Will polar ice melt more quickly than previously thought?
Hugh Easton
susceptible to melting than previously supposed. The Larsen ice shelf now
appears to be in the process of breaking up: a huge iceberg has broken off
it, but more importantly a large part of the ice shelf has disintegrated
altogether, leaving open water between James Ross island and the Antarctic
peninsula for the first time since Antarctic exploration began. Also there
have been reports of a 40-mile long crack opening up in the Larsen ice
shelf, apparently after the iceberg broke off.
Temperatures in the Antarctic peninsula (where these events and the earlier
breakup of the Wordie ice shelf took place) have risen by 2.5 C over the
last 40 years. Although some melting of polar ice has been anticipated as
a consequence of global warming, the speed and scale of recent events in
Antarctica appear to have taken the scientific community completely by
surprise.
Ice is not a very good conductor of heat: the thermal conductivity at
273.15 K (0 C) is 2.26 W/(m.K), but it improves slightly at lower
temperatures - at 260 K it is 2.35, and at 240 K it is 2.50 W/(m.K) [from
table 33 "Thermophysical properties of saturated water substance", Handbook
of heat transfer fundamentals 2nd ed., McGraw-Hill 1985]. Putting a
physical perspective to these figures, this is the amount of heat in watts
per square metre that would travel through a 1m thick sheet of ice with a
temperature difference of 1 C between the top and bottom.
Now apply these figures to an Antarctic ice shelf, such as the Larsen ice
shelf. A typical thickness of ice for these is about 200m, and we have
heard that 2.5 C of warming has taken place. Assuming that it has
penetrated the top 100m of ice already, how long would it take to warm the
remainder?
The 100m of ice which has not yet warmed has a mass of about 10e8
g/m^2, to warm this by 2.5 C requires a heat input of 2.5 x 10e8 J/m^2
(assuming a thermal capacity for ice of 1 J/(g.K) - the actual figure
may well be higher, in which case the heat input required will
increase correspondingly).
The heat flux through 100m of ice is (2.5W/(m.K) / 100m) x 2.5K
temperature difference, or 6.25 x 10e-2 W/m^2. This will take 4 x 10e9
seconds to provide the 2.5 x 10e8 J/m^2 required, or a little over a
century.
Before the ice shelf actually broke up, substantial melting would have to
take place. The latent heat of fusion for the water-ice transition is over
300 J/g. With the heat flux just calculated, it would take over three
thousand years for the bottom half of the ice shelf to melt. No wonder
glaciologists are not too concerned about the prospect of the polar icecaps
melting in the near future!
So why is the Larsen ice shelf breaking up? I think it is because of a
physical property of ice that has hitherto been overlooked - transparency.
Water is extremely opaque to infrared radiation of any wavelength, but the
same is not necessarily true of ice. The molecules in water can vibrate or
rotate freely, while those in ice are bound into a rigid crystal lattice
and so cannot move. The various different vibrational and rotational modes
of the water molecule occur at the right frequencies to make it a good
absorber of infrared; when they are suppressed, infrared absorbtion no
longer occurs.
Imagine for a moment that ice is completely transparent to infrared. We saw
earlier how, using regular heat conduction, it would take a century or more
for heat to get through a 100m thickness of ice. Using the same example
given earlier but including the effects of infrared transmission:
for a 2.5K temperature difference at 260K the net heat transmission
is s.T2^4 - s.T1^4 = 269.2 - 259.1 or about 10 W/m^2
(where s is the Boltzmann constant, 5.67 x 10e-8 W/(m^2.T^4), T1 is
260K, T2 is 262.5K).
This is nearly 200 times more than the heat flux calculated earlier.
Instead of taking 3000 years to melt, the ice shelf is gone in little more
than a decade.
With the limited time and resources at my disposal, I have so far been
unable to find any research which conclusively proves or disproves whether
ice is sufficiently transparent in the relevant parts of the infrared
spectrum (from about 4um to 120um) for infrared heat transmission to work.
