date: Thu May 3 17:14:20 2001
from: Mike Hulme <m.hulmeatXYZxyz.ac.uk>
subject: Re: Sea level question
to: "Simon Torok" <S.TorokatXYZxyz.ac.uk>
At 19:31 02/05/01 +0100, you wrote:
I have made changes to the Sea Livel article based on David's suggestions. A
question that I haven't been able to find an answer to in the IPCC WGI
summary is the following: if sea level has not been accelerating through the
20th century, what is the basis for concluding that rising sea levels are
very likely to have been caused by global warming? My guess would be that
the evidence is the link between warming and expansion, and that it is the
inertia of the oceans that means that the expansion has not led to rises as
dramatic as the warming in the latter part of the 20th century. But if this
is the case, what was causing the rise in sea levels in the 19th century?
The 3 parts of the article in question are:
Rises have been at an average rate of 1 to 2mm per year, and there is no
evidence that this rate has accelerated during the 20th century [this seems
to imply that sea-levels are not increasing as a result of global warming;
is that what you intend?].
But SL in 20C has risen more than in 19C. There is a likely background rate of dSL due to
isostatic adjustment. This is less than the 20C rate. There is no acceleration during the
20C since the global T increase to date has been patchy, except for the last 30 years and
this extra heat will not have penetrated far into the oceans yet.
The latest IPCC report by Working Group I concluded, "it is very likely that
the 20th century warming has contributed significantly to the observed
sea-level rise". [but what was causing sea-level rise previously? what is
the scientific basis on which they claim this 'likelihood'].
Isostatic adjustment to last glaciation. Scientific basis is modelling and observation of
worldwide glacier melt and also modelling of thermal expansion process in oceans.
Over the last 3000 years, global sea-level rose 0.1 to 0.2mm a year- about a
tenth the rate of rise in the last 100 years [this suggests that the rate of
sea-level rise is accelerating -- does that contradict what you said a
couple of paragraphs previously?].
My understanding would suggest that if the background rate has been constant over last few
100 years, then the human-induced additional rise would only kick in during 20C - there is
a large lag effect between warming and dSL - and since warming has been patchy during 20C
(mid-century plateau) then there will not yet have been an exponential curve of dSL - which
is what would suggest acceleration.
Do you have any suggestions on how to phrase these to answer David's
Thanks for any suggestions, Simon.
If you have time, you could put the revised text to Sarah Raper in CRU. She knows quite a
bit about this and she is employed 10% time by Tyndall Centre so you could ask her to
comment in that capacity for Tyndall Centre.
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