Velocity vs Melt Greenland

Zwally et al (2002)

Rapid, dynamic GRIS response to climate warming

via coupling btwn surface melting & ice sheet flow

Based on satellite obvs

Higher melt rates coincide w/higher max velocities

Shortsighted:

Does not mean higher ice mass loss

Higher melt rates do cause higher velocities

Important Q

Motivation: What contribution will dynamic change of GRIS make to sea level rise ?

Does increased surface melting

lead to increased ice flow?

Leverett
Glacier

GPS receivers installed @ varying elevations

from land terminating margin

to top of ablation zone

Observations taken over 4 years

GPS derived velocities

Very accurate

Poor spatial coverage

Leverett Melt
vs Velocity
(Sole 2013)

Stronger melting yields

1. Summer speedup - faster summer velocities & greater displacement

2. Winter Slowdown - Slower motion during subsequent winter

  1. Greater melt @ lower elevation
    ∴ higher summer speedup

Confirmed melt vs seasonal velocity relation
(Zwally hypothesis)

Summer melt & annual velocity Tedstone et al (2013)

No significant positive relation
between summer melt & annual velocity

Evidence
= 2012

Massive melt rate in mid ablation zone (vs 2009)

Yet annual velocity = slightly slower

Key Limitation

Annual Velocity Importance

Overall displacement = what matters

i.e. what dictates total ice loss to sea

Not episodic velocity

summer speedup / high velocities for short periods --> have little effect on ice loss

= Lack of long term observations over large areas

Steady increasing flow - delivers more ice to sea

insufficient if annual average = low

Surface melting msmts GRIS

Past 2 decades - unprecedented melt increase

Problem: GPS velocity msmts only ~ past decade

Enter : Landsat imagery

Decadal Evo
Ice Velocity

Apply sophisticated feature tracking

∴ Extract more velocity info
from old low resolution Landsat imagery

Correlates image chips between images

to shitty Landsat images (back to 1980s)

Feature Tracking

Feature tracking Landsat imagery 1985 - 2015 once image pairs found

  1. Annual velocities acquired by applying algorithms
  1. Robust velocities in 1-2 year periods
  1. Velocity timeseries developed 1985 - 2014

Significance velocity timeseries

Provides data required to answer the question

Does higher melt rate cause
slowdown or speedup on annual timescales ?

Dehecq, Gourmelen & Trouve, 2015