Influence of West Antarctic Ice Sheet Collapse on Antarctic Surface

GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1002/,

1 Inﬂuence of West Antarctic Ice Sheet collapse on

2 Antarctic surface climate and ice core records

1,2 3 2 2 Eric J. Steig, Kathleen Huybers, Hansi A. Singh, Nathan J. Steiger, Qinghua Ding,4 Dargan M.W. Frierson,2, Trevor Popp5, James W.C. White6

Corresponding author: Eric J. Steig, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. ([email protected])

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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3 Abstract. Climate-model simulations are used to examine the impact of

4 a collapse of the West Antarctic Ice Sheet (WAIS) on the surface climate of

5 Antarctica. The lowered topography following WAIS collapse produces anomalous

6 cyclonic circulation with increased ﬂow of warm, maritime air towards the

7 South Pole, and cold-air advection from the East Antarctic plateau towards

8 the Ross Sea and Marie Byrd Land, West Antarctica. Relative to the background

9 climate, some areas in the East Antarctic interior warm moderately, while

◦ 10 substantial cooling (several C) occurs over parts of West Antarctica. Anomalously

11 low isotope-paleotemperature values at Mt. Moulton, West Antarctica, compared

12 with ice core records in East Antarctica, are consistent with collapse of the

13 WAIS during the last interglacial period, Marine Isotope Stage (MIS) 5e. More

14 deﬁnitive evidence might be recoverable from an ice core record at Hercules

15 Dome, East Antarctica, which would experience signiﬁcant warming and oxygen-isotope

16 anomalies if the WAIS collapsed.

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1. Introduction

17 Sea level records provide indirect evidence that the West Antarctic Ice Sheet (WAIS)

18 collapsed during the last interglacial period, Marine Isotope Stage (MIS) 5e, ∼130 to

19 116 ka (thousands of years before present). Eustatic sea level was 4 to 6 meters higher

20 than present, possibly more [Dutton and Lambeck, 2012]. Mass loss from the Greenland

21 ice sheet is thought to have contributed about 2 m of that total [NEEM Community

22 Members, 2013]. Full retreat (“collapse”) of the WAIS would produce about 3.3 m of sea

23 level rise [Bamber et al., 2009], which could account for the balance of MIS 5e sea level

24 rise. Sediment cores from beneath the WAIS [Scherer et al., 1998] and the present-day

25 Ross Ice Shelf [Naish et al., 2009] provide more direct evidence of WAIS collapse, several

26 times in the past few million years, but it remains equivocal whether WAIS collapsed

27 during MIS 5e.

28 In this paper, we evaluate whether collapse of the WAIS, if it occurred, would have

29 caused regional climate changes suﬃciently large to be detectable in ice core records,

30 several of which extend to MIS 5e. The idea of using ice core records to determine ice sheet

31 changes has been exploited previously, chieﬂy under the assumption of a ﬁxed climate,

32 with elevation change expressed through the lapse-rate eﬀect on temperature and on the

33 stable-isotope ratios in precipitation [Steig et al., 2001; Bradley et al., 2013]. Here, we

34 consider changes in regional climate induced by a reduced WAIS topograpy. We evaluate

35 the inﬂuence of WAIS collapse on atmospheric circulation and temperature using general

36 circulation model (GCM) experiments at varying levels of complexity. We compare our

37 results with existing ice core records that extend through MIS 5e.

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2. Methods

2.1. Climate Model Experiments

38 We compare GCM control simulations using modern Antarctic topography, with

39 “WAIS-collapse” simulations, in which the Antarctic topography is reduced. In all

40 experiments, climate boundary conditions (greenhouse gases, orbital conﬁguration) are

41 set to preindustrial values and are not changed. The surface characteristics (e.g. albedo)

42 are unchanged in all areas between the full modern topgraphy and the altered topography

43 that simulates WAIS collapse. The ice shelves are unchanged.

44 We conducted aquaplanet simulations with the Gray Radiation Moist (GRaM) GCM

45 [Frierson et al., 2006], and the Geophyical Fluid Dynamics Laboratory Atmospheric Model

46 2 (AM2) [GFDL, 2004], in which the only topography is a “water mountain” having the

47 shape of Antarctica. Both are coupled to a shallow (2.4 m) slab ocean and have no seasonal

48 cycle. In GRaM, radiative ﬂuxes are functions of temperature alone; there is a convective

49 scheme but no cloud parameterization. In AM2, there are more complete radiation and

50 convection schemes and the model includes cloud and water-vapor feedbacks. GRaM was

◦ ◦ ◦ 51 run at T85 (spatial resolution of ∼ 1.4 ), with 25 vertical levels, AM2 at 2 latitude x 2.5

52 longitude with 24 vertical levels. In the WAIS-collapse simulations with these models, all

53 elevations in West Antarctica were reduced to zero elevation; topographic features such

54 as mountains are not retained. We use the last 5 years of 15-year simulations.