However, I have found some supporting evidence in the book "Radiative Heat
Transfer" (Michael F. Modest, McGraw-Hill 1993 ISBN 0-07-042675-9). In
chapter 11, "Radiative properties of semitransparent media", there is a
graph (Fig 11-4, p442) "Spectral absorbtion coefficient of clear water (at
room temperature) and clear ice (at -10 C)". It shows that the absorbtion
spectra of water and ice are almost identical for ultraviolet, visible and
near infrared. However, for infrared wavelengths beyond 10um the spectral
properties of water and ice abruptly diverge - the ice becomes
significantly more transparent than water, so that for a wavelength of 20
um (this is end of the range the graph covers) ice is over 10 times more
transparent than water. Unfortunately, this does not indicate what happens
at longer wavelengths, or when the temperature is lowered or the pressure
increased.
It is also possible that other mechanisms than infrared heat transmission
could give rise to anomalously high rates of long-range heat transfer
through ice - for instance phonons, individual quanta of sound energy, can
probably travel quite long distances through ice and could perhaps
transport significant quantities of heat - they do in liquid helium.
Given the evidence that "bulk" ice such as ice shelves and glaciers appears
to be melting at a faster rate than simple theory would suggest, and given
the catastrophic consequences if the polar ice caps should melt, I think
that the possibility that unconventional modes of long-range heat
conduction operate in ice deserves thorough investigation. The experiments
necessary to conclusively settle this issue one way or the other should not
be that difficult or expensive to conduct. Is anyone reading this message
prepared to carry them out?
--
Hugh Easton <Hu...@daflight.demon.co.uk>
Simen Gaure
It is also possible that other mechanisms than infrared heat transmission
could give rise to anomalously high rates of long-range heat transfer
through ice - for instance phonons, individual quanta of sound energy, can
probably travel quite long distances through ice and could perhaps
transport significant quantities of heat - they do in liquid helium.
Is there any evidence that the ice has actually melted?
It has broken up, yes, but is that due to large scale melting?
It might also be the case that there's now more ice in
Antarctica (e.g. due to increased precipitation).
I'd guess (although I haven't made any computations) that
even a minor increase in ice thickness (say 1 metre) will
result in a lot of ice floating outwards towards the sea.
--
Simen Gaure, Department of Mathematics, University of Oslo
Ivan D. Reid
Simen...@math.uio.no (Simen Gaure) writes...
> It is also possible that other mechanisms than infrared heat transmission
> could give rise to anomalously high rates of long-range heat transfer
> through ice - for instance phonons, individual quanta of sound energy, can
> probably travel quite long distances through ice and could perhaps
> transport significant quantities of heat - they do in liquid helium.
>Is there any evidence that the ice has actually melted?
>It has broken up, yes, but is that due to large scale melting?
>It might also be the case that there's now more ice in
>Antarctica (e.g. due to increased precipitation).
>I'd guess (although I haven't made any computations) that
>even a minor increase in ice thickness (say 1 metre) will
>result in a lot of ice floating outwards towards the sea.
A good point. One must remember that the ice-sheets are dynamic,
even though they move at glacial speed. (Sorry, pun intended...) One case
I have personal knowledge of was at Mawson in 1980. The seismo reckoned
from his traces that there'd been an ice-fall several km along the coast, so
he and I went to look for it. The fall was, from memory, about 200 m across
and a similar distance long. It appeared that it had been projecting out as
an ice sheet into the sea and via a combination of undercutting and flow had
reached the point where its weight was no longer supported by the sea. It
cracked across, and fell down into the sea, hitting the rock bottom and
sliding out maybe 50 m. The destruction was tremendous, an outer region
several hundred metres across where seawater had been thrown out across the
sea-ice, then a few hundred metres of cracked ice, then a hundred metres or
so where the sea-ice (1 m thick) was cracked into small chunks and tumbled
together in confusion. An impressive amount of energy had been liberated!
Oh, the height of the ice-wall left behind was of the order of 20 m above sea
level. (All figures are from memory, so I may be a bit out here and there).
Ivan Reid, Paul Scherrer Institute, CH. iv...@psi.ch
Ben Sharvy
>In article <Simen.Gaure-19...@ma-mac17.uio.no>,
> Simen...@math.uio.no (Simen Gaure) writes...
>>Is there any evidence that the ice has actually melted?
>>It has broken up, yes, but is that due to large scale melting?
