Projecting Antarctic ice

43 Pages·2012·2.22 MB·English

TheCryosphereDiscuss.,6,3447–3489,2012 D
The Cryosphere is
www.the-cryosphere-discuss.net/6/3447/2012/ c TCD
Discussions u
doi:10.5194/tcd-6-3447-2012 s
s
©Author(s)2012.CCAttribution3.0License. io 6,3447–3489,2012
n
P
a
Thisdiscussionpaperis/hasbeenunderreviewforthejournalTheCryosphere(TC). p
e Antarctic ice
r
PleaserefertothecorrespondingfinalpaperinTCifavailable.
discharge from
|
SeaRISE response
D
is functions
c
u
s
s A.Levermannetal.
io
n
Projecting Antarctic ice discharge using P
a
p
e TitlePage
response functions from SeaRISE r
| Abstract Introduction
ice-sheet models
D
is Conclusions References
c
u
A. Levermann1,2, R. Winkelmann1, S. Nowicki3, J. L. Fastook4, K. Frieler1, ss Tables Figures
R. Greve5, H. H. Hellmer6, M. A. Martin1, M. Mengel1, A. J. Payne7, D. Pollard8, ion
T. Sato5, R. Timmermann6, W. L. Wang3, and R. A. Bindschadler3 Pa J I
p
e
1EarthSystemAnalysis,PotsdamInstituteforClimateImpactResearch,Potsdam,Germany r J I
2InstituteofPhysics,PotsdamUniversity,Potsdam,Germany |
Back Close
3Code615,NASAGoddardSpaceFlightCenter,GreenbeltMD20771,USA D
4ComputerScience/QuaternaryInstitute,UniversityofMaine,Orono,ME04469,USA isc FullScreen/Esc
5InstituteofLowTemperatureScience,HokkaidoUniversity,Sapporo060-0819,Japan us
s
6Alfred-Wegener-Institute,Bremerhaven,Germany io
n Printer-friendlyVersion
7BristolGlaciologyCentre,UniversityofBristol,UniversityRoad,Clifton,BristolBS81SS,UK P
a
8EarthandEnvironmentalSystemsInstitute,PennsylvaniaStateUniversity,UniversityParkPA p InteractiveDiscussion
e
16802,USA r
3447 |
Received:1August2012–Accepted:6August2012–Published:23August2012 D
is
c TCD
Correspondenceto:A.Levermann([email protected]) u
s
s
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. io 6,3447–3489,2012
n
P
a
p
e Antarctic ice
r
discharge from
|
SeaRISE response
D
is functions
c
u
s
s A.Levermannetal.
io
n
P
a
p
e TitlePage
r
| Abstract Introduction
D
is Conclusions References
c
u
ss Tables Figures
io
n
P
a J I
p
e
r J I
|
Back Close
D
is
c FullScreen/Esc
u
s
s
io
n Printer-friendlyVersion
P
a
p InteractiveDiscussion
e
r
3448 |
Abstract D
is
c TCD
u
The largest uncertainty in projections of future sea-level change still results from the s
s
potentially changing dynamical ice discharge from Antarctica. While ice discharge can ion 6,3447–3489,2012
alter through a number of processes, basal ice-shelf melting induced by a warming P
a
p
5 ocean has been identified as a major if not the major cause for possible additional ice er Antarctic ice
flow across the grounding line. Here we derive dynamic ice-sheet response functions discharge from
|
forbasalice-shelfmeltingusingexperimentscarriedoutwithintheSea-levelResponse SeaRISE response
to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic Dis functions
ice-sheet models. As used here these response functions provide separate contribu- c
u
tions for four different Antarctic drainage regions. Under the assumptions of linear- ss A.Levermannetal.
