Influence of West Antarctic Ice Sheet Collapse on Antarctic Surface
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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. DRAFT March 25, 2015, 10:32am DRAFT X - 2 STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE 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. D R A F T March 25, 2015, 10:32am DRAFT STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE X - 3 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. DRAFT March 25, 2015, 10:32am DRAFT X - 4 STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE 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 DRAFT March 25, 2015, 10:32am DRAFT STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE X - 5 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 DRAFT March 25, 2015, 10:32am DRAFT X - 6 STEIG ET AL.: CLIMATE RESPONSE TO WAIS COLLAPSE 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.