>>It might also be the case that there's now more ice in
>>Antarctica (e.g. due to increased precipitation).
>>I'd guess (although I haven't made any computations) that
>>even a minor increase in ice thickness (say 1 metre) will
>>result in a lot of ice floating outwards towards the sea.
Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
ice masses are retreating. 2+2=4.
--
Live Globally, Die Locally. Witches Heal. So Do Blowjobs. Liberate the
Weirdos and You Liberate the Squares.
Simen Gaure
In <21APR199...@erich.triumf.ca> iv...@erich.triumf.ca (Ivan D.
Reid) writes:
>In article <Simen.Gaure-19...@ma-mac17.uio.no>,
> Simen...@math.uio.no (Simen Gaure) writes...
>>Is there any evidence that the ice has actually melted?
>>It has broken up, yes, but is that due to large scale melting?
>>It might also be the case that there's now more ice in
>>Antarctica (e.g. due to increased precipitation).
>>I'd guess (although I haven't made any computations) that
>>even a minor increase in ice thickness (say 1 metre) will
>>result in a lot of ice floating outwards towards the sea.
Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
ice masses are retreating. 2+2=4.
Glaciers in Norway are advancing, rapidly. To the extent that
nearby houses are threatened. So are glaciers in
New Zealand. Rising sea level isn't evidence for large scale
melting of Antarctic ice, it may also be due to expansion
(due to global warming). 2+2=5.
My point was simply that one should have some evidence that ice
in Antarctica is actually melting, not just assume that it is.
After all, it's well below freezing most of the time in Antarctica,
ice simply doesn't melt then. (Under normal pressure).
Len Evens
>In <21APR199...@erich.triumf.ca> iv...@erich.triumf.ca (Ivan D. Reid) writes:
>
>>In article <Simen.Gaure-19...@ma-mac17.uio.no>,
>> Simen...@math.uio.no (Simen Gaure) writes...
>
>>>Is there any evidence that the ice has actually melted?
>>>It has broken up, yes, but is that due to large scale melting?
>>>It might also be the case that there's now more ice in
>>>Antarctica (e.g. due to increased precipitation).
>>>I'd guess (although I haven't made any computations) that
>>>even a minor increase in ice thickness (say 1 metre) will
>>>result in a lot of ice floating outwards towards the sea.
>
>Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
>ice masses are retreating. 2+2=4.
>
It is a bit more complicated than that. Warming produces sea level
rise just by thermal expansion. See my recent previous posting.
Leonard Evens l...@math.nwu.edu 708-491-5537
Dept. of Mathematics, Northwestern Univ., Evanston, IL 60208
Len Evens
Simen Gaure <Simen...@math.uio.no> wrote:
>In article <3n9iin$n...@efn.org>, bsh...@efn.org (Ben Sharvy) wrote:
>
> In <21APR199...@erich.triumf.ca> iv...@erich.triumf.ca (Ivan D.
>Reid) writes:
>
> >In article <Simen.Gaure-19...@ma-mac17.uio.no>,
> > Simen...@math.uio.no (Simen Gaure) writes...
>
> >>Is there any evidence that the ice has actually melted?
> >>It has broken up, yes, but is that due to large scale melting?
> >>It might also be the case that there's now more ice in
> >>Antarctica (e.g. due to increased precipitation).
> >>I'd guess (although I haven't made any computations) that
> >>even a minor increase in ice thickness (say 1 metre) will
> >>result in a lot of ice floating outwards towards the sea.
>
> Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
> ice masses are retreating. 2+2=4.
>
>nearby houses are threatened. So are glaciers in
>New Zealand. Rising sea level isn't evidence for large scale
>melting of Antarctic ice, it may also be due to expansion
>(due to global warming). 2+2=5.
>
>My point was simply that one should have some evidence that ice
>in Antarctica is actually melting, not just assume that it is.
>After all, it's well below freezing most of the time in Antarctica,
>ice simply doesn't melt then. (Under normal pressure).
>
>--
>Simen Gaure, Department of Mathematics, University of Oslo
This is basically correct. In fact, warming, through increased
precipitation could result in an increase in the Antarctic ice sheet.
