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On Mon, Jul 1, 2013 at 10:26 AM, Robert G. Brown wrote:
The ocean is (approximately) isostatic -- certainly there is little
reason to think that it is on average anything but isostatic -- and we
HAVE no baseline for the depth of the middle ocean. It is damn
difficult to measure ocean level even at the shore -- and I say this
watching the tide come up outside my window. The tide varies constantly
with the moon, the sun, the wind, the local air pressure, and more.
Differentiating a long run increase at all is pretty difficult. But I
certainly trust the tide gauge record more than anything else because it
is a single, reasonably consistent collection of measurements mostly
made long before any thumbs were applied to scales to support "AGW",
catastrophic or otherwise.
The point you do not make as solidly as you might is that the failure of
tide gauge data to "accelerate" suggests that there has been no
acceleration of the rate of land-based ice melt, the only source of
actual additional real volume of the ocean. I have asserted for some
time that the ocean itself is the best global thermometer we've got,
because precisely as you note, coastal levels represent an UPPER BOUND
on the expansion warming of coastal waters. All of them. This in turn
is a boundary condition on the entire ocean. If there is land ice melt
going into the oceans, this actually DECREASES one's expectation of
coastal warming as then the increase in SL at the coasts has to be split
between land ice melt and thermal expansion, which strictly reduces the
contribution from thermal expansion. Land ice melt is the only "real"
increase in oceanic volume, and the only thing likely to significantly
impact coastal SLR in the long run, because the ocean is isostatic and
cooling in one hemisphere where it is warming in another so that even
local thermal expansion in one place is balanced by small changes in
prevailing currents to maintain isostasis.
The fundamental problem is that the actual rate of EITHER tide gauge
data OR satellite data projects out to around a foot of SLR by 2100,
utterly ignorable, just as the 9+ inches we have had since 1870 was not
even noticed by the people living on the sea. I have asked my
neighbors, who have lived on the ocean where I'm staying in Beaufort for
over 40 years (since their back yards were built up from a sandspit
island with a small sea wall), if they have observed any increase in SL.
In principle, they should have had around 3-4 inches over 40 years, but
they cannot see it -- their docks are still out of water on the highest
of spring tides, their yards are still underwater on a storm surge from
a hurricane (which happens here as it has happened here before over all
40 years, every four or five years), it is still impossible to launch a
boat from the ramp in my backyard at anything but high tide. The
barnacles on the piers of their docks and the sea wall itself tell the
same story -- the line where they survive has not visibly increased over
the entire multi-decade lifetime of dock pilings or the sea wall.
Personally, I think Hansen's assertion of 5 meter SLR by 2100 to be
criminal, the equivalent of yelling fire in a movie theater because you
think that the theater MIGHT catch on fire in 90 years because you think
that the movie theater's projector MIGHT overheat by then. I think
taking steps now as if there will be even 1 meter of SLR by 2100 is
silly. Right now, the sensible rate to use as a projection of the
future is one inch per decade. Even Trenberth, hardly a skeptic,
acknowledges as much -- SLR even according to the probably inflated
satellite record is a whopping 30-35 cm/century.
If this rate really does change, it isn't going to do so overnight.
There will be DECADES to react. Spending money as if we know what SLR
is going to do 80 years from now when it isn't doing it now is just
Robert G. Brown
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305