In their article ‘Mass gains of the Antarctic ice sheet exceed losses’ Zwally and others (2015) choose Vostok Subglacial Lake as an exemplary region to demonstrate their inference of surface height change rates from a portion of the ICESat mission’s laser altimetry data (2003–08). In their appendix, they discuss some of the remarkable differences between their results and those reported by Richter and others (2008, 2013, 2014). However, the selective consideration of our works and the misleading or incorrect interpretation of our results call for clarificationFil: Richter, Andreas Jorg. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Technische Universitaet Dresden; AlemaniaFil: Horwath, M.. Technische Universitaet Dresden; AlemaniaFil: Dietrich, R.. Technische Universitaet Dresden; Alemani

Dietrich, R.

Horwath, M.

Richter, Andreas Jorg

Journal of Glaciology

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A. RICHTER

M. HORWATH

R. DIETRICH

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Comment on Zwally and others (2015)-Mass gains of the Antarctic ice sheet exceed losses

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Journal of Glaciologyhttp://journals.cambridge.org/JOGAdditional services for Journal of Glaciology:Email alerts: Click hereSubscriptions: Click hereCommercial reprints: Click hereTerms of use : Click hereComment on Zwally and others (2015)-Mass gains of the Antarctic ice sheet exceed lossesA. RICHTER, M. HORWATH and R. DIETRICHJournal of Glaciology / FirstView Article / May 2016, pp 1 - 3DOI: 10.1017/jog.2016.60, Published online: 03 May 2016Link to this article: http://journals.cambridge.org/abstract_S0022143016000605How to cite this article:A. RICHTER, M. HORWATH and R. DIETRICH Comment on Zwally and others (2015)-Mass gains of the Antarctic ice sheetexceed losses. Journal of Glaciology, Available on CJO 2016 doi:10.1017/jog.2016.60Request Permissions : Click hereDownloaded from http://journals.cambridge.org/JOG, IP address: 190.50.29.106 on 04 May 2016LetterComment on Zwally and others (2015)-Mass gains ofthe Antarctic ice sheet exceed lossesIn their article ‘Mass gains of the Antarctic ice sheet exceedlosses’ Zwally and others (2015) choose Vostok SubglacialLake as an exemplary region to demonstrate their inferenceof surface height change rates from a portion of the ICESatmission’s laser altimetry data (2003–08). In their appendix,they discuss some of the remarkable differences betweentheir results and those reported by Richter and others(2008, 2013, 2014). However, the selective considerationof our works and the misleading or incorrect interpretationof our results call for clarification.1. Richter and others (2014) present results of in-situobservations of surface height changes in the area ofVostok station, by repeated kinematic GNSS profiling.These observations were first carried out in 2001 and wererepeated in 2012 and 2013. In 308 crossovers, covering anarea of ∼6 km2, a mean value of surface height change of1 ± 5 mm a−1 was determined, indicating a stable surfaceheight for more than a decade including the entire ICESatmission period. This observational result contradicts the con-clusion of Zwally and others (2015), of a sustained increaseof surface height over Vostok Subglacial Lake. The claimedmean rate of +20.2 mm a−1 implies a surface rise exceeding20 cm between our kinematic surveys. This is not reconcil-able with the accuracy of our surface height profiles.Although Zwally and others (2015) cite the Richter andothers (2014) work in their review, at no point do theyaddress this discrepancy.2. Richter and others (2014) discuss in detail, the fact thatthe ice sheet above Subglacial Lake Vostok is in hydrostaticequilibrium (also Ewert and others, 2012; Schwabe andothers, 2014). This means that the local surface heightchange observed at a point within the lake area reflects pri-marily the mean ice-thickness change over the lake area.Spatial variations in the ice-thickness change are attenuatedby a factor (ρw− ρi)/ρw (with water density ρw and ice densityρi), thus propagating only 8% of the difference between thelocal and the mean ice-thickness change to the surfaceheight.The stability of the surface height observed around Vostokstation is therefore to a large extent valid for the whole lake.In particular, it would require an implausibly large localanomaly in ice-thickness change close to Vostok station tocounterbalance a mean ice-thickness growth of 2 cm a−1implied by the remote sensing results as interpreted byZwally and others (2015).In the light of the hydrostatic balance, the significantspatial variation in height change rates across the lake areain Figure 5 of Zwally and others (2015), e.g. the jumps inICESat profile 0330 and slopes in profiles 1312 and 0077,need further explanation.3. Zwally and others (2015) misinterpret Figure 4 ofRichter and others (2014) writing: ‘Similarly, ΔHi (i.e. ΔH(ti)in Figure 4 of Richter and others, 2014) mostly overlaprather than separate in time as they would if bias adjustmentswere applied.’ This figure is intended to reveal surfacedeformations instead of height changes over time.Therefore, any effect of inter-campaign biases is eliminatedbeforehand. Richter and others (2014) explain this in detail.4. Richter and others (2008) document the accumulationrates and snow densities observed in-situ around Vostokstation. These are challenged by Zwally and others (2015).It may be that the authors prefer models and remotesensing results rather than local observations. We preferlong-term, solid field observation as evidence of actual be-haviour of the system for the following reasons.First, the accumulation map derived by Arthern and others(2006) and evoked by Zwally and others (2015) has alreadybeen reported to overestimate the accumulation rates in theVostok Subglacial Lake region by ∼10 mm w.e. a−1 (i.e. by25–50%), thus underestimating the regional N–S accumula-tion gradient, and providing error estimates by a factor of2–5 too small in this region (Richter and others, 2013).In addition, Zwally and others’ statement, ‘Richter andothers (2008)… did not actually measure the rise of thesnow surface in the vicinity of their GPS markers’ is nottrue. However, given the stochastic nature of the spatio-tem-poral distribution of snow buildup, especially in an areaaffected by snow dune drift and anthropogenic activity, thelocal snow heights measured at a few GPS markers over afew years is unlikely to yield a snow build-up rate of ad-equate precision and reliability for a comparison with themuch more homogeneous, densification driven vertical par-ticle velocity. Robust long-term accumulation rates fromdated tephra layers in snow pits or snow build-up ratesobserved over decades in the array of 79 accumulationstakes at Vostok are far better suited for this purpose.Instrumental accumulation monitoring (e.g. in arrays of accu-mulation stakes) is usually too limited in its spatial extensionand resolution to account rigorously for the spatial compo-nent of snow build-up variation. A pragmatic approach toreduce the noise introduced by this spatial variation is to in-crease the temporal integration interval. For this reason, andconsidering the significant 50 a period in temporal accumu-lation variation established in the Vostok station area(Ekaykin and others, 2004), Richter and others (2008) pre-ferred the 200 a mean accumulation rate over that derivedfrom 25 a of stake measurements.Third, when determining the accumulation rate – either bystake measurements or from dated layer depths in snow pitsand boreholes – the primary, observed quantity is an accu-mulation layer thickness (corrected for densification).Subsequently, this thickness is converted to an accumulationrate (e.g. in terms of w.e. height a−1) using measured dens-ities. It is therefore inappropriate to choose an arbitrarydensity value to reconstruct the snow build-up rate from pub-lished accumulation rates, as exercised by Zwally and others(2015) in their attempt to invalidate the conclusion of a stablesurface height from Richter and others (2008). The densityvalue of 0.33 g cm−3 adopted by Richter and others (2008,2014) represents the best estimate for the surface snowlayer combining the results from numerous snow pits andmeasurements in the Vostok accumulation stake farm(Ekaykin and others, 2003; personal communication fromEkaykin, 2007). This value was used to derive theJournal of Glaciology (2016), Page 1 of 3 doi: 10.1017/jog.2016.60© The Author(s) 2016. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.accumulation rate of 22.9 ± 1.8 mm a−1 from the snowbuildup observed in the Vostok stake farm in the period1970–1995 (Ekaykin and others, 2004), and there is neitherreason nor evidence given by Zwally and others (2015) forwhy a density of 0.30 g cm−3 is ‘a better value’ for theVostok station area. The density variation within the upper-most 20 cm of the stacked snow-pit profile representsrandom scatter rather than a dependence on depth(Ekaykin and others, 2003; personal communication fromEkaykin, 2007). The stake farm observations at Vostok,extended to the 40 a period, 1970–2010, yield a meansurface density of 0.335 g cm−3 and a (densification-cor-rected) snow build-up rate of 67.8 mm a−1 (Ekaykin andLipenkov, 2010). If preference is given to this instrumental(in situ) result instead of the long-term mean accumulationrate from snow pits and shallow cores (Ekaykin and others,2004), the surface height change implied by the GPS obser-vations around Vostok station (Richter and others, 2008)amounts to 67.8 mm a−1–62.1 mm a−1=+5.7 ± 5.2 mma−1. Accumulation rates and density values that, togetherwith the GPS observed vertical particle velocity of −62.1mm a−1, yield surface height change rates of +14.2 mma−1 or even +61.2 mm a−1 as proposed by Zwally andothers (2015) are very much incompatible with the resultsof decades of high-quality glaciological fieldwork at Vostok.5. Zwally and others (2015) challenge the stability of theGPS markers presented by Richter and others (2014), describ-ing them as ‘poles placed in the firn to some unspecifieddepths’ and claiming: ‘An unspecified potential source oferror is possible motion of their GPS markers within thefirn’. Richter and others (2014) state: ‘The markers consistof wooden stakes or aluminum tubes initially emplaced atleast 50 cm deep in the snow.’ Furthermore, Richter andothers (2014) refer their readers to Richter and others(2013) for additional details concerning the marker monu-mentation and observation procedure. In that paper, it isexplained: ‘Each of these markers was complemented bytwo wooden reference stakes (10 m apart) arranged in a tri-angle. Before and after the GNSS occupations, the stabilityof the GNSS marker was verified by tape and levelling mea-surements within the triangle.’ These local measurementshave proven in all cases that the ‘possible motion of theirGPS markers within the firn’ is indeed zero. Finally, Richterand others (2014) presented vertical velocities of 56 GNSSmarkers distributed over the whole lake area. The observedvelocities are not only consistent between markers close toeach other, but reveal a coherent pattern all over the lake.The velocities range from −50 to −60 mm a−1 in the south-ern part of the lake up to −80 to −100 mm a−1 in the nor-thern part. Richter and others (2014) discuss in detail thatthis is a combined effect of a regional N–S accumulation gra-dient, proven independently by glaciological data and thehydrostatic balance of the floating ice.6. Based on the observational results demonstrating thestability of the ice surface height above Vostok SubglacialLake throughout the ICESat mission’s duration, Richter andothers (2014) derived relative ICESat intercampaign biaseswithin the lake area. They used the GLAS 12 data productin release 633 and applied the G–C correction (Borsa andothers, 2014) as mentioned by Zwally and others (2015).This is equivalent to the data release 634 used in the bias es-timation by Zwally and others (2015), since some minor add-itional modifications in the newer release are negligible, atleast in their impact on surface elevations over VostokSubglacial Lake. Both bias sets, however, disagree signifi-cantly in their temporal evolution (Fig. 1). In particular, thetemporal trend of the biases, which crucially affects the in-ferred mass-balance rates, differs substantially betweenboth sets. Zwally and others (2015) do not address this dis-crepancy. In our opinion, estimates of surface height trendsand ice-volume change generated for East Antarctica andelsewhere with the inter-campaign biases presented byZwally and others (2015) are therefore highly questionable.ACKNOWLEDGEMENTSWe thank two anonymous reviewers for their comments. Partof our research was supported by the German ResearchFoundation (DFG) under contract DI 473/38-1.A. RICHTERM. HORWATHR. DIETRICHTechnische Universität Dresden, Institutfür Planetare Geodäsie, 01062Dresden, GermanyE-mail: andreas.richter@tu-dresden.deFig. 1. Comparison of ICESat inter-campaign biases (2003–09) estimated by Richter and others (2014) and those by Zwally and others (2015)after subtraction of their mean value.2 Richter and others: CorrespondenceREFERENCESArthern RJ, Winebrenner DP and Vaughan DG (2006) Antarcticsnow accumulation mapped using polarization of 4.3 cm wave-length microwave emission. J. Geophys. Res., 111(D6), D06107(doi: 10.1029/2004JD005667)BorsaAA,MoholdtG, FrickerHAandBrunt KM (2014)A rangecorrec-tion for ICESat and its potential impact on ice-sheet mass balancestudies. Cryosphere, 8, 345–357 (doi: 10.5194/tc-8-345-2014)Ekaykin AA and Lipenkov VY (2010). Forty years of snow accumu-lation instrumental measurements in central East Antarctica. InPresented at XXXI SCAR Open Science Conference, 3 August2010, Buenos AiresEkaykin AA, Lipenkov VY, Petit JR and Masson-Delmotte V (2003)50-letniy tsikl v izmeneniyah akkumulyatsii i isotopnogo sostavasnega na stantsii Vostok [50-year cycle in changes of snow accu-mulation and isotope composition at Vostok Station]. Mater.Glyatsiol. Issled./Data Glaciol. Stud., 94 [In Russian with Englishsummary]Ekaykin AA and 5 others (2004) The changes in isotope compositionand accumulation of snow at Vostok station, East Antarctica, overthe past 200 years. Ann. Glaciol., 39, 569–575Ewert H and 5 others (2012) Precise analysis of ICESat altimetry dataand assessment of the hydrostatic equilibrium for subglacial LakeVostok, East Antarctica. Geophys. J. Int., 191(2), 557–568 (doi:10.1111/j.1365-246X.2012.05649.x)Richter A and 9 others (2008) Observational evidence on the stabil-ity of the hydro-glaciological regime of subglacial Lake Vostok.Geophys. Res. Lett., 35, L11502 (doi: 10.1029/2008GL033397)Richter A and 10 others (2013) Ice flow velocities over VostokSubglacial Lake, East Antarctica, determined by 10 years ofGNSS observations. J. Glaciol., 59(214), 315–326 (doi:10.3189/2013JoG12J056)Richter A and 12 others (2014) Height changes over subglacial LakeVostok, East Antarctica: insights from GNSS observations. J.Geophys. Res. Earth Surf., 119, 2460–2480 (doi: 10.1002/2014JF003228)Schwabe J, Ewert H, Scheinert M and Dietrich R (2014) Regionalgeoid modelling in the area of subglacial Lake Vostok,Antarctica. J. Geodyn., 75, 9–21 (doi: 10.1016/j.jog.2013.12.002)Zwally HJ and 5 others (2015) Mass gains of the Antarctic ice sheetexceed losses. J. Glaciol., 61(230), 1019–1036 (doi: 10.3189/2015JoG15J071)MS received 12 December 2015 and accepted in revised form 3 April 20163Richter and others: Correspondence

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