Net Robert oceanography Sea and Gree

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From: bobg@Radix.Net (Robert Grumbine)
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Subject: Sea Level, Ice, and Greenhouses -- FAQ
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Summary: Survey of physical processes affecting sea level.

Archive-name: sea-level-faq
Version: Sea Level FAQ v 7.1 1997/10/23 19:35:00 bobg 

  The relation between sea level, ice, and greenhouses (or more
Much of the underpinning, is alas misunderstood or garbled.  This
FAQ tries to bring a little coherence to the discussion.  It does
not address the matter of detailed predictions or attempts at
n observing sea level change.  These are subjects for a different

  What this FAQ _does_ address are the primary physical mechanisms
those mechanisms which are either commonly mentioned, or which
addressing relatively timeless issues, the references may appear a 
bit dated.  This is not the case.  The basic processes are well known,
and haven't changed their spots in the years since I first issued the

  This FAQ is archived and is available on the web at 
are some of Jan Schloerer's articles, which I strongly recommend to all.

  This article is copyright protected.  Write me regarding further use
of the material.

Robert Grumbine

Revision: October 1997
  New introduction 
  Some wording changes in text

Revision: June 1994  
  Explicit copyright notice
  Minor wording changes
  Note of future modifications
  Please e-mail me corrections (with citation preferably) if you find an
error.  This FAQ does not contain everything relevant to the problem of
of investigation on sea level.  The basic principles are outlined, no
more.  This note has been cross-posted with the default followup set to

  This article is copyright 1993, 1994, 1997 by Robert W. Grumbine.  All

Begin, at last, the article:

  There are two ways of changing sea level on the human time scale.  We
can change the amount of water in the oceans, or we can make the water
there is occupy more or less volume.  The first corresponds to changing
the mass of ice on land.  The second can be done by warming or cooling 
the ocean.  Colder water is denser, so the same mass of water occupies 
less space.  In considering sea level changes, an important 
consideration is the rate at which they occur.  1 meter in 1 day is
quite disastrous.  1 meter in a million years would be irrelevant on
the human scale.

  Water has a small but nonzero expansion as it warms.  The expansion is
approximately 2E-4 per degree of warming, at the temperatures of the
upper ocean.  To convert that into a sea level change, we need to 
multiply by the amount of warming and the thickness of the ocean that
modelling.  Let's consider a warming of 1 K for simplicity.  The central
question for the oceanographers is then how deep a layer of the ocean

  This is a difficult question.  The challenge lies in the fact that 
the atmosphere heats the ocean at the top.  Obvious.  Not obvious is
that this impedes warming much of the ocean.  Warm water is less dense,
affected, about the top 100 meters.  There is mixing within the ocean,
effect is the fact that water from the deep ocean (which is cold)
thickness that gets warmed is approximately the same as that which is
already warm.  That is approximately 500 meters.  For the 1 degree
time scale over which this occurs is the length of time it takes to mix
the upper ocean, and is on the order of decades.

  In terms of the ice, there are five identifiable reservoirs, only one
of which is expected to be able to have catastrophic effects on sea
level.  They are sea ice, mountain glaciers, the Greenland ice sheet,
the East Antarctic ice sheet, and the West Antarctic ice sheet.  The one
expected to be potentially catastrophic is West Antarctica. 
Catastrophic is taken to mean meters of sea level in a few hundred years
or less.

  First, why can't the other four be catastrophic?  Sea ice cannot
change sea level much.  That is can do so at all is because sea ice is
not made of quite the same material as the ocean.  Sea ice is much
fresher than sea water (5 parts per thousand instead of about 35).  When
the ice melts (pretend for the moment that it does so instantly and
little higher than the local sea level.  The amount of extra height
corresponds to about 2% of the thickness of the original ice floe.  For
thickness of 2 meters (the Arctic ice is about 3 meters, the Antarctic
s about 1), the corresponding change in global sea level would be 2
(meters) * 0.02 (salinity effect) * 0.10 (fraction of ocean covered by
ce), or 4 mm.  Not a large figure, but not zero either.  My thanks to
chappell@stat.wisc.edu (Rick Chappell) for making me work this out.

  Mountain glaciers appear to have already made their contribution.  
Further collapse of them seems unlikely, and they are too small to be 
major elements in sea level change (even should they double their size).

  The three ice sheets can change sea level significantly, depending on
floating on the ocean.  They are grounded on land.  Sometimes, which 
ncome minus outgo.  The income is from snow fall -- accumulation. The
outgo (ablation) is primarily melting and the calving of icebergs.  