10 io
responsetheoryweprojectfutureice-dischargeforeachmodel,eachregionandeach n
P
ofthefourRepresentativeConcentrationPathways(RCP)usingoceanictemperatures a
p
from19comprehensiveclimatemodelsoftheCoupledModelIntercomparisonProject, e TitlePage
r
CMIP-5, and two ocean models from the EU-project Ice2Sea. Uncertainty in the cli-
| Abstract Introduction
matic forcing, the oceanic response and the ice-model differences is combined into an
15
D
uncertainty range of future Antarctic ice-discharge induced from basal ice-shelf melt. is Conclusions References
c
Theadditionalice-loss(Table6)isclearlyscenario-dependentandresultsinamedian u
of 0.07m (66%-range: 0.04–0.10m; 90%-range: −0.01–0.26m) of global sea-level ssio Tables Figures
n
equivalentforthelow-emissionRCP-2.6scenarioandyields0.1m(66%-range:0.06–
P
0.14m; 90%-range: −0.01–0.45m) for the strongest RCP-8.5. If only models with an a J I
20 p
e
explicit representation of ice-shelves are taken into account the scenario dependence r J I
remains and the values change to: 0.05m (66%-range: 0.03–0.08m) for RCP-2.6 and
|
0.07m (66%-range: 0.04–0.11m) for RCP-8.5. These results were obtained using a Back Close
D
timedelaybetweenthesurfacewarmingsignalandthesubsurfaceoceanicwarmingas is
c FullScreen/Esc
25 observed in the CMIP-5 models. Without this time delay the ranges for all ice-models us
s
changes to 0.10m (66%-range: 0.07–0.12m; 90%-range: 0.01–0.28m) for RCP-2.6 io
n Printer-friendlyVersion
and0.15m(66%-range:0.10–0.21m;90%-range:0.02–0.53m)forRCP-8.5.Allprob- P
a
ability distributions as provided in Fig. 10 are highly skewed towards high values. p InteractiveDiscussion
e
r
3449 |
1 Introduction D
is
c TCD
u
The future evolution of global mean and regional sea-level is important for coastal s
s
planning and associated adaptation measures. The Fourth Assessment Report (AR4) ion 6,3447–3489,2012
of the Intergovernmental Panel on Climate Change (IPCC) provided sea-level projec- P
a
p
5 tions explicitly excluding change in dynamic ice-discharge from both Greenland and er Antarctic ice
Antarctica(Solomonetal.,2007).Whilethepartoftheice-sheetdirectlysusceptibleto discharge from
|
ocean water on Greenland is limited, marine ice sheets in West Antarctica alone have SeaRISE response
the potentialto elevate sea level globallyby 3.3m (Bamber et al., 2009). Previous pro- Dis functions
jections of the Antarctic ice-sheet mass-balance have used fully coupled climate-ice- c
u
s
sheet models (e.g. Huybrechts et al., 2011; Vizcaino et al., 2010). These simulations s A.Levermannetal.
10 io
include feedbacks between the climate and the ice sheet and thereby provide very n
P
valuable information especially on multi-centennial time scale. On shorter, i.e. decadal a
p
to centennial, time scales the direct climatic forcing is likely to dominate the ice-sheet e TitlePage
r
evolution.For21st-centuryprojectionsitmightthusbemorefavorabletoapplytheout-
| Abstract Introduction
put of comprehensive climate models as external forcing to the ice sheet, neglecting
15
D
feedbackswhilepossiblyimprovingontheaccuracyoftheforcinganomalies.Herewe is Conclusions References
c
follow this approach. u
In order to meet the relatively high standards that are set by climate models for the ssio Tables Figures
n
oceanicthermalexpansionandglacier-andice-capmodelswhichusethefullrangeof
P
state-of-the-artclimateprojections,itwouldbedesirabletousedifferentice-sheetmod- a J I
20 p
e
els for a robust projection of the sea-level contribution. While changes in basal lubri- r J I
cations, ice-softening from surface warming and changes in surface elevation through
|
altered precipitation can affect dynamic ice-discharge from Antarctica, changes in dy- Back Close
D
namic ice loss is likely to be dominated by changes in basal melt underneath the ice is
c FullScreen/Esc
25 shelves. Here we combine the dynamic response of five ice-sheet models to changes us
s
in basal melt with the full uncertainty range of future climate change for each of the io
n Printer-friendlyVersion
Representative Concentration Pathways (RCP, Moss et al., 2010; Meinshausen et al., P
a
p InteractiveDiscussion
e
r
3450 |
2011b)using thecurrentsimulations fromtheCoupled ModelIntercomparisonProjec- D
is
tion, CMIP-5 (Taylor et al., 2012). c TCD
u
TheresponsefunctionsforthesefivedifferentAntarcticicesheetmodelsarederived ss
io 6,3447–3489,2012
fromastandardizedmeltingexperiment(M2)fromtheSea-levelResponsetoIceSheet n
P
Evolution(SeaRISE)intercomparisonproject(Bindschadleretal.,2012).Thiscommu- a
5 p
nity effort gathers a broad range of structurally different ice-sheet models to perform a e Antarctic ice
r
climate-forcing sensitivity study for both Antarctica (Nowicki et al., 2012b) and Green- discharge from
|
land(Nowickietal.,2012a).Asuiteofprescribednumericalexperimentsonacommon SeaRISE response
D
setofinputdatarepresentsdifferenttypesofclimateinput,namelyenhancedsub-shelf is functions
c
melting, enhanced sliding and surface temperature increase combined with enhanced u
10 s
net accumulation. sio A.Levermannetal.
