Gareth wrote:
> It seems that Hansen has good grounds for his concerns: as this Science
> Daily (http://www.sciencedaily.com/releases/2006/09/060920193210.htm)
> report on the latest Velicogna and Wahr paper in Nature demonstrates.
>
> ""Our results correlate well with independent observations of glacier
> acceleration in the south of Greenland," said Wahr, also a professor of
> physics at CU-Boulder. "It's a fairly straightforward story -- the ice
> loss in Greenland increased dramatically in 2004, particularly in the
> south, and it is continuing."
>
> How do the ice sheet models cope with that?
Well, of course, that is the point, the ice sheet models don't do
bupkis (well at all...) on known rapid paleosea level changes. To
quote from the notes that accompanied Hansen's presentation:
"23. Ice Quakes on Greenland
A. Seismometers around the world are detecting an increasing number of
earthquakes on Greenland. These earthquakes all occur near outlet
glaciers and are of magnitude 4.6 to 5.1.
Apparently a chunk of the ice sheet surges forward and then grinds to a
halt. The number of quakes doubled from 7 in 1993 to 14 in 1999 and in
the last 6 years it doubled again. Iceberg flux and quakes have
increased together, but we don't have accurate enough data on iceberg
flux prior to the gravity satellite to determine a relationship between
quakes and iceberg flux.
24. Paleoclimate Sea Level Data
A. Paleoclimate data indicate that ice sheet collapse can be
catastrophic. For example, in Meltwater Pulse 1A, about 14,000 years
ago, sea level went up about 20 meters in 400 years, an average rate of
1 meter every 20 years. There are numerous other cases in the record of
rapid sea level rise of many meters.
B. Existing ice sheet models cannot produce these rapid changes,
because they do not contain the physics of the ice streams. Our best
guide is the history of the Earth, which shows that warmings are rapid
because of multiple positive feedbacks that occur with melting.
C. It might be argued that most ice sheet disintegrations in the past
occurred at lower latitudes, so perhaps the Greenland and Antarctic ice
sheets are safer. However, on the other hand, the human-made forcing is
much stronger and it is changing more rapidly.
25. Summary: Ice Sheets
A. BAU global warming of ~3°C, with probably twice that warming on the
ice sheets, would leave Greenland and West Antarctica bathed in summer
meltwater. It is impossible for me to imagine that that these ice
sheets could survive more than centuries, at the most. The common
presumption that it requires millennia for ice sheets to respond is
based on inferences from changes that occurred in response to very weak
forcings that changed over millennial time scales.
B. The last time that the Earth was 3°C warmer than today, during the
Middle Pliocene about 3 million years ago, sea level was about 80 feet
(± 25 feet) higher than today.
C. Once ice sheet disintegration begins, there is the possibility of a
system out of our control. We cannot tie a rope around a collapsing ice
sheet."
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