A bit sideways, this one, but just something I noticed. Here is a new paper (under discussion), from The Cryosphere. The subject matter is the mass balance of the McCall glacier, Alaska.
The paper is interesting in itself, and at only 25 pages, worth a read, not least for the work on internal accumulation and its relation to mass balance, which seems to be quite original.
But the main reason for posting it id to draw your attention to the graphics in the latter part of the paper. No surprise to learn that the glacier has been shrinking for some time now (100 years, in total), nor that the rate of change of mass balance has accelerated since the 1980’s. What is interesting is that the graphs bear an uncanny resemblance to others I have seen recently, such as the ones showing long-term ice extent trends for the Arctic on Cryosphere Today.
In fact, the shape of the graph is similar to very many of those showing changes related to climate in the Arctic. On the principle that coincidence may be plausible for two discrete measurements, but not for a whole set, it may be acceptable to conclude - tentatively - that the consistency carries for a number of measures and data. This, again, probably comes as no surprise to those of you who observe the progress of the Cryosphere.
But it does have implications. The shape is suggestive of the first part of a hyperbolic curve, where increase goes from negligeable, to noticeable, to substantial, in a logarithmic (?) relation. The question we could be asking, then, is whether the line of best fit for likely future changes in glacier mass balance, sea-ice area, GIS mass balance, should continue along this curve.
I cannot say that this should be the case, nor claim that it is likely, but it is plausible. If so, this suggests that the impacts of climate change for glaciers, sea ice, and other elements of the cryosphere, is starting to accelerate rapidly. This in turn has implications for the estimates of 21st Century sea level rise, and also for water resource availability in the next ten-twenty years.
I would be interested to know if there are scientists out there, or knowledgeable amateurs, who are willing to speculate that this is the case, and, if so, what the implications might be.
If you decide to post a comment, it won’t go up ’til Sunday, as I’m off line for two days, but don’t let that stop you if the mood takes you: I’ll update as soon as I get back.
Enjoy the weekend.
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November 9, 2007 at 7:57 pm
Aaron Lewis
Is this the curve you are looking for?
“Ice is a viscoelastic material with a nonlinear flow law. When shear stress is applied to a single crystal of ice, it undergoes plastic shear strain easily parallel to the basal plane, which is perpendicular to the hexagonal c-axis. In other directions, the stress needed to produce plastic shear deformation is much higher. When polycrystalline ice is subjected to stress, it immediately deforms elastically, followed by transient creep, and finally steady viscous flow called secondary creep is reached. For high stresses in excess of 400 kPa, the creep curve accelerates which is called tertiary creep. Several physical processes are responsible for these deformations: movement of dislocations, sliding along grain boundaries, and recrystallization. The steady-state secondary creep rate sigma in secondary creep is related to the stress sigma by the flow law
d /dt = A n e-Q/kT,
where A is a temperature-independent constant, Q is the activation energy for creep, n is the nonlinear exponent, k is the Boltzmann constant, and T is the absolute temperature. Values of A, Q, and n depend to some extent on the grain size, grain orientation distribution, and impurity content of the polycrystalline material. At temperatures between 0 and -10 C and for stresses between 100 and 250 kPa, A = 5 x 10-15 s-1 kPa-3, Q = 139 kJ mol-1, n = 3. At stresses lower than 100 kPa, n = 2. “
the above from http://skua.gps.caltech.edu/hermann/ice.htm
My feeling (as an old ice climber) is that (outside of the lab) ice becomes fractured. In a cold environment, the fractures heal, and modeling is possible. As the temperature approaches freezing, heat is advected through the fractures, and the ice becomes discontinuous. One problem with the cited model is that in nature, the temperature of water in the fractures may be above the pressure melting temperature. At some point, the hydraulic head of the water in the fractures may exceed the strength of the ice, or the head becomes sufficient to float some portion of the ice. Conventional models do not capture this behavior very well. Hansen has used the term “explosive” for these conditions. This may be the only point I have ever disagreed with Hansen on. I say the ice-water slurry remains sub-sonic.
See http://query.nytimes.com/gst/abstract.html?res=F20E15F9345D17738DDDAA0994DF405B8285F0D3
November 13, 2007 at 7:17 am
Hank Roberts
I think there’s some modeling being done (or maybe it’s just field observation) on this. I posted some tidbits in the “quarter inch” thread at Prometheus back in the day.
Here are a few snipped from there about what’s being studied. This isn’t ’simple’ ice breaking and healing, it’s complex motion in space and time, and that may keep cracks from resolidifying so a shelf can crumble after it’s internally broken up over time and tide.
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http://scrippsnews.ucsd.edu/article_detail.cfm?article_num=685
forced flexure zone between fully grounded. continental ice and fully floating ice shelf … rift propagation on Amery Ice Shelf, East Antarctica …
http://www.igpp.ucsd.edu/PDF/research/2006/people/fricker_helen_06.pdf
Antarctic Ice Shelves and Ice Sheet Evolution: … New Insight into Ice Shelf Rift Propagation from Geodetic and Seismic Monitoring … Rifts in Antarctic ice shelves are large through-cutting fractures …
http://www.agu.org/meetings/fm05/fm05-sessions/fm05_C12B.html
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