breaking with tradition: dealing with unavoidable but ... · breaking with tradition: dealing with...
TRANSCRIPT
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G. Comer Foundation
Breaking with tradition: Dealing with unavoidable but imperfectly observed physics
in ice-sheet models and sea-level riseR.B. AlleyNov. 28, 2018The Future of Earth System Modeling: Polar Climates. CaltechPlease note: I work for Penn State University, and help UN IPCC, NRC, etc., But I am not representing
them, just me.
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Forcing is moving outside instrumental data
May move outside paleoRCP 8.5 deglacial-sized
warming in 100 yrs, not 10,000, to Pliocene-plus?
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Then, no matter how good the data assimilation,
No matter how true to instrumental data,
Extrapolations tend to underestimate changes
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Fig. 1, Shoji, H. & A. Higashi, 1978, A deformation mechanism map of ice, J.Glac. 21(85), 419-427
Deformation maps
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Fig. 1, Shoji, H. & A. Higashi, 1978, A deformation mechanism map of ice, J.Glac. 21(85), 419-427
Speed of sound2x stress109x defm
2x stress, 10x defm2x stress, 2x defm
Deformation maps
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Fig. 1, Shoji, H. & A. Higashi, 1978, A deformation mechanism map of ice, J.Glac. 21(85), 419-427
Deformation maps
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Linear
process
Cubi
c pro
cess
Cartoon of deformation mechanismsAll are always active, but in broad
regions, only one is significant
Stress
Stra
in ra
te
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Linear
process
Cubi
c pro
cess
Sum
Stress
Stra
in ra
teCartoon of deformation mechanismsAll are always active, but in broad
regions, only one is significant
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Linear
process
Cubi
c pro
cess
Sum
Stress
Stra
in ra
teLow-stress calibration sensitive
to linear, misses cubic, underestimates high-stress deformation
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Linear
process
Cubi
c pro
cess
Sum
Stress
Stra
in ra
teLow-stress calibration sensitive
to linear, misses cubic, underestimates high-stress deformation
High-stress calibration sensitiveto cubic, misses linear, underestimates low-stress deformation
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àMany deformation mechanismsàRates~Aoexp[-Q/(RT)]tn
àn=1 (several processes) to ~30 (subcritical crack growth), wide range of QàFor some (T, t) one process dominatesàData are sensitive to only that processàIf extrapolated too far, some other process always grows to dominateàAnd model always underestimates rate
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àExtrapolations underestimate rateàVery clear for ice deformationàEspecially at onset of fractureàProbably for basal sliding as wellàI’m happy to discuss (Iken limit, till deformation viscous-plastic, role of ice deformation in sliding; R-channels unclear but likely don’t change answer)
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If you want buildings, bridges and ice sheets to persist, then rule is:All extrapolations are overly optimistic
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https://earthobservatory.nasa.gov/images/88701/glacier-bay-national-park-preserve
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https://earthobservatory.nasa.gov/images/88701/glacier-bay-national-park-preserve
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https://soundwaves.usgs.gov/2001/07/glacierbaymap.gif
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Muir Glacier, Alaska, August 13, 1941, photo by W.O. Field
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Muir Glacier, Alaska, August 31, 2004, photo by B.F. Molnia
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C.F. Larsen et al. / Earth and Planetary Science Letters 237 (2005) 548–560
D Ice thickness~1500 m maxNOT by flow
to Icy Strait105 km retreatPeak ~1 km/yr?Shallow, narrow
compared to polar outlets
Icy Strait
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Margold, Stokes & Clark, QSR, 2018
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Margold, Stokes & Clark, QSR, 2018
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Margold, Stokes & Clark, QSR, 2018
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Margold, Stokes & Clark, QSR, 2018
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Margold, Stokes & Clark, QSR, 2018
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Margold, Stokes & Clark, QSR, 2018
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Big issue: Do prognostic models need calving and cliff physics to accurately estimate warming-induced sea-level rise?
