1. Complex Antarctic ice shelf thickness
change revealed by 18 years of satellite
radar altimetry
Fernando S. Paolo*
Helen A. Fricker
Laurie Padman
*fpaolo@ucsd.edu
AGU Fall Meeting 2014

2. 18 years of continuous satellite
radar altimetry over the ice shelves
1994 2012
ERS-1 ERS-2 Envisat

3. How and what
satellite radar
altimetry measures
ocean tides
atm pressure
sea-level rise
surf. densification
surf. penetration
...
thickness change
∂h/∂t :

4. High crossover density
3-month time bins
30-km spatial cells
We averaged many observations in time and space
Averaging data points
35 days of
crossovers
# of crossovers per cell
per 3 months

5. We constructed several records and stacked
them to enhance the signal-to-noise ratio
Incoherent
The spatial distribution of
crossing points changes with
time due to “repeat” satellite
tracks not being exact, miss-
pointing errors, etc.
Averaging time series

6. 1994 2012
18 years of volume change in
East Antarctic ice shelves
Volume
1994 2012
Is the trend a
Line?
Quadratic?
Cubic?
Volume
How do we represent trends?

7. Tibshirani, 1996
misfit
bias-variance tradeoff
We select λ by
Cross-Validation
min || h − t β ||2 + λ || β ||1Using Lasso:
constrains
bias
constrains
variance
We fit polynomials: ĥ(t) = ∑ βn tn + ε

8. Regional
time series
of thickness
change
Sustained ice loss
since 1994
East Antarctica
ice shelves in phase?
Ice-loss acceleration
5-year straight-line fit (ICESat period)
18-year polynomial fit
We derived
average rate of
change from
the18-year
polynomial fits

9. %-Thickness change 1994-2012
5%loss
5%gain
500 km
25 10 0 10
Rate of thickness change
(m/decade)
1994
1997
2000
2003
2006
2009
2012
¡1500
0
+1500
Volume change (km3
) West East All
Queen Maud
Amery
Wilkes
Amundsen
Bellingshausen
Larsen
Filchner-Ronne
Ross
ROE
ROW
WIT
SUL
NIC
GET
DOT
CRO
THW
PIN
COS
ABB
VEN
STA
BAC
WIL
GEO
LAB
LAC
LAD
RON
FIL
BRU
RII
FIM LAZ
BAU
PRI
AME
WES
SHA
DIBMER
COO
REN
MAR
DRY
West
Antarctica
East
Antarctica
TOT
MOS
HOL
Some ice shelves have lost up to
18% of their thickness in 18 years

10. 1994
1997
2000
2003
2006
2009
2012
¡1500
0
+1500
Volume change (km3
) West East All
Amundsen
Ross
ROE
ROW
WIT
SUL
NIC
GET
DOT
CRO
THW
ABB
REN
MAR
DRY
West
AntarcticaWest Antarctic ice shelves:
rate of volume loss increased
by ~70% in the past decade East Antarctic ice shelves:
earlier increase in volume
stopped in the past decade
All Antarctic ice shelves: rate of volume change
-25 km3/yr (pre-2003) ➔ -310 km3/yr (post-2003)

11. Average rate
for different
4.5-year
intervals
Short-term rates
are highly variable,
and depend on the
chosen time interval!

12. Volume loss from Antarctic ice shelves
accelerated in the last decade
Rate of volume loss from WAIS increased by
~70% in the 2000s
West Antarctic ice shelves have lost up to 18% of
their thickness in less than 2 decades
Single satellite missions are insufficient to
infer the long-term state of the ice shelves
Summary

13. -9 deg isotherm
moving southward
Cook &Vaughan, 2010
Regional atmospheric
warming trend?
Different forcings within each environmental setting?

14. Pritchard et al., 2012
Characteristic signature
of ocean-induced
basal melting
Faster thinning rates near the grounding lines ➔ increased CDW inflow

15. A reliable and complete ice shelf mask is a problem.
So we (Geir Moholdt) created our own using all data
available: MOA (Scambos et al. 2007),ASAID
(Bindschadler et al. 2011), InSAR (Rignot et al. 2011),
ICESat (Fricker/Brunt et al. 2006-10)

16. We explore all possible
time combinations
These are elevation
changes with respect to
different epochs
D J F M A M J J A D J F
summer autumn winter spring
h(t0) h(t1) h(t2) h(t3) h(tN)
Δh0,1 Δh0,2 Δh0,3 Δh0,... Δh0,N
Δh2,3 Δh2,... Δh2,N
Δh1,2 Δh1,3 Δh1,... Δh1,N
Δh(t,t0)
Δh(t,t1)
Δh(t,t2)
...
Why can we do this? The spatial distribution of
crossovers changes with time

17. As a consequence of hydrostatic balance...
This limits detection of changes
in the vertical component
≈