The (1990) IPC Report in fact concludes that changes in the Antarctic
ice sheet in the past century have probably resulted in a drop in
sea level, but this conclusion is far from certain.
With respect to mountain glaciers, however, anecdotal evidence from
Norway or New Zealand doesn't tell us anything. Again, if I
understand the IPCC Report, it is generally believed that overall
mountain glaciers have been retreating in the past 100 years.
In any case, as Simon Gaure points out, all this ignores thermal
expansion which is probably the major factor in sea level rise.
Ben Smith
This looks like too much fun to pass up!!!!!
Not that I am right, but please ponder on a few ideas...
First, As others have pointed out themal expansioin of an incredibly
large body of water (oceans) can have an effective rise of a whopping 1
mm /year, at that rate I better move away from the coastline. Have you
considered the effects of erosion? Or the fact that the plates
(tectonics) float on the mantle! Well this could mean that any
particular point is just a reference, and that reference is subject to
sinking and rising. Many geologist beleive that the land under the great
ice sheets that now are uncovered will rise just like an added weight on
a floating boat. Okay, well this causes a problem when someone puts a
stick in the ground and records sea rise over time, ranted the continent
is not going to rise a few feet per year byt this mechanism, but the
mechanism must be considered.
Next, global warming is an odd term. Lets say the globe warms. The
tropical zones on earth will increase (one theory) the semi arid regions
will decrease and the frigid areas will grow. The earth is a big dynamic
system, constantly shifting energy from here to there. The last ice age,
for example is beleived to have been composed of large tropical and artic
regions, with very little semi arid regions. How can this happen, well
the increase in heat, evaporates more water that moves to the equator and
the poles via trade winds. Then the evaporated water precipitates as
rain or snow and ice, depending on the region, vegetation grows or
glaciers grow. A glacier can exist anywhere the rate of deposition of
snow/ice excedes the melting of snow/ice.
Also, something else puzzles me, I heard some talk on the tube that the
sun dishes out enough energy to power (something, wasn;t paying enough
attention). Well, that sunlight, a few hundred years ago, was used to as
fuel for things to grow. Now it is used to heat things up (skysrapers,
highways, parking lots, etc). How would this change the scenario of
global warming and the dreaded green house gases! Anyway this was just
for fun,
the world is a little too complicated for 2+2 = 4!!!!!
Ben Smith smi...@weiss.che.utexas.edu
(The above statments may not be my employer's!)
Hugh Easton
Simen...@math.uio.no "Simen Gaure" writes:
>
> In article <3n9iin$n...@efn.org>, bsh...@efn.org (Ben Sharvy) wrote:
>
> In <21APR199...@erich.triumf.ca> iv...@erich.triumf.ca (Ivan D.
> Reid) writes:
>
> >In article <Simen.Gaure-19...@ma-mac17.uio.no>,
> > Simen...@math.uio.no (Simen Gaure) writes...
>
> >>Is there any evidence that the ice has actually melted?
> >>It has broken up, yes, but is that due to large scale melting?
> >>It might also be the case that there's now more ice in
> >>Antarctica (e.g. due to increased precipitation).
> >>I'd guess (although I haven't made any computations) that
> >>even a minor increase in ice thickness (say 1 metre) will
> >>result in a lot of ice floating outwards towards the sea.
>
> Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
> ice masses are retreating. 2+2=4.
>
> Glaciers in Norway are advancing, rapidly. To the extent that
> nearby houses are threatened. So are glaciers in
> New Zealand.
Bruce Hamilton mentioned this some weeks back in another thread on
sci.environment. From the responses to his posting it emerged that this
is actually an indication that the glacier is melting - it results from
increased lubrication from meltwater underneath the glacier, and loss of
mechanical strength in the ice making up the glacier.
> Rising sea level isn't evidence for large scale
> melting of Antarctic ice, it may also be due to expansion
> (due to global warming). 2+2=5.