  It is believed that in a warmer climate, the amount of precipitation 
more than temperature.  The mechanism for the increase is that warmer
temperatures put more water into the atmosphere (inarguable) so that 

  But, Greenland is already quite snowy and quite warm.  A warming is 
likely to increase the melting far more rapidly than the accumulation. 
A small bit of graphics would help here.  Draw an arc that opens 
the way to the peak of the arc, draw a horizontal line through the 
accumulation through the year.  Below the line, there is net ablation 
through the year.  In a warming, the snow line moves upwards.  Three 
things happen then.  First, in the area that is melting increases.  
Second, the melting rate increases.  Third, the area of accumulation 
ncrease in the area that does have net accumulation.  But we have
Outgo definitely increases, and income probably decreases or at best 

  These mechanisms set up the possibility for an accelerating collapse 
of the ice sheet.  Namely, this excess ablation lowers the ice sheet in
that region.  Since the lower elevations are even warmer, the ablation
of the ice sheet and decreases the accumulation.  Together, the
accumulation is decreased and the ablation is increased.  This is the
elevation-ablation feedback.  It is believed to be operating in
Greenland already.  Under present climatic conditions, the Greenland
ce cap could not be regrown.  It is simply too warm there.  (Odd
thought for Greenland, I know, but glaciologists have unusual

  But, how fast would it melt away?  This is our major question for 
Greenland ice cap is surrounded by mountains.  These have the general
effect of damming up the ice sheet (which is part of the reason it
collapse would take on the order of several hundred years.  The sheet
under the rates of sea level rise experienced during the end of the
last ice age (around 20 mm/year), so is not ecologically unprecedented.
Such rises have occurred several times in the last 2 million years.

  What about East Antarctica?  The ice sheet there is extremely large, 
about 70 meters of sea level.  Get a map for a minute.  East Antarctica
s the part of Antarctica that lies between 15 W and 165 E as you move
clockwise.  It is the vast majority of the Antarctic ice and land mass. 
Antarctica is so cold already that a slight warming will not raise the
zone.  East Antarctica is also ringed by mountains, so that the ice
mechanism of mass loss is for ice to flow through passes in the
transantarctic mountains over to West Antarctica.

  Having little means to lose mass, East Antarctica would seem to be a 
but it runs into the problem that precipitation is also highly 
nefficient over the East Antarctic plateau (arguably the driest desert
n the world).  The best estimates place the rate of increased
accumulation over East Antarctica at right about the same as the 
ncreased ablation on Greenland.  That would be a wash for sea level.
Some redistribution of water from north to south, but no net effect.  

  West Antarctica is the joker in the deck.  Sea ice we can ignore (for
to balance each other's effects.  But West Antarctica represents 6
meters of sea level that _can_ collapse rapidly as glaciologists
measure things.

  The collapse mechanisms rely on the peculiar geometry of the West 
Antarctic ice sheet.  The first major feature of West Antarctica is 
that it includes two large ice _shelves_.  These are masses of ice 
approximately the size of France, approximately 500 meters thick.  They
float on the ocean, so cannot directly change sea level if they were
lost.  The peculiarity of having ice shelves is that ice shelves are
to advance all the way to the edge of the continental shelf, or to
collapse to include no ice shelf.

  Why should we worry about the presence or absence of the ice shelves?
They can't change sea level if they disappeared.  But the ice shelves
Weddell Sea) and the Ross Ice shelf (in the Ross Sea) act as buttresses
to the West Antarctic ice sheet.  Without these buttresses, the West
Antarctic ice sheet will collapse into the ocean on a time scale of

  The ice shelves contribute to ablation both through melting (at their
bases more than the surface) and through iceberg calving.  Some notably
large bergs have calved in the last few years, including a couple
larger than the state of Rhode Island.  So through either a warmer
ocean providing more ablation or through an increase in calving 
(arguably observed), the West Antarctic ice shelves could collapse.

  That West Antarctica can collapse much faster than Greenland relies 
on another oddity of the West Antarctic geometry.  Most of the ice 
mportant oddity is that as you move further inward, the land is
further below sea level.  So, consider a point near the grounding line 
(the point on land where the ice shelf meets the ice sheet).  Ice flows from the
mass loss in the ice shelf means that the shelf becomes thinner (and
lower) so more ice flows in from the ice sheet.  This makes the ice
floor.  Without that mass, what used to be ice sheet begins to float. 
Since the sea floor slopes down inland of the grounding line, the area
of ice sheet that turns into ice shelf increases rapidly.  More ice

