polar ice sheets and ice shelves: mass balance, uncertainties, and potential improvements robert h...
TRANSCRIPT
Polar Ice Sheets and Ice Shelves:Mass Balance, Uncertainties, and
Potential Improvements
Robert H Thomas…etc
Sea-level equivalents
0
10
20
30
40
50
60
70
80
90
100
1
Volume (V); Accumulation (A); Contribution (C)
Perc
enta
ge
V
V
V
A A
A
C
C
C
"Small Glaciers"
Greenland
Antarctica
Estimating ice-sheet mass balance:techniques
• Mass-budget, compares total snow accumulation with losses by ice discharge and melt runoff
• Repeated altimetry, to estimate volume changes
• Temporal changes in gravity, to infer mass changes
Mass budget: uncertainties
Antarctica Greenland
(Gt a-1)
Snowfall + 130 (7%) + 25 (5%)
Ice flow + 93 (5%) + 25 (5%)
Melt runoff Very small + 30 (10%)
Mass balance
(mm/yr SLE)+ 160
(+ 0.5)
+ 46
(+ 0.15)
Altimetry: uncertainties
SRALT ICESat ATM
Topography effects
Laser pointing,
scattering, saturation
Changing dielectrics
Spatial/temporal cover
(clouds)
Basal uplift
Changing snow-densification rates
GRACE: uncertainties
Measurement errors
Spill-over effects
Atmosphere Basal uplift
Greenland Small
(few Gt a-1)
Very Small + ~12 Gt a-1
Antarctica
*V & W# R et al
+ ~13 Gt a-1 + ~9 Gt a-1 + ~72 Gt a-1*
+ ~25 Gt a-1 #
Low-resolution results refer to entire ice sheets, but are seriously limited by short temporal coverage
Greenland Ice Sheet: rates of surface-elevation change (dS/dt)Above (red), and below (blue) ~ 2000-m elevation
Greenland Mass Balance
____________________________________________________________________ ERS SRALT ATM(5) ICESat/ATM Interval: 1992-2003(8) 1992-2002(9) 1993/4-98/9 1993/4-2004 1998/9-2004 _____________________________________________________________________ Elevation (m) >2000 +65+/-4 +48+/-2 + 7+/-10 + 24+/-10 +38+/-10 <2000 +25+/-7 -14+/-12 - 77+/-17 -153+/-17 -212+/-17 >1500 +64+/-5 +42+/-5 - 2+/-10 + 12+/-10 + 16+/-10 <1500 -20+/-9 -56+/-14 -120+/-30 -257+/-30 -363+/-30 All +54+/-2 +27+/-3 - 30+/-11 - 45+/-11 - 56+/-11 ________________________________________________________________________ Table I: Rates of surface-elevation change (dS/dt) in mm/yr derived from ERS data compared with those from laser-altimeter surveys.
ERS wavefront over Jakobshavn
Antarctic Surface Elevation Change
PIG
AP
LIS
RIS
FRIS
Crane Glacier after Larsen breakup
airborne lidar profiles measured in 2002 &
2004
Antarctic ice-sheet mass balance
-250
-200
-150
-100
-50
0
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
Observation Period
Rat
e of
mas
s in
crea
se (G
t/yr)
Orange: Zwally Blue: GRACE Black: Rignot
Recent mass-balance of Greenland and Antarctica.
Greenland Antarctica
Area (M km2)
1.7 12.3
Volume (M km3)*
2.9(7 m SLE)
24.7(57 m SLE)
Total accumuln. (Gt a-1)#
500(1.4 mm SLE)
1850(5.1 mm SLE)
Mass Balance Since ~1990: Thickening above 2000 m, at an accelerating rate; thinning at lower elevations, also accelerating to cause a net loss from the ice sheet of perhaps > 100 Gt a-1 after 2000.
Since early 1990s: slow thickening in central regions and southern Antarctic Peninsula; localized thinning at accelerating rates of glaciers in Antarctic Peninsula and Amundsen Sea region. Probable net loss, but close to balance.
Causes
o post-glacial memoryo variability/trends in snowfall/meltingo changes in glacier velocities
- ice shelves- basal lubrication- ????
Sea-level equivalents
0
10
20
30
40
50
60
70
80
90
100
1
Volume (V); Accumulation (A); Contribution (C)
Perc
enta
ge
V
V
V
A A
A
C
C
C
"Small Glaciers"
Greenland
Antarctica
Sea-level change and the cryosphere, June 2006QUESTION ACTION
-How long will glacier acceleration continue,
resulting from existing perturbations?
- Extend time series of observations- Model development and comparison with truth
- Relative importance of ice-shelf weakening and melt-water lubrication? - As above plus field measurements - East Antarctica: is this also vulnerable to
“perturbation weakening”?
- Remote-sensing surveys, and time series to identify current status
- Why are ice shelves thinning? - Ocean measurements near/beneath ice shelves
- Model development and comparison with truth
- Possible causes for ice-shelf breakup? - Field observations and modeling - Importance of oscillations and trends in surface accumulation? - Ice cores, accumulation radar, and modeling - Quantify the impact of changes in summer melt rates?
- AWS, field work including percolation, and modeling
- Relationship to the “Big Picture”: the link to prescribed scenarios of future climate change
- Climate-warming scenarios must include parameters important to the ice sheets: accumulation; summer temperatures; ocean conditions near ice sheets and beneath ice shelves