>
While we're on the subject, that rate of 1.2 mm/yr for sea level rise
that Ivan Reid mentioned is substantially less than the observed rate
over the last two years. Quoting from NASA press release 94-202:
The insights from TOPEX/Poseidon add to data collected from
tide gauges over the last century which suggest that average sea level
has been rising at a rate of about .04 to .08 inches (1 to 2
millimeters) per year, roughly equivalent to the rate expected from
global warming, Nerem said. "The data (from Dec. 1992 to Sept. 1994)
show a rise in average sea level of about .12 inches (3 millimeters)
per year, which is in reasonable agreement with the tide gauge
results," Nerem explained. However, tide gauge measurements can
be affected by the movement of land masses and are too sparsely
distributed to provide global coverage.
The tidal gauge data shows a 1-2mm/yr sea level rise over the last century,
and the sea level rise over the last two years was 3mm/yr. Seems to
indicate that the rate of sea level rise has increased substantially in
the last few years, which is what you would expect if polar ice was now
beginning to melt.
> My point was simply that one should have some evidence that ice
> in Antarctica is actually melting, not just assume that it is.
> After all, it's well below freezing most of the time in Antarctica,
> ice simply doesn't melt then. (Under normal pressure).
>
It does start to lose its strength well below freezing point though. Also,
the ice near the bottom of a glacier or an ice sheet is under considerable
pressure, which lowers its freezing point.
> --
> Simen Gaure, Department of Mathematics, University of Oslo
>
--
Hugh Easton <Hu...@daflight.demon.co.uk>
Ivan D. Reid
hu...@daflight.demon.co.uk writes...
>> In article <3n9iin$n...@efn.org>, bsh...@efn.org (Ben Sharvy) wrote:
>> In <21APR199...@erich.triumf.ca> iv...@erich.triumf.ca (Ivan D.
>> Reid) writes:
>> Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
>> ice masses are retreating. 2+2=4.
>While we're on the subject, that rate of 1.2 mm/yr for sea level rise
>that Ivan Reid mentioned is substantially less than the observed rate
>over the last two years.
I'd just like to point out that it was bsh...@efn.org (Ben Sharvy)
who wrote that -- although he included an attribution to me, he completely
snipped all of my posting to reply to the post I had quoted. I wouldn't
want anyone to get the idea I'm an expert on global climate!
Simen Gaure
Evens) wrote:
With respect to mountain glaciers, however, anecdotal evidence from
Norway or New Zealand doesn't tell us anything. Again, if I
understand the IPCC Report, it is generally believed that overall
mountain glaciers have been retreating in the past 100 years.
Certainly, local evidence tells very little about the global
situation, I posted that in response to the general claim that
"glaciers are retreating". (And it's certainly not "anecdotal").
My main concern is that there's some evidence of
a global warming going on, we then need precise information
to create and adjust our models, we don't need statements
generalized from John Doe's personal experience with his freezer
during the power fallout of '67.
When people make statements like "a consequence of global warming
is that glaciers retreat", then, the sheep farmer watching
his nearby glacier demolishing the stone fence his great grandfather
built, will conclude that there's certainly not any global warming
going on. This becomes critical when politicians jump to the
same conclusion. (They're not always very eager to listen to
proper scientists, sadly enough.)
Simen Gaure
Bruce Hamilton mentioned this some weeks back in another thread on
sci.environment. From the responses to his posting it emerged that this
is actually an indication that the glacier is melting - it results from
increased lubrication from meltwater underneath the glacier, and loss of
mechanical strength in the ice making up the glacier.
Now, wait a second. Now both retreat and advance of a glacier have
become evidence for melting. This doesn't immediately add up.
However, we may make it add up.
The facts are that there have been massive snowfalls during
the last decade. This has caused two things:
1 The glaciers have become heavier, causing an increase in the pressure.
This again causes two phenomena.
1a Some ice at the bottom melts, causing increased lubrication.
1b The ice, even if it doesn't melt, moves downwards due to
its "liquid" nature.
2 The new snow on top of the glacier hasn't turned into hard ice yet;
hence it's more likely to melt. This may also cause
increased lubrication.
In this way it's correct to say that there's melting going on, but
the glacier still has grown larger. However, the statement
"the glacier melts" is quite misleading.
Of course, the increased snowfall may very well be evidence for
global warming, but I don't think one should insist that the
"glacier melts" just to convince laymen that global warming
is taking place.
[...]
