The Antarctic Ice Sheet and
its role in present-day
sea-level change
Sea level change
Sea level change
Sea level change – where does it come from?
Glaciers/ice
sheets
Source: IPCC 4th Assessment, 1961 to 2003 and 1993 to 2003
Why we need to know where it comes from

Understand processes ->
improved predictions
Source: IPCC 4th Assessment
Why we need to know where it comes from

Understand processes ->
improved predictions

Sea level change
has a
spatial pattern
Source: IPCC 4th Assessment
Location matters I
Greenland mass loss equiv to
1mm/yr global sea level rise
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West Antarctic mass loss equiv
to 1mm/yr global sea level rise
Pacific islands feel the brunt, regardless
Northern Europe feels less than average due to Greenland melt
Why location matters: gravity and rebound
Tamisiea & Mitrovica, 2011
So where does it come from?

Greenland quite well understood. Not so for Antarctica
Courtesy Jonathan Bamber,
2010
So where does it come from?

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No consensus
Even on the sign
Differences are largely
technique-specific
Systematic differences are
greater than the
uncertainties
Some uncertainties not
sufficiently well-known to
be quantified
Each technique needs
work – our work focuses
on GRACE
The Gravity Recovery and Climate
Experiment (GRACE)
GRACE estimates of rate of total mass
change (2002-10)
(atmospheric and
oceanic mass
“removed”)
mm/yr water
equivalent; signal
not particularly
linear in some
regions
Area integrated
sum ~0 Gt/yr
Signal is
dominated by ice
mass change
plus signal within
the solid earth
Glacial Isostatic Adjustment (GIA): A (basic)
primer

A simple view as

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
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Earth rotational feedbacks
Gravitational feedbacks
-> Global feedbacks
Modelling critically requires


Time-steps of spatial distribution of
ice mass
Knowledge of how Earth responds to
loading: (3d) Earth structure
(notably, lithospheric thickness, lower
& upper mantle viscosity)
One model of Antarctic ice evolution since
Last Glacial Maximum
Ross Ice Shelf
Filchner-Ronne
Ice Shelf
Antarctic
Peninsula
GIA model estimates expressed in terms
of vertical displacement
Ice history component varied only;
Earth model adds further variation – this uncertainty
swamps GRACE estimates of ice mass change
Large differences =
uncertainty! How to
validate?
Earth structure and rheology poorly
understood


Percent variations of modelled
shear wave velocity with respect to
anisotropic PREM. Morelli & Danesi,
GPC, 2004
Only broad-scale
features known
Not straightforward
to relate seismic
measurements to the
information needed in
GIA modelling
Glacial Isostatic Adjustment (GIA): A (basic)
primer

A simple view as




Earth rotational feedbacks
Gravitational feedbacks
-> Global feedbacks
Modelling critically requires


Time-steps of spatial distribution of
ice mass
Knowledge of how Earth responds to
loading: (3d) Earth structure
(notably, lithospheric thickness, lower
& upper mantle viscosity)
Glacial Isostatic Adjustment (GIA): A (basic)
primer

A simple view as




Earth rotational feedbacks
Gravitational feedbacks
-> Global feedbacks
Modelling critically requires


Time-steps of spatial distribution of
ice mass
Knowledge of how Earth responds to
loading: (3d) Earth structure
(notably, lithospheric thickness, lower
& upper mantle viscosity)
Glacial Isostatic Adjustment (GIA): A (basic)
primer

A simple view as




Earth rotational feedbacks
Gravitational feedbacks
-> Global feedbacks
Modelling critically requires


Time-steps of spatial distribution of
ice mass
Knowledge of how Earth responds to
loading: (3d) Earth structure
(notably, lithospheric thickness, lower
& upper mantle viscosity)
GRACE estimates of ice mass change degraded
Improving our understanding of Antarctic
ice mass balance

Generate more accurate models of GIA
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Primary need: more accurate ice evolution from 20ka BP to
present
Secondary need: more accurate earth structure
Data to constrain the models
An ability to test the models and measure improvement
(if any)
How to validate/constrain GIA models?

