Wednesday, March 28, 2012

2918.txt

date: Sat, 17 Jul 2004 20:42:33 -0600
from: Tom Wigley <wigleyatXYZxyz.ucar.edu>
subject: more GSIC
to: Sarah Raper <sraperatXYZxyz-bremerhaven.de>, Sarah Raper <s.raperatXYZxyz.ac.uk>

Sarah,

You are right that all the GSIC will melt even for zero warming
from today. Our old model was better in that regard because it
had a different eventual melt for different warmings. But this is a
tricky thing for me even to conceptualize. My intuition says that
if we stabilized temperature at today's level then not all of the
GSIC ice would melt -- but how much would remain? It also
seems reasonable that there is some warming amount that would
ensure that virtually all the GSIC ice would melt --- 3degC,
5degC, ???

So the next step would be to try to get some realism here, but I
really have no idea what would be realistic.

However, I think what we have is fine out to 2400 for most cases.
For zero warming from now, sea level rises at about 5cm/century
initially and then more slowly later -- so it would take 1000 years
or more for all the GSIC ice to melt. We never get far enough
for things to look silly. This melt rate (%cm/century) is proportional
to (0.15 + T(1990)), so the 0.15 has an important effect. Climate
sensitivity has an effect too (in MAGICC) since this influences
T(1990) -- a model artifact. (The TAR says that the 0.15 comes
from Zuo and Oerlemans, which I have added to the refs.)

So this leads to a question -- at present the GSIC model,
embedded in MAGICC, uses T(1990) from MAGICC. Perhaps
it would be better to use an observed value (like the 0.65 I use
in the text example)? For most cases this would have little effect,
but it has a noticeable effect for cases with very little post-1990
warming. Arguing against this, to add another add hoc correction
like this would just make things messier. I'm just thinking out loud
here.

As it happens, the analytic result I gave was for zero warming
from 1990, not (as the text implied) for linear warming. The linear
warming result is quite complicated -- but all I needed to illustrate
the points made in the paper was/is the zero warming case. I
have corrected this.

I have also done the A proportional to V**n case, and dumped
the n=1 results. The refs you gave were just what I needed, and
I have cited these. n.NE.1 has little effect out to 2100, but does
lead to greater melt by 2400. I have revised the text to cover all
this, but need to re-do the diagrams. I hope to send this new
version to you tomorrow. I think it is a significant improvement
even though the results are pretty much the same. Some of the
bibliographic holes are filled -- I can imagine that someone like
Oerlemans would criticize anything that didn't liberally cite his
work (so there are now more Oerlemans refs).

Best wishes,
Tom.

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