55 We also conducted simulations with the ECHAM4.6 atmospheric model [Roeckner et al.,

◦ 56 1996] at T42 ( ∼2.5 ) with 19 vertical levels coupled to a slab ocean with thermodynamic

57 sea ice. We included the ECHAM4.6 isotope module [Hoﬀmann et al., 1998] to obtain

58 simulations of changes in the oxygen isotope ratios in precipitation. We use the ﬁnal

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59 30 years of 40-year-long simulations. In these experiments, we use global topography,

60 and realistic topographic changes in Antarctica. The “WAIS-collapse” topography is

61 given by the ice-sheet model simulations of Pollard and DeConto [2009], as shown in

62 Fig. 1. The model topography accounts for isostatic adjustment to ice thickness changes.

63 Major topographic features such as Hercules Dome and the mountain ranges of coastal

64 Marie Byrd Land do not change elevation, though moderate (50–100 m) lowering occurs

65 as an artifact of the smoothing of the high-resolution topography to match the GCM

66 resolution. There are elevation changes in the major East Antarctic drainage basins

67 with reductions of more than 100 m extending well inland.

68 Finally, we used the fully coupled ocean-atmosphere GCM, Community Climate System

69 Model version 4 [CCSM4, Gent et al., 2011], with the same topography as for the

◦ ◦ 70 ECHAM4.6 experiments, at a land/atmosphere resolution of 1.9 x 2.5 with 26 vertical

◦ 71 levels and an ocean resolution of 1 ; the sea ice is fully dynamic. The WAIS-collapse

72 simulation was branched from the control simulation at year 700, and run for an additional

73 230 years; the climatology of the ﬁnal 30 years is compared to that of the control.

2.2. Ice Core Data

74 We restrict our analysis of ice core data to the conventional stable isotope ratio of

18 75 oxygen (δ O) in the ice, a well-established proxy for temperature, facilitating comparison

76 with the climate model output. Isotope-ratio records that include MIS 5e are currently

77 available at six locations in East Antarctica (Fig. 1): at Dome C [EDC, Jouzel et al.,

78 2007], Taylor Dome [Steig et al., 2000], Talos Dome [Stenni et al., 2011], Vostok [Jouzel

79 et al., 1993], Dronning Maud Land [EDML, EPICA Community Members, 2006] and

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80 Dome F [Watanabe et al., 2003] and one in West Antarctica, at Mount Moulton, Marie

81 Byrd Land [Dunbar et al., 2008; Korotkikh et al., 2011]. We also discuss Hercules Dome in

82 East Antarctica. Hercules Dome is of interest because it is close to West Antarctica, and

83 owing to the bounding bedrock topography, would have remained at a similar elevation

84 and ice thickness even if WAIS collapsed [Jacobel et al., 2005]. Only a shallow (72-m)

85 ice core has been recovered from Hercules Dome but radar proﬁles indicate that ice from

86 MIS 5e may be recoverable there [Jacobel et al., 2005]. Locations are shown in Fig. 1,

87 and isotope records in Fig. S1 in the supporting information.

88 The Mt. Moulton record is the only currently-available West Antarctic record that

89 includes MIS 5e. It is not strictly an ice “core” record; it was developed from a transect

90 along an exposed section on the southern shoulder of Mt. Moulton, in the Flood Range

91 of Marie Byrd Land, where ice ﬂow brings old ice to the surface. The exposed section is

◦ ◦ 92 at ∼76 S, 134.7 W, at an elevation of 2820 m above sea level. The Mt. Moulton record

93 was dated radiometrically by a series of volcanic tephra layers that span the ∼600 m long

94 exposed section [Dunbar et al., 2008; Korotkikh et al., 2011]. The isotope data we use here

95 are from the Ph.D. thesis of Popp [2008].

