Week 1: Importance of glaciers Flashcards
Glacier =
Snow and ice mass which, if accumulates to sufficient thickness, deforms under own weight and flows (Sugden 1994)
Cover 10% Earth’s surface
75% freshwater
IMPORTANCE OF GLACIERS
Glacio-EUSTASY (global)
Glacio-ISOSTASY (local)
Local/global climate systems
Ice cores/palaeoclimate records
Hazards
Glacial ‘respiration’ and nutrient fluxes
Source of water
Tourism
Sediment resources (sands/gravels)
Glacio-isostasy
England/Scotland rises 102mm/yr
Will take 10-20,000 years to re-equilibriate
Local/global climate systems
Greenland meltwater –> Labrador Sea
1) reduce surface convection in N Atlantic due to freshwater flux
2) slows gyre
3) affects NW Europe’s summer climate
Hazards
Outburst floods (GLOF)
Surges
Ice avalanches
–> position important e.g. earthquakes, geothermal heat flux
Glacial respiration and nutrient fluxes
Hodsen et al 2007: CO2 flux of cryconite across an entire arctic suprglacial ecosystem
Size facts
AIS 30.1 million km3
GIS 2.28 million km3
EQUIVALENT TO 70M OF SEA LEVEL
Other smaller make up 180,000 km3
Controls on glacier distribution (and explained)
LATITUDE
- low solar energy = less energy to melt ice
ALTITUDE
- thinner air holds less heat energy
- -> glaciers near equator!
CONTINENTALITY
- no moisture source = drier air
- rain shadow effect
- ALSO sea ice reduces moisture from oceans e.g. Europe
Types of classification
SIZE/RELATIONSHIP TO TOPOGRAPHY
BASAL THERMAL REGIME
Classification; size/relationship to topography
Largest = ice sheets = unconstrained by topography
Transition from constrained –> unconstrained as grows
Example of constrained: valley/cirque/niche glacier
Classification; basal thermal regime
Cold-based, warm, polythermal
Marine glaciers: ice rise =
Ice moves towards sea, bumps into island and rises over
Marine glaciers: sea-ice ice shelf =
Long term, thickens until shelf forms e.g. N Pole
Direction of ice flow
Acc zone –> ELA –> ablation zone
ELA = (and usual value)
Altitude when acc = abl
Usually 0.6 i.e. contour below 60% of total glacier area
Acc > abb
+ve mass balance
Low temps +/ greater snowfall
Decrease ELA
Acc < abl
-ve mass balance
High temps +/ less snowfall
Increase ELA
Identifying mass balance
Annual misbalance
Aerial photos/satellite images
Accumulation Area Ratio (AAR)
Methods of acc (major to minor)
Snow Wind blown drift Avalanches Condensation (rime = rapid frost formation) Freezing rain/external water
Methods of abl (major to minor)
Calving (wet/dry, DOMINANT) Melting (top down thinning) Wind blown drift (scoured from top) Avalanches Evaporation Sublimation (e.g. Antarctica) DEFLATION
How does ice form?
Reduce voids and increase density
e.g. New snow = 50-70kg/m2
Glacier ice = 830-917kg/m2
Firn =
Snow which has survived a melt season
Forms ice when air can’t transit in/out of system
Types of snow zone
Warm/wet
Cold/dry
Warm/wet snow zone, how ice forms
Melting Refreezing Rounding Joining ... of grains
Cold/dry snow zone, how ice forms
Sintering = bonds form under heat and pressure
Why is englacial ice not a simple layer cake stratigraphy?
Disruptions to layers relating to annual acc of snow
e.g. partial avalanches ‘jumble’ = new structures due to compression/extension
Controls on ice formation
Temperature
Accumulation
Temperature: control on ice formation
Warmer = faster
Important control on transformation
Accumulation: control on ice formation
More snow = faster formation
Examples of differing ice formation
ALASKA
Warm/wet with lots of snow into system
High densities at shallow depths = faster formation
Ice at 13m
GREEENLAND
Colder/drier
Takes longer for ice to form
Ice at 66m
Controls on calving (and explained)
1) Water depth
Hf = (pw/pi)Dw
“Critical floatation depth” - will float if less than = calving feedbacks
2) Water T
Bottom up thinning
3) Tidal variation
Flexing
4) Glacier crevassing
5) Water discharge from glacier
Freshwater density
1000 kg/m3
Seawater density
1030 kg/m3
Ice density
900 kg/m3
Grounding line =
Point at which glacier begins to float
Calving front =
Where calving occurs
If calving front = grounding line –> “tidewater glacier”
N.B. Doesn’t occur in cold polar environment e.g. Antarctica; 100s km from grounding line
Energy balance at glacier surface:
Qm = Qs + Ql + Qh + Qe
Qm = energy available to melt ice
Qs = solar shortwave radiation
Ql = longwave (emitted) radiation
Qh = sensible heat transfer
Qe = latent heat transfer
What affects energy balance at glacier surface?
ALBEDO i.e.
Supraglacial debris cover
(- albedo affected by surface texture/material made up of/debris cover)
Typical albedos of clean ice vs debris covered ice
34-51%
10-15%
THIN debris =
Increased absorption/re-radiation = more ablation
e.g. Cryconite holes = thin patches, accelerate abl not thick enough for blanket effect
THICK debris =
Less conduction = increased protection = less ablation
N.B. ?False impression of +ve mass balance
Can bury glaciers for decades!
Measuring glacial mass balance
DIRECT METHODS
GEODETIC (REMOTE SENSING)
HYDROLOGICAL
CLIMATIC CALCULATIONS
GRAVITY CHANGES
KINEMATIC MASS BUDGET
Measuring glacial mass balance; direct methods
Accumulation - snow pits = depth/density
Ablation - stakes = ice surface changes
Measuring glacial mass balance; geodetic
Aerial photos = area/altitude changes different years = vol changes
- DIFFICULT
Repeat satellite radar altimetry (radio waves) and LIDAR (Light Detection and Ranging - laser pulses)
= thickness changes within mm
- v useful for remote areas/large continental sheets
Measuring glacial mass balance; hydrological
Net balance = option - runoff - evaporation
:) water resources/prediction/management
- favours small glaciers b/c data collection required
- can’t distinguish changes in snow/ice vol vs water storage
Measuring glacial mass balance; climatic calculations
Meteorological data –> calculate surface energy balance and ablation
Statistical methods based on T
POSITIVE DEGREE DAYS (Braithwaite 1995) = sum of mean daily T for which T>0’C
- assumes uniform conditions across large area
- ignores calving
Measuring glacial mass balance; gravity changes
Gravity Recovery And Climate Experiment satellites
Ice has a gravity influence
e.g. Alaska has summer/winter fluctuations but long term trajectory = -ve mass balance
Measuring glacial mass balance; kinematic mass budget
FOR MARINE TERMINATING GLACIERS
1) Surface mass balance (climate model)
2) Mass lost from marginal flow (remote sensing)
Steep mb gradient =
High rates acc/abl
Moist mid latitudes e.g. Norway
Shallow mb gradient =
Low rates acc/abl
Arid/polar latitudes e.g. Arctic, Canada
Why isn’t mb gradient linear
Should be due to lapse rate (0.4-0.9’C/100m)
- acc increases with altitude UNTIL too cold for pption
- debris cover
- shading by topography
- avalanching
