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Velocity vs Melt Greenland (Zwally et al (2002) (Shortsighted: (Does not…
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
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
=
Lack of long term observations
over large areas
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
insufficient if annual average = low
Steady increasing flow - delivers more ice to sea
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
to shitty Landsat images (back to 1980s)
∴ Extract more velocity info
from old low resolution Landsat imagery
Correlates image chips between images
Dehecq, Gourmelen & Trouve, 2015
Feature Tracking
Feature tracking Landsat imagery 1985 - 2015 once image pairs found
Annual velocities acquired by applying algorithms
Robust velocities in 1-2 year periods
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 ?