Contemporary Evolution Of Ice Sheets Notes
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Contemporary Evolution Of Ice Sheets Revision
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Contemporary Mass Balance Change Ice Sheets Human effects: Cruikshank 2005: Do glaciers listen? Bamber et. al. 2007: Soon to be felt far away. 33, 000, 000 km^3 in two ice sheets, representing 70m sea level rise Antarctica WAIS GRIS
13.5 m km^2, 25.4mn MB negative = -48 MB?
<4.8 km thick
<3km high 57m sea level rise 6m sea level rise 10% total freshwater 15cm accumulation/year
6.5m/sea level rise (half Greenland) 30cm accumulation/year Growing knowledge through technical advances in remote sensing: Laser altimetry, radar altimetry, InSAR, GPS: IceSat US laser altimeter (up to 80 ), CryoSat European radar altimeter. Optical imagery: temperature variability, albedo, lake formation. GRACE: gravity recovery + climate experiment
Flow of marine-terminating glaciers in general: General calving processes: Benn et. al. 2007 - Relationships between thinning, acceleration + calving retreat. Tidewater glaciers: grounded calving margins: Model of dynamic response: Fluctuation with the tidal cycle = around 4x variation in water depth. Direct response through thinning, enhanced by calving, enhanced further by retreat over grounding-line shoals into troughs (who?). Meier & Post 1987 - cyclic advance/retreat. Thought to be independent of direct climate forcing. Pfeffer 2007: Rapid terminus retreat in Columbia Glacier. Ice Shelves/Floating Ice Tongues Calving through gradual bending + high magnitude events. Effect of backstress. Collapse of ice shelves leading to exposure of calving face…
But traditionally focused in Alaska, not studied re. anthropogenic climate change, mechanisms of direct climate forcing. New forms of ice-ocean interaction investigated in Greenland/Antarctica.
Greenland Ease of access, visibility to satellites. Measurements including InSAR (Rignot &
Kangaratnam 2005), and GRACE (Van den Broeke et. al. 2009):
- More than 3x increase in negative MB 1995-2005.
- ½ from increased runoff, ½ from enhanced glacier flow. In particular, thinning retreat of marine-terminating outlet glaciers:
- W. Retreat from 1998
- Eastern from 2002-5 Synchronous + speedups in summer. Shows need to understand. Series of scientific problems. 3x amplified Arctic warming? (Holland & Bitz 2003) Surface Meltwater Zwally 2002 GPS measurements on "Swiss Camp". Hydrologically-assisted fracture propagation. Das et. al. 2008. Supraglacial melt - lake drainage. Creating icequakes (?) . Hansen 2002 Scientific Reticence: "The whole ice sheet is going to fall through that hole!" Currently, seasonal variation on slow moving ice north of Jakobshavn Isbrae: seasonal speedup correlated with summer melt rates (10-25%). Parizek + Alley 2004: 10-25%
ice loss from global warming, and increasing freshwater input into the ocean. IPCC
2007. Potential contributor to further mass loss.
Joughin et. al. 2004: Little meltwater percolation to ice front, and speedups correlated to ice front position not meltwater penetration. (Mair et. al. 2010. Kangiata Nunata Sermia - 36-72km from calving front mostly due to hydrological fluctuations.) Dynamic thinning of marine-terminating glaciers
>160 tidewater glaciers in Greenland, mostly in fjords [Alaska]. Grounded calving faces / floating ice tongues. Meier & Post 1987 - cyclic advance/retreat - thinning or retreat reduced backstress; Possible mechanisms: Atmospheric or oceanic forcing causing thinning, retreat or ungrounding: instant speed increase from reduced backstress - propagating up-glacier causing dynamic adjustment and steepening according to Glen's Flow law.. Possibility of retreat into depressions. Need to understand this better in Greenland. Helheim Grounded calving front 600-700m below sea level for 30km upstream. Helheim: 2001-2003 - 3.25km calving retreat, stabilizing by 2006. Kangerlugssuak: 2004-2006: beginning to readvance. glacial earthquakes (Nettles & Ekstrom 2010). Howat et. al. 2005 - Ungrounding suggesting calving front not meltwater caused increase. Joughin et. al. 2008: Classic pattern of tidewater retreat (Meier & Post 1987). Enhanced calving, retreat into topographic low. Echoing tidewater retreat patterns in Alaska ie Columbia. Linked to ocean dynamics: Straneo et. al. 2010: Sermilik Fjord at head of Helheim glacier subtropical waters continually replenished throughout the year as fresh outflow balanced by saltier subsurface inflow. Direct pathway subpolar NAO to glacial fjords. Again, a sustained readvance could occur due to mass balance of ice sheet (Joughin et. al. 2008). Seasonality. Jakobshavn Isbrae Drains 6.5% of GRIS, moving 20m/day, 30km^3 annually. Previously had 15km floating ice tongue. Thinning began in 1997 in lower reaches and spreading inland. Sudden collapse of tongue in 2002-3, further acceleration. Thomas et. al. 2003: Dynamic thinning - surface melting, ocean warming?
Joughin et. al. 2008: Explained it due to weakening of the marine portion of the glacier: 1) Collapse of floating ice tongue exposes calving face directly to warm ocean water, and increasing seasonal fluctuations. Initial speedup now progressing gradually into slower moving ice. Has reached an area of reverse bed slope. Will ice flux keep it from retreating? Or could thinning further destabilise front position and cause it to retreat into deep 60km trough?
Role of ocean warming: Holland et. al. 2008 correlated beginning of speedup to warm water pulse arriving in Disko Bay in 1997, originating in Irminger Sea in the mid 1990s. General evidence that they were responsible for retreat events
- Increasing consensus that marine thinning is responsible. Moon & Joughin 2008 : Land-terminating glaciers saw almost no change. Suggests marine thinning from atmosphere and ocean - reduced sea ice extent / surface meltwater processes / thinning from loss of surface ice.
- Dominance of marine forcing: Howat et. al. 2008: Synchronous retreat +
acceleration of 32 SE glaciers - beginning with temperature anomaly in 23. Yashayayev et. al. 2005: Major warming in Irminger Sea However, uncertainties as to future evolution:
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