Modelling Antarctic and Greenland volume changes during the 20th and 21st centuries forced by GCM time slice integrationsIntroductionThe ice sheet modelClimate forcingClimate anomaly patternsClimatic time seriesPattern scaling techniqueResultsContributions to global sea-level changePatterns of ice thickness changeSensitivity of the results to the model setupComparison with observationsSummary and conclusionAcknowledgementsReferencesModelling Antarctic and Greenland volume changes during the20th and 21st centuries forced by GCM time slice integrationsPhilippe Huybrechtsa,b,*, Jonathan Gregoryc,d, Ives Janssensb, Martin WildeaAlfred-Wegener-Institut fu¨r Polar- und Meeresforschung, Postfach 120161, D-27515 Bremerhaven, GermanybDepartement Geografie, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, BelgiumcHadley Centre for Climate Prediction and Research, Meteorological Office, London Road, RG12 2SY Bracknell, UKdDepartment of Meteorology, University of Reading, Earley Gate, P.O. Box 243, RG6 6BB Reading, UKeInstitute for Atmospheric and Climate Science ETH, Swiss Federal Institute of Technology, Winterthurerstrasse 190,CH-8057 Zu¨rich, SwitzerlandReceived 7 May 2003; received in revised form 19 September 2003; accepted 21 November 2003AbstractCurrent and future volume changes of the Greenland and Antarctic ice sheets depend on modern mass balance changes andon the ice-dynamic response to the environmental forcing on time scales as far back as the last glacial period. Here we focus onmodel predictions for the 20th and 21st centuries using 3-D thermomechanical ice sheet/ice shelf models driven by climatescenarios obtained from General Circulation Models. High-resolution anomaly patterns from the ECHAM4 and HadAM3Htime slice integrations are scaled with time series from a variety of lower-resolution Atmosphere –Ocean General CirculationModels (AOGCM) to obtain the spread of results for the same emission scenario and the same set of ice-sheet modelparameters. Particular attention is paid to the technique of pattern scaling and on how GCM based predictions differ from olderice-sheet model results based on more parameterised mass-balance treatments. As a general result, it is found that the effect ofincreased precipitation on Antarctica dominates over the effect of increased melting on Greenland for the entire range ofpredictions, so that both polar ice sheets combined would gain mass in the 21st century. The results are very similar for bothtime-slice patterns driven by the underlying time evolution series with most of the scatter in the results caused by the variabilityin the lower-resolution AOGCMs. Combining these results with the long-term background trend yields a 20th and 21st centurysea-level trend from polar ice sheets that is however not significantly different from zero.D 2004 Elsevier B.V. All rights reserved.Keywords: Polar ice sheets; Climate change; Sea level rise; Greenhouse warming; Numerical modeling; Mass balance1. IntroductionBy far the largest amount of continental water isstored in the ice sheets of Antarctica and Greenland,which would add some 70 m to global sea level rise ifthey were to melt entirely. The average rate of massexchange between these ice sheets and the oceanscorresponds to about 6.5 mm/year of sea level change,0921-8181/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.gloplacha.2003.11.011* Corresponding author. Alfred-Wegener-Institut fu¨r Polar- undMeeresforschung, Postfach 120161, D-27515 Bremerhaven, Ger-many. Tel.: +49-471-4831-1194; fax: +49-471-4831-1149.E-mail address: [email protected](P. Huybrechts).www.elsevier.com/locate/gloplachaGlobal and Planetary Change 42 (2004) 83 –105or 65 cm per century, implying that even relativelysmall imbalances between the average yearly snowfalland mass loss by surface melting and ice flow acrossgrounding lines can have a significant effect, both forsocieties and the environment (Church et al., 2001).Despite considerable progress in observational dataover the last decade, in particular from remote sensingplatforms, the question of whether the polar ice sheetsare in balance with the present-day climate can stillnot be answered with confidence (Rignot and Thom-as, 2002), although large overall imbalances areincreasingly considered unlikely. Large uncertaintiesare associated with future model predictions of icesheet respon se, related to issues such as the evolutionof greenhouse gas concentrations, the climate sensi-tivity to these changes, and the way such climatechanges will affect the surface mass balance compo-nents of snow accumulation and meltwater runoff,which primarily determine volume changes on timescales less than a century (Huybrechts and de Wolde,1999).From a modeling point of view, it is convenient todistinguish between four components determiningcurrent and future volume changes of the polar icesheets. The first component is the long-term back-ground evolution as a result of ongoing ice-d ynamicadjustment to past environmental changes as far backas the last glacial period. Superimposed on this long-term trend is the effect of modern mass-balancechanges during the 20th and 21st centuries. A nydeviation from their long-term average has an imme-diate effect on ice volume and thus on sea level. Inaddition, there is the ice-dynamic response to thesemodern surface mass-balance changes due to varia-tions in the velocity field associated with changes inice thickness and surface slope. As a fourth compo-nent one should also consider the possibility of‘unexpected ice-dynamic responses’, which may ormay not be related to contempo rary climate changes,and which find their origin in variations at the icesheet base or at the grounding line. Examples are theinferred thinning of the Pine Island and Thwaitessectors of the West Antarctic ice sheet (Shepherd etal., 2002) or the oscillatory behaviour of the SipleCoast ice streams (Joughin et al., 2002). Linked to thislast category is the possibility of unstable behaviour,most importantly of a collapse of the West Antarcticice sheet, but such behaviour is considered to be veryunlikely during the 21st century (Vaughan andSpouge, 2002; Bindschadler and Bentley, 2002).The distinction between the long-term trend and theresponse to 20th/21st century mass-balance changes isadmittedly somewhat arbitrary, but is convenient tomake from a modeling point of view because detailedforcing can only be generated for