Flashcards in // lecture 28 Deck (41):
there are two ice sheets in the world:
- Greenland: 7 m sea level equivalent
- Antarctica: 61 m sea level equivalent
ice sheets gains
mass from snow on top and loses mass from melting and calving of icebergs.
- both loss and gain are increasing (increased precipitation at high latitudes).
- Greenland is losing mass though.
overall, ice sheet losses
are exceeding gains in Greenland. Also losses from calving have accelerated in the last 10 years.
Greenland is very big though -
it took tens of thousands of years to grow and at the current rates it would take many thousands of years to melt.
- now it's contributing less than 10% of the sea level rise.
Could Greenland ice loss accelerate?
maybe because more melting causes larger melt ponds meltwater can fall through the cracks and make it to the bedrock below.
- water dripping through could cause it to stick to the bottom less and flow faster (moulin - hole in ice).
- still unclear how relevant this is in causing recent accelerations of calving.
A 2 C temp. rise would almost certainly
melt Greenland (eventually). this would likely take centuries though.
Currently, Greenland melting is a
small contribution to sea level rise. this is likely to become a larger percentage in the future though.
Antarctica has two giant
and many smaller, ice shelves.
float, but are connected to the sheet.
acceleration of melt has occurred in
w. antarctica recently. even more acceleration than in greenland.
glaciers along W. Antarctica's Amundsen Sea would
raise sea levels by 1.2 m if they melted.
Antarctica has experienced relatively
small warming to this point, except the bits near South America.
- this was long ago forecast by climate models.
- strong jet stream keeps warmer air out of high latitudes.
- also ocean nearby takes up a lot of heat.
- no sea ice decrease has happened in Antarctica either. although, nearly all Antarctic sea ice melts each summer anyways.
East Antarctic is not
warming, but West Antarctic is warming.
Ice shelves reduce
calving by buttressing the ice.
Antarctica's icy surfaces are
very cold, but warming in the surrounding ocean is a problem for the shelf base.
Ice shelf collapse can
then lead to acceleration of calving.
Antarctica has the most
land ice (61 m of potential sea level rise), but East. Antarctica (the big part) is thought to be safe.
West Antarctica is potentially dangerous:
5 m sea level rise.
- much of the w. antarctic ice sheet is under sea level so warmer ocean water could get in and melt much more after melting starts.
- recent studies (modeling) suggest that the melting of several large ice shelves is underway and unstoppable. Twaites would account for 3-4 m of the sea level rise.
- would take hundreds up to a thousand years to complete the process.
UW and NASA work indicate that the Twaites ice shelf is already
unstable and will slide into the sea and melt, but it might take 500 years. 1-2 m of sea level.
Twaites ice stream and Pine Island Glacier
are being undermined by ocean water.
each day 8 cubic miles of warm ocean water circulate under the Pine Island ice shelf. that water melts and thins the ice from beneath, causing the glacier to slide more quickly into the ocean.
Presently melting mountain glaciers are contributing
more to sea level rise than Greenland and Antarctica combined. probably won't be the case in 50 years (ice sheets are much bigger).
mass balance of a mountain glacier:
gain due to snowfall, loss due to melting, sublimination.
mountain glaciers don't lose mass due to
breaking off chunks into the sea.
mountain glaciers are rapidly
losing mass in most places. increased melt is outpacing snowfall in all but a few places.
Himalayan glaciers may be melting from
black carbon deposition too.
much of the snow on glaciers fell
a long time ago.
melting of glaciers has revealed:
inca mummies, 550 year old ice man, planes from WWI and WWII, and a british plane from the 1940s that was rumored to have gold in its cargo.
Kilimanjaro snow is
disappearing from sublimination because the atmosphere there is drier now. may still be caused by humans, but not directly by warming.
ground that's frozen year-round. typically has a thin layer on top that melts in the summer: "the active layer."
ice is usually
underground. can be visible at river channel.
ice expands upon freezing so this causes potholes in roads in winter, etc. Frozen ground causes weird formations. when permafrost thaws, trees point in all directions.
widespread evidence of permafrost thaw causing:
damage to roads and buildings, methane to be liberated, vegetation to increase.
to melt all the permafrost will take
permafrost thawing will be accelerated if
summer sea ice continues to retreat.
will thawing permafrost release or take up greenhouse gases?
- snow-free season is short so plant growth rates are low. plants are cold adapted, which is their strategy for survival. not adapted for much competition. likely more vegetation will come in after thaw.
- soils are very carbon rich (decaying plants and animals). some fear methane release from thaw. others say greater vegetation will draw down CO2.
methane hydrates (calthrates)
water ice cages enclosing methane. methane caged in a matrix oc ice at high pressure or low temp.
- carbon stored is 500-10,000 Gtons C (coal is 3,200 Gtons C). need high pressure - under ice massive in permafrost or ocean sediments.
risk from methane hydrates
the risk form them this century is speculative. however, their release would possibly cause a positive feedback.
55 million years ago methane hydrates appear to have been
released to the atmosphere at a time when the earth also warmed 2-3 C. there are pockmarks in ocean sediments as indication of past catastrophic release.
snow covered land is by far the largest area component of the
cryosphere. snow is part of the ice albedo feedback cycle, such that if snow melts earlier in spring, it causes spring warming and soil drying. snow is an important reservoir for water in some communities.