Glacial geology
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

Moraines
Cosmogenic exposure dating
Erosional trim lines
Biological markers
Ice core data
Marine Geophysics
Geodesy
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Earth rotation/polar motion
Gravity field change
Change in Earth’s shape
From O’Cafaigh, et al, GRL, 2002
Using GPS to measure change of shape
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Problems:


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No rock! (almost) [but
enough to be useful]
No power! [partly
overcome]
Cost and logistical difficulty
of access (up to ~1800km
from base)
Spot the wind
generator…
Existing GIA models &
empirical uplift estimates
ICE-5Gv1.2 (VM2)
(Peltier, 2004)
IJ05 (‘average mantle’)
(Ivins and James, 2005)
Riva et al., 2009
Empirical GIA uplift estimate
Conventional GIA models
Thomas et al.,
GRL, 2011
Improving on the existing GIA models
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Some details in all models look good, but clearly no model is
fit for purpose for all W or E Antarctica
Ideally would be driven by glaciological model which fits
available ice history (e.g., geological) constraints and have
uncertainties of both ice and earth models
Ice model: community ice sheet model (GLIMMER)
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Some limitations at present, but significant advance on existing
Earth model: Symmetric (1d) model with elastic and density
structure taken from PREM, linear Maxwell rheology. Three
free parameters: lithospheric thickness, and upper and lower
mantle viscosity
GIA model code from Glenn Milne
Ice extent constraints
LGM ice
extent
constraints
5ka BP ice extent
constraints
Marine core sites from
Livingstone et al. (2012).
Courtesy Pippa Whitehouse
Ice sheet reconstruction
Ice thickness change 20 ka BP
to present ⇒ ~8m ESL rise
20 ka BP
10 ka BP
15 ka BP
5 ka BP
Comparing model output to
observations of past ice extent
Whitehouse et al., QSR, 2012
Differences between ice sheet
reconstructions
Peltier, 2004
Whitehouse et al., 2012
Source: King, 2013
Golledge et al., in review
Paleo Sea Level Implications
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Paleo sea level does not
close
Other reconstructions
are suggesting values as
low as 6m (Gomez et al.
in prep)
ICE6G is >10m
Ivins et al., 2013
New GIA model (W12)
uplift rates vs GPS
Whitehouse et al., GJI, 2012
Note: GPS
rates corrected
for elastic
effects
Remaining misfit

Nield et al., GRL, 2012
Antarctic Peninsula most obvious

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Ice constraints loose here
Know there has been recent
accumulation increase in last 150
years; add 200m ice in last 500 years
W12
W12a
Nield et al., GRL, 2012
Summary: new
GIA model
wrms:
grey = East Antarctica
magenta = West Antarctica
black = all Antarctica
Whitehouse et al., GJI, 2012
Solve for present-day ice-mass change using
GRACE
We follow the forward modelling
approach of Wouters et al. (2008,
GRL):
“What ice mass change would be
required to reproduce the GIAcorrected GRACE data?”
King et al., Nature, 2012
Present-day ice-mass
change
Random errors
Associated with fitting linear trend and
acceleration rate to time series for each
basin
Systematic errors
Associated with uncertainty in the GIA
correction
Rate @ 2006.9
Accel
2σ uncertainties
King et al., Nature, 2012
Range due to systematic errors
All Antarctica [-126,-29] Gt/a
West Antarctica [-128,-103] Gt/a
East Antarctica [+7,+89] Gt/a
King et al., Nature, 2012
Regional mass trends after
correcting for GIA using the W12a
model
Linear rates and accelerations quoted
with 2σ uncertainty
• Previous estimates of linear mass
change rates range from -31 to -246
Gt/a
• Our estimates lie towards the upper
(less negative) end of this range
• We find negligible basin-total mass gain
in West Antarctica outside the
Amundsen Sea basins
• Quoted uncertainty due to random
errors
• Systematic errors are larger
Regional and basin-by-basin
acceleration rates
• Only basins shown in magenta have
statistically-significant (1 sigma)
accelerations:
Amundsen Sea basins and Coats Land
King et al., Nature, 2012
Towards “consensus”

NASA and ESA convened
a group of experts to
resolve major differences
between techniques


Ice Sheet Mass Balance
Intercomparison
Experiment (IMBIE)
www.imbie.org
Our team drafted in during
2011
Comparison to other GRACE estimates
King, 2013
Update to new GRACE data release and
extend time period by ~2 years
RL04 (2002.7-2010.9)
RL05 (2003.25-2012.5)

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Series still very short; certainly do not extrapolate beyond
data period
GIA error bounds are unchanged as they are systematic
and not included within the stated uncertainties
Conclusions
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W12a is a major advance made in modelling Antarctic GIA
Led to a significant reduction in estimated ice mass change
using GRACE
Overall mass contribution 0.19mm/yr
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East Antarctica growing quickly – acceleration or accum anomaly?
West Antarctica losing mass more quickly – ocean-driven mass loss
that is likely long-term
Significant uncertainty remains in GRACE secular rates due to
uncertainty in GIA in East Antarctica
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East Antarctica [+7,+89] Gt/a (RL04; range is shifted for RL05)
Post-LGM ice history – need paleo accum, ice extent, ice dynamics,
RSL curves & present-day uplift measurements
Earth structure and rheology