n
The spread in the response of the participating models to these experiments results
P
from differences in the stress-balance approximations, the treatment of grounding line a
p
e TitlePage
motion,theimplementationofice-shelfdynamics,thecomputationofthesurface-mass r
balance,andinthecomputationaldemandwhichsetsstronglimitsonthespin-uppro-
15 | Abstract Introduction
cedure. This approach allows for the identification of the sensitivity of the response
D
of state-of-the-art ice-sheet models to changes in different types of climate-related is Conclusions References
c
boundary conditions. An interpolation analysis of the results is performed in (Bind- uss Tables Figures
schadler et al., 2012) in order to provide a best-guess estimate of the future sea-level io
n
20 contribution from the ice sheets. Here we process the SeaRISE-A−n1tarctica results of Pa J I
one of the experiments (M2: uniform, constant melt-rate of 20ma applied to all ice p
e
shelves) in the framework of linear response theory in order to provide projections of r J I
ocean-warming-induced ice loss from different drainage basins for the different RCP |
Back Close
scenarios. These methods have been used before, for example to generalize climatic D
25 response to greenhouse gas emissions (Good et al., 2011). isc FullScreen/Esc
u
s
s
io
n Printer-friendlyVersion
P
a
p InteractiveDiscussion
e
r
3451 |
2 Brief description of the ice-sheet models D
is
c TCD
u
All models used here are described in more detail by Bindschadler et al. (2012) (Table s
s
2). Here we provide a brief summary for the purpose of this paper referring to relevant ion 6,3447–3489,2012
publications from which even more detailed descriptions can be obtained. P
a
p
e Antarctic ice
r
2.1 AIF
5 discharge from
|
SeaRISE response
The Anisotropic Ice-Flow model is a 3-D ice-sheet model (without ice shelves) incor- D
porating anisotropic ice flow and fully coupling dynamics and thermodynamics (Wang is functions
c
u
etal.,2012).Thisisahigher-ordermodelwithlongitudinalandverticalshearstresses. s
s A.Levermannetal.
Themodelusesfinitedifferencemethodtocalculateice-sheetgeometryincludingiso- io
n
static bedrock adjustment, 3-D distributions of shear and longitudinal strain rates, en- P
10 a
hancement factors which account for the effect of ice anisotropy, temperatures, hori- p
e TitlePage
r
zontal and vertical velocities, shear and longitudinal stresses. The basal sliding is de-
termined by Weertman sliding law based on a cubic power relation of the basal shear | Abstract Introduction
stress. Ice sheet margin moves freely and the rounding line is detected by the float- D
is Conclusions References
ing condition. As the model lacks of ice shelves, the prescribed basal melt rates are c
15 u
applied to the ice-sheet perimeter grid-points only with a bedbelow sea level. The ice- ss Tables Figures
io
sheetmargin,i.e.thegroundingline,movesfreelywithinthemodelgrid-pointsandthe n
P
grounding line is detected by the floating condition without sub-grid interpolation. a J I
p
e
r J I
2.2 Penn-State-3D
|
Back Close
20 The Penn State 3D ice sheet model uses a hybrid combination of the scaled Shallow D
Ice (SIA) and Shallow Shelf (SSA) equations for shearing and longitudinal stretching isc FullScreen/Esc
u
flow respectively. The location of the grounding line is determined by simple flotation, s
s
with sub-grid interpolation as in (Gladstone et al., 2010). A parameterization relating io
n Printer-friendlyVersion
ice velocity across the grounding line to local ice thickness is imposed as an internal P
a
25 boundary-layercondition,sothatgrounding-linemigrationissimulatedreasonablywell pe InteractiveDiscussion
r
3452 |
without the need for very high, i.e. of the order of 100 m, resolution (Schoof, 2007). D
is
Ocean melting below ice shelves and ice-shelf calving use simple parameterizations, c TCD
u
s
along with a sub-grid parameterization at the floating-ice edge (Pollard and DeConto, s
io 6,3447–3489,2012
2009; Pollard and Deconto, 2012) n
P
a
p
5 2.3 PISM er Antarctic ice
discharge from
The ParallelIce Sheet Model (www.pism-docs.org) usedhere is based onversion sta- |
SeaRISE response
ble0.4,whichincorporatesthePotsdamParallelIceSheetModel(PISM-PIK)(Winkel- D
is functions
mann et al., 2011; Martin et al., 2011). Ice flow is approximated by a hybrid scheme c
u
incorporating both the SIA and SSA approximations (Bueler and Brown, 2009). An ss A.Levermannetal.
