Lecture 4 - marine terminating glaciers

Question 1

Joughin and Alley (2011)

Answer 1

world warm –> g shrink size –> raise sea level

thinning can be caused by ice streams speeding up and widening: can lead to ice stream stagnation (basal water supply>supply of meltwater –> increased basal resistance)

once ice crossed grounding line the contribution to sea level is negligible, but discharge across GL is influenced by buttressing and therefore melting and iceberg calving

sub ice shelf melting; sinking of saline water to PMP, melts ice which is freshwater, mixes then melts more on its way up

high surface melting and ponding of water - associated with catastrophic ice shelf break up; growth of crevasses through hydrofracturing, waters greater density causes it to fracture, toppling of the broken blocks

Question 2

Joughin and Alley (2011) WAIS

Answer 2

much of WAIS grounded ice lies on a bed that deepens inland and extends well below sea level - v vunerable to MISI

ocean and atmosphere warming threaten to reduce the floating ice shelves that buttress the ice sheet at present –> loss of ice shelves would accelerate flow of non-floating ice near coast

troughs that channelize ice from the centre of the continent

ice loss through calving into the sea or in situ melting by the warmer ocean, surface melt is negligible

increased basal melt = reduces thick ness and thus buttressing effect of the ice sheet, increases flow of inland ice

forcing from atmospheric and oceanic warming could lead to large contributions to future SL rise

increasing rate of loss due to changes in thickness at the grounding line buttressing

Marine ice sheet collapse in WAIS would contribute more than 3m to global sea levels

Question 3

Alley et al (2005)

Answer 3

Greenland and Antartica - potential to rise sea levels by 70m, freshwater fluxes would also affect global oceanic circulation, and thermal expansion

using geological record show past changes in carbon dioxide associated with changes in ice volume and global SL

buttressing theory of ice shelves = warming induced reduction of ice shelf restraint triggered flow acceleration

Larsen B 2002 collapse led to speed up of tributary glaciers

meltwater penetration to the bed near instant communication between surface forcing and basal ice dynamics

loss of Greenland ice sheet would be irreversible, loss via surface melt, iceberg calving and sub ice shelf melting (all similar loss)

ice sheets mass loss by warming oceans > mass gain from enhanced snowfall –> sea level rise

Question 4

Howat et al (2007)

Answer 4

recent discharge and mass loss at two of Greenlands largest outlet glaciers

variations in ice sheet MB might be dominated by ice dynamics rather than variation in surface balance

ice flow of many tidewater Gs has increase

small perturbations to thickness can induce retreat in calving Gs

Question 5

shepherd et al (2001)

Answer 5

retreat of PIG (&acceleration of WAIS interior)

grounded PIG thinned recently (contributing 0.001mm/yr of eustatic SL)(greater than any fluctuation in surface mass balance), effecting the inland glacier - could be due to increased velocities within the central trunk

PIG largest discharge of all WAIS ice streams

bedrock topography deepens inland - MISI

lack of extensive ice shelf

if loss of mass continues at same rate will be afloat in 600years

Question 6

Van der Veen (2002)

Answer 6

calving

sufficiently cold climates can form floating ice shelves
and tongues which do not go beyond confinements of fjord or topography

local climate>thermal limit of ice sheet viability = no floating tongue maintained, position of terminus determined by thickness in excess of floatation

if terminal region thins (MB deficiency) retreat is initiated with the calving front retreating to where the thickness is slightly in excess of floatation

calving is a factor of rapid ice sheet changes

Jakobshavn effect = powerful instability mechanism = small perturbations are amplified, trigger for retreat of tidewater glaciers would be increase in surface melt and calving rates

water depth model

depth of flotation

Question 7

Benn et al (2007)

Answer 7

calving

• rates increase when increase in velocity or retreat of the G margin
• consequence of propagation of crevasses in response to stress blocks becoming isolated from the main glacier

strain rate (from spatial variation in velocity) controls location and depth of surface crevasses

tendency for Gs to stabilise at pinning points

does calving influence velocity or is it the other way round

calving rate in fresh water glaciers is lower than in tidewater glaciers