18 96 We are speciﬁcally interested in the diﬀerences among the ice core δ O records during

97 MIS 5e. To minimize artifacts arising from errors in the ice core age vs. depth

98 relationships, we use the common AICC2012 timescale [Veres et al., 2013; Bazin et al.,

99 2013], and adjust the preliminary timescale [Popp, 2008] of the Mt. Moulton record by

100 maximizing its correlation with EPICA Dome C record (Fig. S2). This adjustment is

101 conservative with respect to the calculations we make when comparing the records.

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3. Results

102 Fig. 2 shows the temperature and potential-temperature response of the climate models

103 to the changed Antarctic topography, as compared to the climatology from the control

104 runs. The largest temperature increases occur in areas with reduced elevation, but there is

105 also substantial warming over areas of East Antarctica where the topography is unchanged.

106 In all model experiments there is warming over parts of the East Antarctic ice sheet areas

107 adjacent to West Antarctica, including Hercules Dome and coastal Dronning Maud Land

108 (though not extending as far inland as the EDML ice core site). There is also cooling

109 over parts of East Antarctica and signiﬁcant cooling in West Antarctica along the Siple

110 Coast (eastern Ross Sea), and either cooling or no warming in coastal Marie Byrd Land.

111 In the experiments with ECHAM4.6 and CCSM4, there are also areas of greater warming

112 in East Antarctica corresponding to, and upslope from, the areas of reduced topography

113 in the major ice drainage basins there. Temperature changes oﬀ the Antarctic continent

114 vary, but all models agree in showing warming over the Ronne-Filchner Ice Shelf and the

18 115 South Atlantic sector of the Southern Ocean. The spatial pattern of the δ O response is

116 very similar to the temperature response (Fig. S3).

117 Fig. 3 shows the response of the surface wind ﬁeld to the changed topography. All

118 the models show the same pattern, with increased cyclonic ﬂow over areas where the

119 topography is lower. There is commonality between the patterns of wind and temperature

120 anomalies, with greater onshore ﬂow towards areas of West Antarctica and East Antarctica

121 that warm, and greater ﬂow oﬀ of the East Antarctic continent towards the Ross Sea and

122 Marie Byrd Land – both areas that cool. Changes in the large scale circumpolar winds

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123 are less consistent between the models, and in all cases these changes are small compared

124 with those over the West Antarctic region and adjacent areas of East Antarctica.

18 125 Fig. 4 compares the δ O record from Mt. Moulton with the average of the East

126 Antarctic records (excluding the low-resolution Taylor Dome record), normalized to the

18 127 peak warmth (maximum δ O values) of the early Holocene. The Mt. Moulton record

18 128 is distinctive in having a smaller maximum in δ O during MIS 5e. This diﬀerence is

129 signiﬁcant (more than 2 standard deviations below the mean) for most of MIS 5e.

4. Discussion

4.1. Climate Dynamics

130 The large scale wind and temperature changes over West Antarctica can be understood

131 as consequences of relatively simple dynamics. The cyclonic circulation anomaly over

132 West Antarctica that occurs when the topography is lowered would be expected from

133 potential vorticity conservation, in that stretching of the air column requires that it spin

134 up in the same direction as the planetary vorticity, i.e. cyclonically. James [1988] showed

135 that linear, barotropic dynamics causes an anticyclonic anomaly over topography when

136 an idealized Antarctic-like continent is added in a barotropic model; thus, removal of

137 the topography causes a cyclonic anomaly. Watterson and James [1992] found further

138 support for this purely dynamical mechanism in a primitive equation model. Parish and

139 Cassano [2003] show that there is little sensitivity of the winds to strong longwave cooling

140 of the continent, implying that the katabatic component of the observed large-scale winds

141 is weak. These considerations explain why all four models show such similar responses to

142 the removal of topography, despite quite diﬀerent model details.

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143 Adiabatic warming would be expected as a direct response to the thicker atmosphere

144 where the topography is lowered. The changed atmospheric circulation then accounts

145 for the warming observed over the Ronne-Filchner Ice Shelf and the Atlantic sector of

146 the Southern Ocean, as well as areas of the ice sheet where the topography has not

147 changed, including along coastal Dronning Maud Land, and in the vicinity of Hercules

148 Dome towards the South Pole from West Antarctica. Cooling over the Ross Sea and

149 towards coastal areas of Marie Byrd Land reﬂects advection of cold-air anomalies from

150 the East Antarctic plateau. The importance of advection in explaining the temperature

151 pattern is particularly evident in the potential temperature (Fig. 2); if the warming were

152 strictly adiabatic, there would be no changes in potential temperature.

