Input-Output Method

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

Requirements

  1. Surface mass balance
  1. Ice velocity (from sat imagery)
  1. Grounding line location
  1. 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:

  1. Surface
    Mass Balance

Must be sourced from regional climate model

  1. No satellite methods for accumulation / melt rate msmts
  1. To ensure data is independent from velocities

Models = calibrated with sparse ground msmts

Greenland/ Antarctica SMB

Greenland

Antarctica

Positive SMB

Very little surface melt (only signif on Peninsula)

Most positive - Amundsen & Bellinshausen regions

Negative @ margins - lotsa melt there

Except SW Greenland (see subglacial lecture)

Positive in interior

  1. Grounding Line Position

Lotsa uncertainty & no reason to trust models
--> except extensivelyy tested w/ground msmts

Two methods

1. Interferometric differencing
for detection of tidal flexure (Rignot 2016)

2. Surface Slope Break Detection
Manually: optical imagery
Automated: altimetry
(hogg 2017)

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

b. Surface Slope Break Detection

Break in slope often observed
@ transition btwn grounded & floating ice

Detecting slope break

  1. Altimetry = automated
  1. Optical images = manual

Error can be problematic

Small error ---> big efecct

Consistency of g.l. location / depth = important

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

Positive mass balance
--> Ross Sea

West most coast

Antarctic Peninsula/ Amundsen/Bellingshauesn Seas

related to ocean

Accumulation increasing in Antarctica
--> warmer air carries more moisture thus snow fall increased