Cryosphere Lecture 5

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Direct contact needed for a direct impact from the ocean
Marine ice sheets Ice streams (bounded by slower moving ice) Outlet glaciers (bounded by topography) Grounding line – where ice changes from being grounded to floating Ice shelves & calving rate Tidewater glaciers
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How the ocean can impact on ice:
Basal melt rate in ice shelves/tongues is very sensitive to a change in ocean temperature. Temperature, salinity, tides can affect calving rate Sea level rise or fall can impact on ice sheet extent
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In the western hemisphere, hence
West Antarctica’ - West Antarctic Ice sheet
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Marine ice sheet Marine Ice Sheet Instability Hypothesis (MISIH)
Bed below sea level Reverse slopes
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Retreat of grounding line
Greater ice thickness at grounding line Greater discharge of ice Grounding line retreat into deeper ocean Greater ice thickness at grounding line Greater discharge of ice
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Amundsen Sea ice streams: Pine Island Glacier & Thwaites Glacier
Main ice streams draining this sector
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Amundsen Sea ice streams : Weak underbelly’ (Hughes, 1981) :
Reverse slopes No large ice shelf
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Observation on the amundsen sea ice stream
Rapid thinning (e.g. Shepherd et al., 2001, 2002, Pritchard et al., 2009), Velocity increase post 1994 (Pine Island Glacier, Joughin et al., 2003), Grounding line retreat (Pine Island Glacier, Rignot 1998)
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Ice flow modelling demonstrated that thinning could be driven by
changes in the ice shelf
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Suggests oceanic driving force
Intrusion of warm[er] Circumpolar Deep Water (CDW) Causes higher melt rates => grounding line retreat => surface thinning
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Circumpolar deep water
Travels around Antarctica Cold (~2 deg C) But warmer than surrounding water
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Pine Island Glacier
(PIG)
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Pine Island Glacier was an
Acceleration event in 1994 Ocean modelling suggests more CDW on continental shelf in 1994, coinciding with velocity increase (Thoma et al., 2008) Driven by changes in the wind stress
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‘Amundsen Sea Low’
Further south & west in 1994 Caused intrusion of CDW
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WAIS
West Antarctic Ice Sheet
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Areas of WAIS susceptible
3 m global sea level rise (Bamber et al., 2009) How quickly? Will it happen at all? Has it happened in the past?
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ANDRILL project
Suggests obliquity paced oscillations (~40,000 years) (Naish et al., 2009)
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Greenland has seen surface elevation lowering which has been
widespread particularly strong over outlet glaciers (> 10 m/yr)
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Greenland
Mass balance change Cannot account for all the thinning at the coasts => must be another process involved Talked about impact of meltwater on Greenland velocity (Zwally effect) Impact of the ocean?
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Greenland - observations
Outlet glacier retreat Many in deep fjords, below sea level with floating tongue / tidewater Direct influence of ocean Helheim Glacier Jakobshavn Isbrae
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Greenland Observations :Outlet glacier speed up example
Jakobshavn – after 1996, velocity doubled
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Marine terminating glaciers retreating more than
terrestrial ones
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Impact of meltwater is more a
seasonal effect than a long term effect?
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Changes observed in ocean currents (Holland et al., 2008)
possibly driven by North Atlantic Oscillation (NAO) this could be driving temperature too.
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Impact of freshwater flux on thermohaline
Greenland is losing mass, both from surface melting & dynamic thinning Influx of fresh water would disrupt thermohaline circulation Salinity of the water is an important driving factor in the thermohaline circulation
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Impact of freshwater flux on thermohaline
From ice & sediment records, e.g. Heinrich events (iceberg calving events) Dansgaard-Oeshger events (change over decades) Meltwater floods (e.g. Lake Agassiz leading to Younger Dryas)
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Theory-of the impact of freshwater flux on thermohaline
Influx of fresh water prevents deep water forming and stops the driving force behind the thermohaline / deep ocean circulation
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Timescale -of the impact of freshwater flux on thermohaline
Could potentially cause very abrupt climate change (decadal scale). Evidence for such abrupt climate change in Greenland ice core records (see Broecker 1997 for a summary)
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Evidence for it happening now?
Mixed picture – some reports suggest freshening of circulation, some don’t. Bryden et al. 2005 suggest 30 % decrease in rate of overturning But a high level of variability reduces confidence in their result... Other studies suggest no overall trend (
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Card 2

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How the ocean can impact on ice:

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Basal melt rate in ice shelves/tongues is very sensitive to a change in ocean temperature. Temperature, salinity, tides can affect calving rate Sea level rise or fall can impact on ice sheet extent

Card 3

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In the western hemisphere, hence

Back

Preview of the front of card 3

Card 4

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Marine ice sheet Marine Ice Sheet Instability Hypothesis (MISIH)

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Card 5

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Retreat of grounding line

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