAIS: West Antarctic Ice Sheet. EAIS:
East Antarctic Ice Sheet. RF: Ronne-Filchner Ice Shelf. AP: Antarctic Peninsula. Bell.:
Bellinghausen Sea. Wedd.: Weddell Sea. Blue circles: ice-core locations discussed in the text.
Blue cross: South Pole. Green contour: 2000 m above sea level. Contour interval 250 m.
(B) WAIS-collapse topography for the fully-coupled model experiments with CCSM4. For the
aquaplanet experiments with GRaM and GFDL-AM2, all of the West Antarctic sector (shaded)
is set to zero elevation, and East Antarctic elevations are unchanged. Ice core locations (named)
and contours as in (A). (C) Difference between control (A) and WAIS-collapse topography (B)
for the CCSM4 experiments. Red contour = 1000 m elevation change. Contour interval = 250
m. The 100 m elevation-change contour is also shown (gray).
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STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE X - 21
CCSM
4GR
aMGF
DL-
AM2
A
-8 -6 -4 -2 0 4 862
Temperature Change (°C)
Surface Temperature Potential temperature
B
C D
E F
G H
ECHA
M 4
.6
Figure 2. Response (“WAIS-collapse” minus control) of surface temperature (left column) and
potential temperature (right colum) to changes Antarctic topography as shown in Fig 1b. A,B)
GRaM and C,D) GFDL-AM2 aquaplanet experiments; all of West Antarctica is removed in the
WAIS-collapse simulations. E,F) ECHAM 4.6 with a slab ocean. G,H) CCSM4 fully-coupled
ocean and atmosphere. Circles show locations of ice cores discussed in the text.
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X - 22 STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE
Wind speed change
G
ECHA
M 4
.6
C D
CCSM
4AM
2
GRaM
BA
5 m/s
Figure 3. Surface-wind response to elevation changes from WAIS collapse from the GCM
experiments using A) GRaM, B) GFDL-AM2, C) ECHAM4.6, D) CCSM4.
D R A F T March 25, 2015, 10:32am D R A F T
STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE X - 23
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average δ18O anomaly of Talos Dome, EDML, Vostok, Dome Fuji, and EDC records from East
Antarctica (black). Dark gray shading: ± one standard deviation (σ) about the East Antarctic
mean, relative to the early-Holocene peak value at ∼10 ka. Light gray shading: ±2σ. The ice
core records are shown individually in Fig. S1 in the supporting information.
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