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

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.

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

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.

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

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

Mass Balance Estimates Challenging to get accurate estimates Net rates range -230 to -680 Gt/yr Equivilant of 0.6-0.8 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 0.6 -0.8 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.

Future Scenarios and Policy Implications Three main scenarios Basal melt and constant flow, thins the ice draining over 500-700 years Ice streams readjust to negative mass balance and slow Rapid thinning of ice shelves, increased ice flow and hydrostatic uplift/collapse in 250-400 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

Questions?