On 6/20/07, William M Connolley <[EMAIL PROTECTED]> wrote:
> On Tue, 19 Jun 2007, Michael Tobis wrote:
> > It would be like popping a cork on a bottle; the ice sheet
> > would just spill out the opening.
> >
> > This is summarized in Hansen's "rising sea level helps unhinge the ice
> > from pinning points".
> >
> > I'm not sure this is regarded as likely, but it at least it's not very
> > unlikely.
>
> Is it "can't be eliminated" or " not very unlikely" or what? Really quite
> improbable would be my view. I don't think ice sheet behave like champagne.
> But
> since H isn't saying this, we're getting a bit off track.
I don't say it behaves like champagne, but it does behave like
ketchup. It's very viscous, but it does flow. Viscous flow flux
depends crucially on the width of the flow. A very wide stream of
ketchup can pour quite quickly, as can a very wide glacier. It can
also statically sustain a gravitational gradient at the margins. Such
a condition can switch suddenly from a static to a dynamic state with
small changes in boundary conditions.
The question is whether an Antarctic basin which rests on a
sub-sea-level floor can fail in a very large glacierlike flow mode.
Greenland has to fail by icequakes and moulins and lots of messy
stuff, but the West Antarctic simply has to find an outlet and spill
out of it. This is almost certainly the failure mode of ice sheets
that rest on the sea floor. The WAIS is the only remaining bit of
those, and it apparently did disappear for the most part in the
previous interglacial, indicating that its state is precarious
already.
Right now it's pinned up against a ridge. If the edge gets slushy and
it retreats behind the ridge, it's not a matter of warming the
interior for it to fail. Gravity is sufficient, though warming will
just make it worse.
The edge is in fact getting slushy, more than local warming would
account for. There are arguments as to why; someone has even tried to
implicate the ozone hole as a player I hear.
So what constrains the maximum mass flux through an unplugged glacier
with a 2 km gravitational head and a several hundred kilometer outlet
width? The impassioned answer is "not much".
The dispassionate answer is "dynamics for which we have no adequate
quantitative model". Which is to say, 800 years, 100 years, these are
pretty close to each other dynamically but pretty far apart in
practical impact. I've only just started to hang around with ice
experts, so my own intuitive guess is sufficiently worthless that I
probably shouldn't mention it.
It is, in fact, quite a serious matter to say that the 100 year 2
meter rise can't be excluded on current evidence. It would be good if
we could do a better job of quantifying this, but insofar as we have
very little observational evidence to constrain the model it does seem
prudent to treat a 2 meter rise in a century as a substantial
possibility. (With a "bigger than 10%" feel in that IPCC-ish
qualitative probability range space.)
Then there's Greenland...
I agree that there is more than a hint of polemics in the
presentation, but I'm not at all convinced that it's inappropriate
given the seriousness of the situation. I don't think anything Hansen
said is substantively out of line with current knowledge.
Have you looked at the possibility of WAIS collapse by super-glacier
dynamics? On what basis, dynamic or observational, do you either a)
constrain the time scale to greater than a century or b) discount the
possibility?
Hansen is questioning the conventional wisdom. "I don't think so"
doesn't constitute a strong defense, especially given that the actual
substantive content of AR4 is "we don't know, really".
mt
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