io
10 enthalpy formulation (Aschwanden et al., 2012) is used for thermodynamics, and the n
P
model employs a physical stress-boundary condition to the shelfy-stream approxima- a
p
tionaticefronts,incombinationwithasub-gridinterpolation(Albrechtetal.,2011)and e TitlePage
r
akinematicfirst-ordercalvinglaw(Levermannetal.,2012)atice-shelffronts.InPISM-
| Abstract Introduction
PIK, the grounding line is not subject to any boundary conditions or flux corrections.
D
Its position is determined from ice and bedrock topographies in each time step via the
15 is Conclusions References
floatation criterion. The grounding line motion is thus influenced only indirectly by the cu
velocitiesthroughtheicethicknessevolution.SincetheSSA(shallowshelfapproxima- ss Tables Figures
io
tion) velocities are computed non-locally and simultaneously for the shelf and for the n
P
sheet, a continuous solution over the grounding line without singularities is ensured a J I
p
and buttressing effects are accounted for. e
20 r J I
|
2.4 SICOPOLIS
Back Close
D
is
The SImulation COde for POLythermal Ice Sheets is a three-dimensional, polyther- c FullScreen/Esc
u
s
mal ice sheet model that was originally created by Greve (1995, 1997) in a version s
io
for the Greenland ice sheet, and has been developed continuously since then (Sato n Printer-friendlyVersion
25 and Greve, 2012) (sicopolis.greveweb.net). It is based on finite-difference solutions of Pa
p InteractiveDiscussion
the shallow ice approximation for grounded ice (Hutter, 1983; Morland, 1984) and the e
r
3453 |
shallow shelf approximation for floating ice (Morland, 1987; MacAyeal, 1989). Special D
is
attention is paid to basal temperate layers (that is, regions with a temperature at the c TCD
u
s
pressure melting point), which are positioned by fulfilling a Stefan-type jump condition s
io 6,3447–3489,2012
at the interface to the cold ice regions. Basal sliding is parameterized by a Weertman- n
P
type sliding law with sub-melt sliding (that allows for a gradual onset of sliding as the a
5 p
basal temperature approaches the pressure melting point, Greve, 2005), and glacial e Antarctic ice
r
isostasy is described by the elastic lithosphere/relaxing asthenosphere (ELRA) ap- discharge from
|
proach (Le Meur and Huybrechts, 1996). The position and evolution of the grounding SeaRISE response
D
lineisdeterminedbythefloatingcondition.Betweenneighbouringgroundedandfloat- is functions
c
ing grid points, the ice thickness is interpolated linearly, and the half-integer auxiliary u
10 s
grid point in between (on which the horizontal velocity is defined, Arakawa C grid) is sio A.Levermannetal.
n
consideredaseithergroundedorfloatingdependingonwhethertheinterpolatedthick-
P
ness leads to a positive thickness above floatation or not. a
p
e TitlePage
r
2.5 UMISM
| Abstract Introduction
D
The University of Maine Ice Sheet Model consists of a time-dependent finite-element
15 is Conclusions References
solution of the coupled mass, momentum, and energy conservation equations using cu
the SIA (Fastook, 1990, 1993; Fastook and Chapman, 1989; Fastook and Hughes, ss Tables Figures
io
1990; Fastook and Prentice, 1994) with a broad range of applications (for example n
P
Fastook et al., 2012, 2011) The 3-D temperature field, on which the flow law ice hard- a J I
p
ness depends, is obtained from a 1-D finite-element solution of the energy conserva- e
20 r J I
tion equation at each node. This thermodynamic calculation includes vertical diffusion
|
andadvection,butneglectshorizontalmovementofheat.Alsoincludedisinternalheat Back Close
D
generation produced by shear with depth and sliding at the bed. Boundary conditions is
c FullScreen/Esc
consist of specified surface temperature and basal geothermal gradient. If the calcu- u
s
s
25 lated basal temperature exceeds the pressure melting point, the basal boundary con- io
n Printer-friendlyVersion
dition is changed to a specified temperature, and a basal melt rate is calculated from
P
a
the amount of latent heat of fusion that must be absorbed to maintain this specified p InteractiveDiscussion
e
temperature. Conversely, if the basal temperature drops below the pressure melting r
3454 |
point where water is already present at the bed, a similar treatment allows for the cal- D
is
culation of a rate of basal freezing. A map-plane solution for conservation of water at c TCD
u
s
the bed, whose source is the basal melt or freeze-on rate provided by the temperature s
io 6,3447–3489,2012
solution, allows for movement of the basal water down the hydrostatic pressure gradi- n
P
ent (Johnson and Fastook, 2002). Areas of basal sliding can be specified if known, or a
5 p
determinedinternallybythemodelasregionswherelubricatingbasalwaterispresent, e Antarctic ice
r
produced either by melting in the thermodynamic calculation or by movement of wa- discharge from
|
ter beneath the ice sheet down the hydrostatic gradient. Ice shelves are not modeled SeaRISE response
D
explicitly in UMISM. However, a thinning rate at the grounding line produced by longi- is functions
c
tudinal stresses is calculated from a parameterization of the thinning of a floating slab u
10 s
(Weertman, 1957). No sub-grid grounding line interpolation is applied. sio A.Levermannetal.