153 The more variable response of the models over the Southern Ocean and most of East

154 Antarctica suggests that the direct eﬀects of WAIS collapse on climate over most of East

155 Antarctica and the Southern Ocean are small, relative to other processes and feedbacks.

156 In the aquaplanet experiments, large areas over the Southern Ocean cool, while in the

157 fully coupled-results with CCSM4, warming occurs over nearly the entire Southern Ocean.

158 The latter can be attributed to dynamical changes in the ocean due to wind forcing, as has

159 also been observed in a recent WAIS-collapse experiment with a coarse-resolution fully

160 coupled model [Justino et al., 2015], though we note that there is also warming over much

161 of the Southern Ocean in the slab-ocean experiment with ECHAM4.6. Meltwater ﬂuxes

162 – not considered in our experiments – may play a role in the ocean circulation response

163 to a real WAIS collapse [Holden et al., 2010], but this would not alter the fundamental

164 atmospheric response.

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165 Other changes in climate boundary conditions that might be associated with WAIS

166 collapse could potentially inﬂuence our results. In particular, we assumed that the

167 albedo does not change where the WAIS has been removed; in reality the land ice

168 and ice shelves might be replaced by ocean water that could be partly free of sea ice

169 in summer. The diﬀerence, however, is evidently not large: Otto-Bliesner et al. [2013]

170 conducted a similar fully-coupled experiment to ours (though with simpler topography)

171 using CCSM3, in which they replaced WAIS land ice and ice shelves with ocean, and used

172 130 ka boundary conditions. Their results show the same pattern of potential temperature

173 and winds changes (Fig. S4). We conclude that cooling along the mountains of Marie

174 Byrd Land, relative to the background climate state, as well as the advection of warm

175 anomalies towards adjacent areas of East Antarctica, are fundamental features of the

176 climate response to a collapse of the WAIS.

4.2. Ice Core Interpretation

177 Our results have important implications for the interpretation of ice-core paleotemperature

18 178 records. Popp [2008] presaged our results by suggesting that the comparatively low δ O

179 values for MIS 5e in the Mt. Moulton record from coastal Marie Byrd Land could

180 be explained by altered atmospheric circulation owing to lowered elevations over the

18 181 WAIS. Our results clearly support this idea. Given a characteristic scaling of δ O with

◦ 18 182 temperature of ∼ 0.8 / C [e.g., Steig et al., 2013], the relatively small δ O anomaly at h

183 Mt. Moulton, about 1–2 less than in the East Antarctic ice cores, is in good agreement h 18 184 with the temperature and δ O anomalies from the GCM experiments.

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185 The distinctive character of the Mt. Moulton isotope record supports other, independent

186 evidence of WAIS collapse during MIS 5e, but there are some caveats to this interpretation.

18 187 The Mt. Moulton record has not only relatively low δ O values during MIS 5e; it also has

18 188 relatively high δ O during the coldest part of the glacial period (∼20–40ka) compared

18 189 with other records (Fig. S1). From our model results, elevated δ O at Mt. Moulton

190 might be expected if the WAIS were thicker than present, but evidence for a substantially

191 thicker WAIS is equivocal [e.g., Steig et al., 2001]. Furthermore other changes associated

192 with MIS 5e climate that are not considered here, including more modest changes in

193 WAIS – short of a full “collapse” – may perhaps be suﬃcient to give rise to the anomalies

194 observed.

18 195 These caveats notwithstanding, δ O records from the East Antarctic ice cores may

196 provide indirect supporting evidence for the interpretation of the Mt. Moulton data in

197 terms of WAIS collapse. Comparison of the model results for surface temperature with

18 198 ice core δ O implicitly assumes a ﬁxed linear relationship between the two, whereas the

199 signiﬁcant circulation changes associated with WAIS-collapse may lead to non-linearities

200 in the isotope-temperature relationship. Sime et al. [2009] suggested that varying

18 18 201 magnitude of the MIS 5e δ O peak could be explained by diﬀerent δ O/temperature

202 sensititives, owing to much higher MIS 5e temperatures than previously thought. However,

203 they did not examine the possibility that atmospheric circulation changes associated with

18 204 WAIS collapse could explain those diﬀerences. In our results with ECHAM4.6, the δ O

205 response is stronger at Dome C than at Vostok, even though the elevation at neither

206 site changes. This is consistent with the observations. Such results are sensitive to the

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207 assumed ice sheet conﬁguration, but it is clear that topography-induced atmospheric

208 circulation changes need to be accounted for in the interpretation of ice core records.