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There are many places on land where you can walk up on the toe of a glacier…
which is what these Greenlandic wild reindeer did, to get away from tundra mosquitoes
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But you can’t paddle your sea kayak to the ice front, jump out and walk up—almost all ice flowing into the ocean ends in a cliff, where fracture dominates
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So, a reasonable hypothesis is that calving and cliff failure will continue to be important
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Fürst et al., 2016, Nature Climate Change
Almost all ice-shelf ice buttresses
Ice-shelf loss thus is important to sea-level rise…
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Fürst et al., 2016, Nature Climate Change
Almost all ice-shelf ice is buttressed
Ice shelves typicallyform behind ice rises and in calving-restraining
narrow fjords (e.g., Alley et al., 2008)
Unbuttressedice breaks off
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Fürst et al., 2016, Nature Climate Change
Almost all ice-shelf ice is buttressed
Ice shelves typicallyform behind ice rises and in calving-restraining
narrow fjords (e.g., Alley et al., 2008)
Unbuttressedice breaks off
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Doake et al., Nature, 1998
Successfully predicted Larsen B collapse
When some buttressing lost, ice shelves tend to break up completely
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January 31, 2002
http
://sv
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x.ht
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12 mi20 km
OceanAntarctic Peninsula (gothic cathedral)
Icebergs
Larsen B Ice Shelf (flying buttress)
Melt ponds
Island
Island
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January 31, 2002
12 mi20 km
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March 7, 2002. 8x tributary flow-speed increase followed
12 mi20 km
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Not much ice behind Larsen B; loss can’t raise sea level much
Many more ice shelves with lots of ice behind them that can raise sea level a lot.
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Jakobshavn Isbrae retreat and speed doubling fromocean-warming-induced ice-shelf loss Ian Joughin (Alley et al., 2005).
Rock
Rock
Ice Flow
Floatingice mess
Berg
5 km19
92
2000
2002
2003 20
04
Ice-shelf loss followed rifting at sides that reduced buttressing(Joughin et al., 2008)
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Rock
We’ve seen ice shelves fall apart, and not grow back Jakobshavn, Helheim try to re-grow, favored by:àcold ice, so strongàmuch thicker than surface-crevasse depthàmoulin drainage inland keeps water out of crevasses (Parizek et al., 2010) ànarrow fjords give stabilizing side shear, restrict wave forcing, aid mélange formationàflow so fast that can grow an ice shelf in a winteràfluctuations in climate give colder years
But shelves still fail to re-grow
Hysteresis—breaking easier than building
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So we see in multiple casesàWarming (air and/or water) reduces shelf buttressingàShelf then breaks off, leaving a grounded calving cliffàIce flow acceleratesàAnd the shelf stays off
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All cliff ice is relaxing by creep, but is breaking faster
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Could higher future cliffs relax fast enough to re-form ice shelves?
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Schulson, 2001, Engr. Fract. Mech.
Schulson—planar loading, for sea ice (and for free face of calving cliff)
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Schulson, 2001, Engr. Fract. Mech.
Doesn’t break (too strong or too confined)
Tension Compression
Com
pres
sion
All else breaks in some way
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Schulson, 2001, Engr. Fract. Mech.
Doesn’t break (too strong or too confined)
All else breaks in some way
This “rock bursts”—spalls, or breaks out of plane
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Schulson, 2001, Engr. Fract. Mech.
Doesn’t break (too strong or too confined)
àFree face of a cliff prevents confinementàBreaking stresses increase with cliff height
(e.g., Hanson and Hooke, 2000; 2003)àMay fail by slumping (Parizek et al.), water wedging (Pollard et al., 2015), rock burst…
This “rock bursts”—spalls, or breaks out of plane
All else breaks in some way
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If creep and subcritical cracking are occurring at a similar rate, 2x cliff heightà2x deviatoric stress causing deformation:creep increases ~10x; cracking increases ~109x
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If creep and subcritical cracking are occurring at a similar rate, doubling deviatoric stress causing deformation:creep increases ~10x; cracking increases ~109x
Brittle-to-ductile or flow-to-fracture transition
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Could higher future cliffs relax fast enough to re-form ice shelves? No
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Instead, higher or wider future cliffs will be more brittle
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But does it matter? Yes...
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~100m
Jakobshavn calves to ~20 m thicker than flotation, reducing drag, speeding flow and thinning (Joughin et al., 2012)
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Stresses caused by cliffs enhance thinning through second-invariant effects in flow law(see Parizek et al., AGU; ms. in prep.)
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With very rapid failure by water-driven crevassing, slumping, spalling for too-tall cliffs
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So…Do prognostic models need calving and cliff physics to accurately estimate warming-induced sea-level rise? Yes
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What can we do? Model calving and cliffs…
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What can we do? Model calving and cliffs…Or:àSet ice-shelf buttressing to zero above some temperatureàSet basal drag to zero a bit inland of flotation (calving to there)àWeaken near-grounding-zone ice (second-invariant cliff effects)àSet flag–if cliff much taller than 100 m, retreat faster than model
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Forcing outside modern requires models that cannot be properly tuned to modern
Paleo, physics can helpPersistent uncertainty remains