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Greenland Subglacial Water (Zwally et al (2002) Theory (Shortsighted:…
Greenland Subglacial Water
Moulin
= cylindrical vertical shaft
carved by supraglacial meltwater
i.e. crevasse enlarged by melt
Connects surface to bed - extends through glacier to bed
Water
beneath GRIS
Source: surface meltwater
(usually)
Supraglacial rivers
flow across surface from melt collecting
Drop through crevasses/ moulins usually
rarely reach sea
melt = transferred from IS surface to bed
Important Q
Motivation: What contribution will dynamic change of GRIS make to sea level rise ?
Does increased surface melting
lead to increased ice flow?
Misconception Positive feedback
Incorrect positive feedback believed for > decade
Increased temps --> Increased surface melt (Higher basal water P)
--> Increased velocities --> Runaway retreat/ice loss
Zwally et al (2002) Theory
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
Seasonal Velocity Variations
(Sundal 2011)
Sundal 2011 Study
5 glaciers SW Greenland
Several years observations
velocity (Insar calculations)
surface melt (modelling)
Very high surface melt rates & associated speedup
Broad correlation melt volume & velocity speedup
High melt years (yellow histogram)
150% increase velocity at peak
Greater % velocity increase
@ peak melt vs low melt years
Curiosity - when view whole year
Low melt year
melt rate & velocity decrease to low rates at end of summer
High melt year
At end of melt season:
Velocity drops below annual baseline
--> Greater Autumn slowdown
(compensatory?)
Higher summer peak velocity speedup
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
(warmer temps there)
Confirmed melt vs velocity relation (Zwally hypothesis)
Unknown
Whether winter slowdown compensates for summer speedup?
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)
Annual velocity = slightly slower
Problem: Lack of long term observations
over large areas
Annual Velocity Importance