Transcript Glaciers (PartI)

Glaciers (Part I)
• What is a glacier?
• Where are glaciers found?
• What is climate effect on glaciers?
What is a glacier?
• Mass of moving glacial ice created by the
accumulation of snow
• glaciers always moving forward at terminus
• ice & water move forward
Typical glacier system in
Cordillera Blanca, Peru
Glacier landscape in Nepal Himalayas
Shorong Yul-lha glacier,
Nepal Himalayas
How are glaciers formed?
• where average temperatures < O deg C.
• Snow accumulates and compressed by
weight of layers
• buried layers slowly form a thickened mass
of ice
• snow grains squashed together-- snow
metamorphosis
Glacial Ice formation
• SNOW: seasonal snow
void spaces
• FIRN (névé): snow that has lasted more
than one year
less void
space
• ICE: compacted, air pores not connected
density > 860 kg/m3
Air bubbles
Transformation of SNOW -->
ICE
• Rate of transformation dependent on
temperature and accumulation rate
• Rate with load
• Rate with Temperature (for a given load)
– temperature determines size of crystals and
amount of snowfall
Thermodynamic classification of
glaciers
• “Cold” glaciers:
– frozen to the rock of their beds
– ice below pressure melting point
– remain well frozen; melting only at surface
• “Warm” glaciers:
– warm based
– thawed from their bed
– slide and flow
Glacier movement
Ice is solid but it flows!
• When glacier
reaches critical mass
(>20m thick)
• flow occurs
How does ice move?
faster
F
Two ways of glacier movement
• PLASTIC DEFORMATION
• BASAL SLIDING
Gravity main driving force,
s = rgh* sina
1. Internal deformation
• Ice > 60m thick
• specific for cold-based
glaciers
(frozen to bed)
Compaction
weight
velocity
Pressure
melting point
Factors controlling rate of deformation:
•depth of ice
•temperature
•slope
2.Basal sliding
• only “warm-based”
glaciers
• glacier slips over the
rock surface
• H2O as lubricator
• less friction
• velocities: 0-300m/day
-water
-sliding
Glacier surge
• velocities of 100m/day
• dramatic increase in
flow rate, 10-100
hundred times faster
than its normal rate
Structures within glacial ice
• Crevasses:
– cracks in the ice due to
different velocities
between
center and edges of
glacier
– formed perpendicular to
direction of flow
• Bergschrund:
crevasse that separates
flowing ice from stagnant
ice at the head of a glacier
Glacier on Shorong
Yul-lha, Nepal
Crevasses
3. Icefall: steep, fast-flowing section of glacier with cracked
and jumbled surface
Khumbu Ice fall, Everest
• Ogives: alternate bands of light and dark ice on a glacier
(winter) (summer)
Geographical and Climatic
conditions
• high snowfall in winter
• cool temperatures in summer
• Moisture important!!!
– Eg: Siberia and parts of Antarctica: low temperatures
meet glacier growth requirements, but
lack of adequate precipitation prevents glacier
development
Glacier distribution & importance
• 10% of earth
covered by ice
– 85% Antarctica
– 11% Greenland
– 4% elsewhere
• Glaciers store about 75% of the world's
freshwater
Glacier Mass Balance
• (Net)Accumulation zone = area where ice
accumulates
• (Net) Ablation zone= area where glacial ice
melts
• Equilibrium line
where accumulation=ablation
balance = 0 (at equilibrium)
Where is the ELA?
Where is the ELA?
Mass balance
• NEGATIVE: glacier gets smaller
• POSITIVE: glacier gets larger
• ZERO MASS BALANCE:
– no change in glacier size (mass,volume)
– GLACIER STILL MOVING FORWARD!!!
ELA and climate
• Cooling -- ELA lower
• Warming -- ELA higher
• Polar glaciers: ELA lower
• Tropical glaciers: ELA higher
How do glaciers reflect
climate change?
• Climate change:
– changes in temperature
– changes in amount of moisture
– Glaciers sensitive to temperature fluctuations
• climate change can cause glaciers to melt
• but the relationship is not straightforward,
– eg. Antarctica:climate change-->warmer-->more
evaporation from ocean ->more water vapor ->
more snowfall!
Ice ages
• Ice ages return every 100,000 years
• approx. 20 ice ages
• Pleistocene = most recent ice age, that
started about 2 million years ago and ended
~10,000 yrs ago
• 4 major advances of ice, most recent ones:
– Laurentide: ended 20,000 yrs ago
– Wisconsin: ended 100,000 yrs ago
– Presently we are in an interglacial period
Causes of ice ages?
Milankovic cycles
long term variations in Earth’s orbit around the Sun:
Glacial ages
• During the last Ice Age, glaciers covered
32% of the total land area.
• Little Ice Age:
– 17th century - late 19th century
– consistently cool temperatures
– significant glacier advances.
Earth’s climate record
Glaciers sensitive to climate
changes: a few facts
• strong warming over the last 50-200 yrs
• increasing CO2 levels
• Alpine glaciers have been experiencing
rapid retreat
• Ice cap on Mt. Kilimanjaro has been
decreasing by 82% in the last 88 years
• Glaciers in the Alps decreased by 50% in
volume
Ice-albedo (positive) feedback
Global warming
+
+
Glacial melt
More energy
absorbed
+
Decrease in surface
of ice
+
Decrease in albedo
+
Climatic responses- scenarios
• winter temperature:
– less, not more, snow
– polar areas get little precip. (cold air)
– if summer ablation same -- glacier retreats
• summer temperature:
– more cloud cover
– less summer ablation
– if winter accumulation same -- glacier grows
Climatic response (cont’d)
• winter precipitation (snowfall)
– if no change in temperature
– some snow survives over summer
– glacier advances
– temperature crucial factor-
Glacier response -summary
• Alpine glaciers and N.Hem. Ice caps
expected to retreat under global warming
scenario
• NOTE: Antarctica expected to grow due to
possible increase in humidity
Monitoring glaciers
• Field measurements
• Aerial photography
• Satellite images
Radar measurements - ice
thickness
Aster image, Patagonia, Chile
Indian Himalayas:
Glacier ablation at
Gangotri, source of
the holy Ganges
• glacier terminus retreated by
3km
ASTER Image courtesy of: NASA EROS
Data Center, Sept. 9, 2001
Climate reconstructions
• Ice core drilling
Drilling tent on the summit
of Cerro Tapado, Chile
Ice Core drilling, Coropuna, 2003
Having fun at high altitide...
..trying to get the generator
to work
Oxygen isotopes:
18O
and
16O
• clues of temperature in the areas where ice
formed
• Ratio of 18O and 16O indicator of
temperature
– 18O/ 16O > - warming signal
– 18O/ 16O < - cooling signal
2. Taking the ice core out
1.drilling the
ice core
3.measuring and storing the ice core
Isotope record
Glacier mummies: climate
records?
500-year old
mummy
found in
Peruvian Andes
Otzi- 5,000 year old
mummy
found in Tyrolean
Alps, Italy
Glacier retreat revealed
mummy