209 Whether diﬀerences among the East Antarctic records can be used to constrain past

210 topographic changes should be examined further with a fully-coupled GCM that includes

211 isotope tracers.

212 Finally, our experiments may be used to guide the selection of new ice core records

213 that could be used to obtain more deﬁnitive evidence for or against WAIS collapse.

214 Comparison between the Mt. Moulton record and East Antarctic sites is complicated

215 by the uncertainty associated with East Antarctic elevation change, and the problem of

216 ice ﬂow for some locations (EDML, Vostok, Talos). Furthermore, Mt. Moulton may be

217 less than ideally situated as an indicator of WAIS elevation changes as it lies (at least

18 218 in our experiments) near the boundary between positive and negative anomalies in δ O.

219 Other blue-ice areas that retain ice from MIS 5e, if they can be identiﬁed further westward

220 along the Marie Byrd Land coast, would be useful. Another interesting site is Fletcher

221 Promontory, which extends from the WAIS into the Ronne-Filchner Ice Shelf; an ice core

222 record there was recently obtained by the British Antarctic Survey and may contain MIS

223 5e ice. However, local elevation changes at Fletcher Promontory, and the possibilty of

224 ice ﬂow from higher elevations, are likely to complicate interpretation of that record. An

225 especially promising location is Hercules Dome, located between the WAIS and the South

226 Pole. Hercules Dome is not likely to have signiﬁcantly changed in elevation in response

227 to WAIS collapse [Jacobel et al., 2005; Pollard and DeConto, 2009], but its proximity to

D R A F T March 25, 2015, 10:32am DRAFT STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE X - 13

18 228 the WAIS means that warmer temperatures and elevated δ O anomalies at that location

229 may be diagnostic of the changed atmospheric circulation associated with WAIS collapse.

5. Conclusion

230 Collapse of the West Antarctic Ice Sheet (WAIS) would cause regional changes in

231 atmospheric circulation, with signiﬁcant temperature anomalies over adjacent land and

232 ocean areas. Robust responses to WAIS collapse include cooling over the Ross Ice Shelf

233 and coastal Marie Byrd Land, and warming over the Ronne-Filchner Ice Shelf, coastal

234 Dronning Maud Land, and at Hercules Dome. Ice core paleotemperature data show a

235 signiﬁcantly smaller diﬀerence between last-interglacial and Holocene temperature at Mt.

236 Moulton, West Antarctica, than at all locations in East Antarctica. This observation is

237 consistent with the climate model response, and thus provides supporting evidence for

238 WAIS collapse during the last interglacial period.

239 Acknowledgments. T. Sowers was the ﬁrst to point out the evident correspondence

240 between our model results and the Mt. Moulton record. We thank C.M. Bitz and E.

241 Maroon for modeling support, L. Sime, E. Wolﬀ, D.S. Battisti, T.J. Fudge and M. Whipple

242 for helpful feedback, J. Pedro and F. Parrenin for assistance with the AICC2012 timescale,

243 and R. DeConto for providing ice sheet model output data. This work was supported by

244 U.S. National Science Foundation (grants 1043092 to EJS and 0230316 to JWCW) and

245 the Deparment of Energy (Computational Science Graduate Fellowship to HAS). We also

246 thank the Leverhulme Trust and the University of Edinburgh for support to EJS. This

247 paper is dedicated to the memory of Stephen C. Porter. All data and model results from

D R A F T March 25, 2015, 10:32am DRAFT X - 14 STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE

248 this paper will be archived at the National Snow and Ice Data Center (nsidc.org), in

249 accordance with the AGU Data Policy.

250

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A West Antarctic Ice Sheet EAIS AP RF

Bell.

Amundsen Sea

Marie Byrd Land Ross Ice Shelf B EDML Dome F Hercules Dome

Moulton Taylor Vostok Talos EDC C

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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Surface Temperature Potential temperature A B M a GR

CD 2 AM - GFDL

EF 4.6 ECHAM

GH 4 CCSM

-8 -6 -4 -2 0 4 862 Temperature Change (°C) 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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A B aM AM2 GR

CD M4 S HAM 4.6

G CC C E

Wind speed change 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.

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2 Holocene 1 0

0 10 20 110 120 Age (ka) Figure 4. Comparison of ice core δ18O records for the Holocene and the last interglacial period (green shading: MIS 5e interval). Purple line: Mt. Moulton δ18O (purple). Black line; 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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