  The collapse mechanism has a mirror-image advance mechanism.  Should 
there be net accumulation, the ice sheet/shelf can ground out to the 
continental shelf edge.  Go back to near the grounding point.  This 
time add some excess mass to the ice sheet/shelf.  This thickens the 
from the ocean ablation zone, which makes the mass balance even more in
favor of accumulation.  So the advance can also be a self- acclerating

  The big question in all this is whether accumulation will go up 
faster than ablation.  The problem is, we don't know how either of them
occurs in West Antarctica at present to satisfactory detail.  From
experience in other polar regions, we would expect the ice shelves and
central West Antarctica to have a fairly high accumulation rate.  They
are almost as dry as East Antarctica.  The ablation from the base of
the ice shelves relies on the mechanisms that get 'warm' water (the
n the melting) from the open ocean to the ice shelf base.  We don't
know enough about how the transfer occurs to be able to say confidently
climate.  Iceberg calving, the other major ablation source, is also not
terribly well understood.

  So, the proper answer to the question "Will sea level rise or fall in
a greenhouse world" is yes.  Warming the ocean will cause a sea level
n an unstable configuration.  It _will_ either advance or retreat.
Current glaciological opinion favors a collapse.  So far, observations
of the major ice sheets (East and West Antarctica, Greenland) are 
nconclusive as to whether the ice sheets are currently growing or
catastrophic sea level rise (which I've taken to be meters of sea level 
n under 500 years).

The players        Size (approx)   Speed (approx)
Sea Ice             0.4 cm         years
Mountain Glaciers  10's cm         decades
Thermal Expansion  20   cm per degree warming, per km of ocean warmed
West Antarctica   500   cm         a few centuries
Greenland         500   cm         several centuries
East Antarctica  7000   cm         several centuries to millenia

  My thanks to chappell@stat.wisc.edu (Rick Chappell), Ilana Stern,
Jan Schloerer, neilson%skat.usc.edu@usc.edu (D. Alex Neilson), Kyle
Swanson, and all others, whose comments (if not addresses) have 

Robert Grumbine

Further Reading:

Climate Change  -  The IPCC Scientific Assessment
Report Prepared for IPCC by Working Group I
Houghton, J.T.,  G.J. Jenkins,  J.J. Ephraums  (eds.)
Cambridge Univ. Press, Cambridge, UK 1990
A look at thermal expansion and sea level:
  Wigley, T. M. L. and S. C. B. Raper  Thermal expansion of sea water
associated with global warming.  Nature, 330, 127-131, 1987.

The Role of glaciers
  Oerlemans, J.  and  J.P.F. Fortuin,  Sensitivity of glaciers
and small ice caps to greenhouse warming,  
Science 258, 115-117 , 1992  
The mass balance of Antarctica:
  Jacobs, S. S..  Is the Antarctic Ice Sheet Growing?  Nature, 360,

Sea level during the last 17,000 years:
  Fairbanks, R. G.  A 17,000 year glacio-eustatic sea level record: 
nfluence of glacial melting rates on the Younger Dryas event and
Classic text on glaciology:
  Paterson, W. S. B. _The Physics of Glaciers_ 2nd ed, Pergamon Press, 
Oxford, New York, Toronto, Sydney, Paris, Frankfurt.  380 pp., 1981.

  Bromwich, D. H.  Snowfall in High Southern Latitudes  Reviews of
Geophysics, 26, pp. 149-168, 1988.  (This issue contains many 
Antarctic Science papers.)

"West Antarctic Ice Sheet Initiative Science and Implementation Plan" 
ed. by R. A. Bindschadler, NASA Conference Publication Preprint.  1991.  

Conference on the West Antarctic ice sheet, including an introduction 
to why West Antarctica is the focus:
  Van Der Veen, C. J. and J. Oerlemans, eds.  _Dynamics of the West 
Antarctic Ice Sheet_  D. Reidel, Dordrecht, Boston, Lancaster, Tokyo.  

Greenland in a Greenhouse world: (also general reference)
  Bindschadler, R. A.  Contribution of the Greenland Ice Cap to 
changing sea level: present and future.  IN: Glaciers, Ice Sheets, and 
Sea Level: Effect of a CO2-induced Climatic Change.  US Dept. of 
Energy Report DOE/EV/60235-1, pp. 258-266, 1985.

Antarctica in a Greenhouse:
  Oerlemans, J.  Response of the Antarctic Ice Sheet to a climatic 

  Weertman, J.  Stability of the junction of an ice sheet and an ice 

An example of the elevation-ablation feedback, triggered by geology.
  Birchfield, G. E. and R. W. Grumbine "'Slow Physics of Large 
Continental Ice Sheets and Underlying Bedrock and Its Relation to the 
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