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Input-Output Method (Requirements (Surface mass balance, Ice velocity…
Input-Output Method
Requirements
Surface mass balance
Ice velocity (from sat imagery)
Grounding line location
Ice thickness (airborne geophysics)
Mass flowing through g.l.
How thick ice is
Where g.l. is
Ice flow velocity through g.l.
Need to know:
b. Surface Slope Break Detection
Break in slope often observed
@ transition btwn grounded & floating ice
Detecting slope break
Altimetry = automated
Optical images = manual
Error can be problematic
Small error ---> big efecct
Consistency of g.l. location / depth = important
What?
Antarctic & Greenland IS reach the sea (sufficiently poleward)
∴ Input/output method may be used (unlike for glaciers)
Most of Greenland & Antarctica terminate below sealevel
Estimate mass balance Δ by considering:
1. Net mass in @ surface
i.e. snow acumulationg & compaction
2. Net mass loss @ grounding line
i.e. melt & flow into ice shelf
N.B. ice shelf = floating
∴ already lost /contributed to RSL rise
Surface
Mass Balance
Must be sourced from regional climate model
No satellite methods for accumulation / melt rate msmts
To ensure data is independent from velocities
Models = calibrated with sparse ground msmts
Lotsa uncertainty & no reason to trust models
--> except extensivelyy tested w/ground msmts
a. Interferometric Differencing
Uses radar & phase differences
Hinge connection btwn grounded & ice shelf = where tidal mvmt is muted / deflected
Method
Compare 2 interferograms
Fringes (wiggles) around hinge line
= where tidal flexure changes
i.e. connection btwn grounded & ice shelf
Greenland/ Antarctica SMB
Greenland
Negative @ margins - lotsa melt there
Except SW Greenland (see subglacial lecture)
Positive in interior
Antarctica
Positive SMB
Very little surface melt (only signif on Peninsula)
Most positive - Amundsen & Bellinshausen regions
Consistency of grounding line:
g.l. location / thickness /velocity
= crucuial
Input/Output Findings
Greenland
Mass balance studied in outlet glaciers around Greenland IS
Since 2000 SMB decreasing
Ice discharge = increasingly positive
Debated whether SMB Δ & mass Δ are related causatively
Antarctic
Negative mass balance = markedly dominant
West most coast
Antarctic Peninsula/ Amundsen/Bellingshauesn Seas
related to ocean
Positive mass balance
--> Ross Sea
Accumulation increasing in Antarctica
--> warmer air carries more moisture thus snow fall increased
Grounding Line Position
Two methods
1. Interferometric differencing
for detection of tidal flexure (Rignot 2016)
2. Surface Slope Break Detection
Manually: optical imagery
Automated: altimetry
(hogg 2017)