Cryosphere (Too Frozen Water)
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Transcript of Cryosphere (Too Frozen Water)
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Cryosphere (Too Frozen Water)
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TOPICS
How ice formsWhat controls ice dynamics & importanceInteraction of sea and continental ice with
atmosphere over different time scales
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IMPORTANCE OF CRYOSPHERE
To climate system – Change in sea ice & snow cover changes albedo &
temperatures Change in glacial ice changes sea level Melting permafrost releases methane Sea-ice formation changes salinity and affects density,
bottom-water formation & deep-ocean circulationTo biosphere -
Challenge of living on frozen ground Meltwater source and reservoir of freshwater
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Components of Cryosphere
Continental ice sheets & ice shelvesMountain glaciersSea iceRiver and lake iceSnow coverPermafrost
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Time scales relevant to the cryosphere
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87% of ice is in Antarctica10% Greenland2.5% ice shelves (mostly Antarctic) If all ice melted sea levels would go up by about 65 meters (213 ft)
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Phase Diagram - Water
Triple Point
Critical Point
Ice grows bydepositionfrom gas to solid (snowflake)
Ice grows bydeposition fromwater to ice(graupel)
Ice grows byagglomeration(ice crystals joinTogether)
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Dendrite: Common six sided ice crystal / snow flake.Influences interaction with climate system
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The shape of ice crystals cause ice to be less dense than liquid water This causes ice to float with about 9% of the ice volume above water (91% below). a.k.a. - the tip of the iceberg.
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Snow Cover
Essential for water resources in the western US
Roughly 75% of the Western US fresh water comes from snow pack.
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NA snow cover – February 2002High albedo – large number of reflecting surfaces return radiation to space (80-90%)What is the effect on regional energy balances?
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January or February have most NH snow cover.10 x more than in summer
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Aside from Antarctica the SH has very little snow cover
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Figure 1. History of global mean surface air temperature, from NASA
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Permafrost
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NH permafrost likely to decrease 20-35% by 2050
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Carbon balance in the tundra
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Methane possibly released from permafrost?
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Methane possibly released from permafrost?
Methane change Since 2005
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Methane change Since 2005
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Methane change Since 2005
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Feedback
Arctic warming faster than rest of planet snow and ice-albedo feedback
Positive feedbackPermafrost
MeltingMethane release
Temperature
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Methane Clathrate Hydrates
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River and Lakes Spring Thaw
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Most rivers are thawing earlier and freezing later.
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GLACIERS AND ICE SHEETS
Glaciers begin when snow accumulation is greater than snow melt
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Glacier Formation
Pressure Sintering –with increasing density snow is compacted and crystals fuse together
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Density Differences
New snow – 50-70 kg/m3
Firn – 400-800 kg/m3
Glacial Ice – 850-900 kg/m3
Low permeability Flows under own weight
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Types of Glaciers
Alpine or Mountain Small Confined to mountainous
valleysContinental
Large Unconfined by topography
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Glacier Flow
Plastic Deformation – Flow due to high pressure
Basal Sliding – glacier slides over the bed due to presence of liquid water or unconsolidated material
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Ice flow speed increases with distance above ground - flow at base equals zeor, ice frozen to the bed
Side view
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top
Top view:
Ice flow increases with distance from valley walls.
Friction at the sides reduces flow
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Basal melt (or unconsolidated material) can provide a lubricant to increase total ice flow
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Ice flow increases with increasing tilt of the mountain
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Air bubbles trapped in ice can reveal atmospheric composition in the past (up to about 800,000 yrs)For long term climate reconstruction – take cores from part of ice cap not moving
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Accumulation: Snow persists through summer &builds upAblation: Melt, sublimation, loss from wind, loss from flowEquilibrium line: Accumulation = Ablation
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When accumulation is greater than ablation in a certain zone the ice sheet :a) Growsb) Shrinksc) Stays the same size
Sublimation or calving
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When accumulation is greater than ablation in a certain zone the ice sheet :a) Growsb) Shrinksc) Stays the same size
Sublimation or calving
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Alpine Glaciers
Milk Lake glacier 1988
Milk Lake 2009
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Glaciers around the world are in retreat
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Greenland Ice Sheet (Land Ice)
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From GRACESee The State of the Greenland Ice Sheet and Gravity Recovery And Climate Experiment
Right: Mass change distribution (2002-2009) across the ice sheet as determined by GRACE observations.
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2010 was an exceptional year for Greenland’s ice cap. Melting started early and stretched later in the year than usual. Little snow fell to replenish the losses. By the end of the season, much of southern Greenland had set a new record, with melting that lasted 50 days longer than average.
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West Antarctic Ice Sheet (Land Ice)East Antarctica is Now Losing Ice
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350 Gt – 1 mm sea level riseBoth Antarctica and Greenland have been losing ice over the past 8 years.
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Sea Ice
What happens to the water level when the ice melts?
a. Overflowsb. Goes downc. Stays the same
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Sea Ice
What happens to the water level when the ice melts?
a. Overflowsb. Goes downc. Stays the same
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Sea Ice
Importance Ice-albedo feedback Bottom water formation Changes affect polar ocean climate
These effects dominate high-latitude response to increasing atmospheric CO2 levels in cgolbal climate models
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Formation of Sea Ice
When temps reach -1.8 °CThickens when new ice freezes onto the
bottom of the ice pack (~1 m every winter)At high latitudes persists during summerPermanent thickness
~5 m in Arctic ~0 m very little survives summer
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Seasonal Ice Cover range
Northern Hemisphere Doubles Summer ice extent
decreasing drastically
Southern Hemisphere 5 fold increase
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From: http://earthobservatory.nasa.gov/Features/WorldOfChange/sea_ice.ph0p The yellow outline on each image shows the median sea ice extent observed by satellite sensors in September and March from 1979 through 2000.
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Sept 2011
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From: http://earthobservatory.nasa.gov/Features/WorldOfChange/sea_ice_south.php
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http://nsidc.org/arcticseaicenews/2011/020211.html
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Sea ice movesIn a constant state of motionMoves faster than land ice2 features of circulation: Transpolar Drift (ice lasts 5 yr) Beaufort Sea Gyre (ice lasts longer)
Sea Ice Drift in the Arctic
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Ice Floes
Ice moved by wind and currents Form pressure ridges, leads and polynii
Always small amount of open water (even in winter) Importance
Open water allows for production of ice Release salt to upper ocean, increasing density
Impacts Arctic energy budget Positive ice-albedo feedback Heat loss (100x that of ice) from water to atmosphere
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Ice-Atmosphere Interactions
Ice cover modifies atmospheric & oceanic circulation
Formation of NADW (drives thermohaline circulation)
Ice-albedo feedback
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