1. Global Warming and the stability of The West Antarctic Ice Sheet
Paper written by Michael Oppenheimer
Auden Schilder

2. Commonly used terms
“collapse”  the loss of majority of ice in short time scale
“unstable”  small perturbations could cause collapse
COLLAPSE  which I use here to mean the loss of most or all of the land-based
(grounded) ice on a timescale that is much shorter than its
accumulation turnover timescale
UNSTABLE  which means
that collapse of currently grounded ice would occur following small
perturbations to its boundary conditions.

3. West Antarctic Ice Sheet
Grounded on land below sea level
Contains two large ice shelves
Ross Ice Shelf and Filchner-Ronne Ice Shelf
Grounding line separates grounded from ice shelves
Other major ice sheets grounded above sea level
Grounded on land below sea level
MEANING THAT IF THE ICE WERE TO BE REMOVED, THE OCEAN WOULD COVER THE LAND
LEAVING ONLY ISLANDS ABOVE SEA LEVEL
 EVEN WITH ISOTATIC REBOUND OVER AND ESTIMATED 10,000 YEARS, THE MAJORITY OF LAND WOULD STILL BE DEPRESSED
Contains two large ice shelves
Ross Ice Shelf and Filchner-Ronne Ice Shelf
MOST OF THE DRAINAGE MAKES ITS WAY TO THIS REGION
Grounding line separates grounded from ice shelves
Other major ice sheets grounded above sea level
SUCH AS GREENLAND, WHICH IS MAINLY SUBJECT TO ATMOSPHERIC CHANGES IN TEMPERATURE

4. Age and Persistnacy of Ice
Overall age of the ice sheet in controversial
At least one period of complete disappearance
Past disintegration could foreshadow future
Over all age of the ice sheet in controversial
WHETHER OR NOT THE ICE HAD BE CONTINUALLY PRESENT OVER THE PAST ~10-15 ma IS UNCERTAIN
THE EXTENT OF GROUNDED ICE DURING THE LAST GLACIAL MAXIMUM WAS SEVERAL HUNDERED KILOMETERS BEYOND PRESENT ICE SHELVES
At least one period of complete disappearance
BASED ON EVIDENCE FROM LAKE SEDIMENTS OF A PLEISTOCENE WARM PERIOD IN ANTARCTICA, THAT WAIS MAY HAVE COMPLETELY DISAPPEARED AT LEAST ONCE.
SUMMARIZED EVIDENCE THAT GLOBAL SEA LEVEL STOOD AT LEAST 6M HIGHER DURING THE LAST INTERGLACIAL PERIOD
Past disintegration could foreshadow future
THESE FINDINGS LED TO THE INFERENCE THAT DISINTEGRATION OF WAIS MAY HAVE CAUSED SEA LEVEL TO RISE AT LEAST ONCE DURING A PERIOD WHEN GLOBAL MEAN TEMPERATURE MAY NOT HAVE REACHED MORE THAN 2 DEGREES C ABOVE THAT OF TODAY.

5. Ice Gain, Loss and Movement
Types of mass loss; evaporation, melt at the surface, and calving of ice bergs
Mass gain; precipitiation
Ice flows in fast moving streams
Some ice streams move over sediment
It loses mass through evaporation, by melting and runoff at its surfaces (top, edges and bottom), and by calving of icebergs at the front of the ice shelves
 MASS LOSS ON UPPER SURFACE IS NEGLIGABLE
IMPORTANT CLARIFICATION…. CALVING ICE ADDS TO MASS LOSS BUT NOT TO INCREASE IN SEA LEVEL RISE …. BECAUSE DISPLACMENT OF WATER = VOLUME OF ICE
Ice flows in fast moving streams
THE FLOW OF ICE FROM THE GROUNDED PART OF WAIS TO THE OCEAN IS CONCENTRATED IN FAST-MOVING ICE STREAMS. ABOUT HALF OF THE ICE FLOWS INTO THE ROSS ICE SHELF
ICE STREAMS AND SEPERATED BY STAGNENT ICE THOUGHT TO BE FROZEN TO THEIR BEDS
Some ice streams move over sediment
ICE STREAMS WERE ORIGINALLY ASSUMED TO SLIDE OVER BEDROCK, LUBRICATED BY MELTWATER FROM THEIR BASE
MOVE OVER A BOUNDARY LAYER OF UNCONSOLIDATED, WATER-SATURATED SEDIMENT UP TO SEVERAL METRESTHICK WHICH LUBRICATES THEIR MOTION

7. Grounding Lines and Ocean Circulation
Grounding line separates ice shelf
Global warming causes ocean warming
Circumpolar Deep Water (CDW)
Water movement creates basal melt
recent studies indicate that significant basal melting already occurs under the large ice shelves. Changes in ocean
circulation and temperature expected to accompany global warming could increase
this circumstance would provide a potential pathway for thinning and disintegration of the large ice shelves
If an ice shelf providing buttressing becomes
unpinned owing to melting or global sea-level rise, the grounded ice
would accelerate its flow, thin, and rapidly float off its bed.
CURRENT MODELS PROVIDED TODAY CAN BE VERY SOPHISTICATED BUT ARE NOT ENTIRLY RELIABLE. THERE ARE TOO MANY UNCERTANTIES ABOUT THE RESPONSE OF THE ICE

8. Mass Balance Estimates
Challenging to get accurate estimates
Net rates range -230 to -680 Gt/yr
Equivilant of mm/yr rise
Estimates revoke the idea of stability
Challenging to get accurate estimates
SOME STUDIES INDICATE POSITIVE GROWTH BECAUSE OF INCREASE IN PRECIPATIATION
MOST STUDIES AGREE THERE IS AN NEGATIVE MASS BALANCE BECAUSE OF THE HIGH MELT RATES UNDER FLOATING ICE
Net rates range from -230 to -680 Gt/yr
THIS COULD ACCOUNT FOR MM/YR OF SEA LEVEL RISE
POTENTIAL FOR 6-8 CM RISE IN SEA LEVEL AT THE CURRENT LOSS RATE
Estimates only revoke the idea of stability
 THIS MASS-BALANCE ANALYSIS SAYS LITTLE ABOUT THE STABILITY OF WAIS BEYOND UNDERCUTTING THE ARGUMENT THAT IT IS STABLE BECAUSE IT IS NOT LOSING MASS.

9. Future Scenarios and Policy Implications
Three main scenarios
Basal melt and constant flow, thins the ice draining over years
Ice streams readjust to negative mass balance and slow
Rapid thinning of ice shelves, increased ice flow and hydrostatic uplift/collapse in years
(4. Catastrophic collapse over the next century)
Three main scenarios ( IN ORDER OF LIKELYHOOD)
Slower method of collapse
There will be internal readjustments made by the dynamics of the ice, causing the streams to slow down
Collapse on a faster rate caused by rapid thinning and hydrostatic uplift of the ice sheet when the ice shelves vanish
Not a convincing argument for many scientists but it has be theorized

10. Questions?