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