n
P
a
p
3 Deriving the response functions e TitlePage
r
InordertousethesensitivityexperimentscarriedoutwithintheSeaRISEproject(Bind- | Abstract Introduction
schadler et al., 2012), we assume that for the 21st century the temporal evolution of D
is Conclusions References
the ice-discharge can be expressed as c
15 u
ss Tables Figures
t io
Z n
S(t)= dτR(t−τ)m(τ) (1) P
a J I
p
0 e
r J I
where S is the sea-level contribution from ice discharge, m is the forcing represented |
Back Close
by the basal-melt rate and R is the ice-sheet response-function. t is time starting from D
a period prior to the beginning of a significant forcing. The responses function R can is
c FullScreen/Esc
u
thus be understood as the response to a delta-peak forcing with magnitude one. s
20 s
io
t n Printer-friendlyVersion
Z P
Sδ(t)= dτR(t−τ)δ(τ)=R(t) ape InteractiveDiscussion
r
0
3455 |
Pleasenotethatweexpressice-dischargethroughoutthepaperinunitsofglobalmean D
is
sea-level equivalent. That means that in deriving the response functions we only diag- c TCD
u
s
noseicelossaboveflotationthatisrelevantforsealevel.Asasimpleconsequencethe s
io 6,3447–3489,2012
response function is unitless. Furthermore the basal-melt signal applied as well as the n
P
ice-discharge signal used to derive the response functions are anomalies with respect a
5 p
to a baseline simulation under present-day boundary conditions (Bindschadler et al., e Antarctic ice
r
2012). discharge from
|
Linearresponsetheory,asrepresentedbyEq.(1),canonlydescribetheresponseof SeaRISE response
D
asystemuptoacertainpointintime;100yrisarelativelyshortperiodfortheresponse is functions
c
of an ice-sheet and the assumption of a linear response is thereby justified. There are u
10 s
a number of ways to obtain the system-specific response function R (e.g. Winkelmann sio A.Levermannetal.
n
and Levermann, 2012). Within the SeaRISE project the switch-on basal-melt experi-
P
mentscanbeusedconvenientlysincetheirresponsedirectlyprovidesthetimeintegral a
p
e TitlePage
of the response function for each individual ice-sheet model. Assuming that over a r
forcing period of 100yr the different topographic basins on Antarctica from which ice
15 | Abstract Introduction
is discharged, respond independently of each other, we diagnose the additional ice-
D
flow from four basins separately (Fig. 1) and interpret them as the time integral of the is Conclusions References
c
response function for each separate basin. The response function for each basin is uss Tables Figures
shown in Fig. 2. The aim of this study is specifically to capture differences between io
n
20 idniffdeivriednutarleicsep-osnhseeeftumncotdioenlss..The fact that the models differ is nicely illustrated by their Pap J I
e
To obtain R we use the response to the temporal stepwise increase in basal melt r J I
−1
by 20ma (denoted M2-experiment in Bindschadler et al., 2012). The ice-sheet re- |
Back Close
sponse to a step forcing is equivalent to the temporal integral of the response function D
25 R with t=0 being the time of the switch-on in forcing isc FullScreen/Esc
u
s
s
Zt Zt io
n Printer-friendlyVersion
S (t)= dτR(t−τ)∆m ·Θ(τ)=∆m · dτR(τ) P
sf 0 0 a
p InteractiveDiscussion
0 0 e
r
3456 |