> My point was simply that one should have some evidence that ice
> in Antarctica is actually melting, not just assume that it is.
> After all, it's well below freezing most of the time in Antarctica,
> ice simply doesn't melt then. (Under normal pressure).
>
It does start to lose its strength well below freezing point though. Also,
the ice near the bottom of a glacier or an ice sheet is under considerable
pressure, which lowers its freezing point.
It does, but at that depth (assuming now land-ice) there's no
way any climate change can have reached yet. However, the pressure
may very well have increased due to increased precipitation.
(Anyway, there seems to be a complete lack of facts about the
precipitation in Antarctica in this group; so it's hard to speculate
any further.)
Len Evens
>Simen Gaure, Department of Mathematics, University of Oslo
I don't disagree with anything you say. Let me say again what I
meant to say. According the the IPCC Report (1990) and Supplement
(1992), the prepoderant evidence indicates that, _on_the_average_,
mountain glaciers have been retreating. Indeed, this is taken as
one argument that some global warming has taken place in the
last century. Of course, that doesn't mean that your favorite sheep
farmer's glacier is retreating.
The latest IPCC report (1994) has just appeared, and I have ordered
my copy. I llok forward to what they say now two years later on this
and other matters.
Len Evens
Hugh Easton <hu...@daflight.demon.co.uk> wrote:
>
>While we're on the subject, that rate of 1.2 mm/yr for sea level rise
>that Ivan Reid mentioned is substantially less than the observed rate
>
> The insights from TOPEX/Poseidon add to data collected from
> tide gauges over the last century which suggest that average sea level
> has been rising at a rate of about .04 to .08 inches (1 to 2
> millimeters) per year, roughly equivalent to the rate expected from
> global warming, Nerem said. "The data (from Dec. 1992 to Sept. 1994)
> show a rise in average sea level of about .12 inches (3 millimeters)
> per year, which is in reasonable agreement with the tide gauge
> results," Nerem explained. However, tide gauge measurements can
> be affected by the movement of land masses and are too sparsely
> distributed to provide global coverage.
>
>The tidal gauge data shows a 1-2mm/yr sea level rise over the last century,
>and the sea level rise over the last two years was 3mm/yr. Seems to
>indicate that the rate of sea level rise has increased substantially in
>the last few years, which is what you would expect if polar ice was now
>beginning to melt.
>
My memory of the satellite data was that it showed an increase of about
2 mm/year, but I could have got that wrong. In any case, one should
be a bit careful about drawing conclusions from just two or three
years of data. More to the point, since greenhouse gas concentrations
have been growing exponentially, it would not be inconsistent to have
a 1-2mm/year increase over the past 100 years and a 2-3 mm/year increase
at present. You don't have to assume the Greenland and Antarctic
ice caps are melting. (The melting of the north polar ice cap should
not lead to sea level rise, as has been pointed out several times in
this thread.)
Steinn Sigurdsson
In article <Simen.Gaure-22...@ma-mac17.uio.no>
Simen...@math.uio.no "Simen Gaure" writes:
> In article <3n9iin$n...@efn.org>, bsh...@efn.org (Ben Sharvy) wrote:
> Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
> ice masses are retreating. 2+2=4.
> Glaciers in Norway are advancing, rapidly. To the extent that
> nearby houses are threatened. So are glaciers in
> New Zealand.
Bruce Hamilton mentioned this some weeks back in another thread on
sci.environment. From the responses to his posting it emerged that this
is actually an indication that the glacier is melting - it results from
increased lubrication from meltwater underneath the glacier, and loss of
mechanical strength in the ice making up the glacier.
It really is quite wonderful. Clearly if glaciers retreat,
then the glaciers are melting and on average the region
is warmer. Now, if glaciers are advancing, this also evidence
they are melting! What one wonders, do glaciers do when they
are growing?
For what it is worth, certain classes of glaciers, "skridjoklar",
can advance when melting, for the reason noted above, they can
also advance when growing. In general, "icecap" glaciers shrink
when melting and grow when cooling. What counts is the
volume of ice, and that has increased in recent years in some
regions.
Dennis Schulze
>My memory of the satellite data was that it showed an increase of about
>2 mm/year, but I could have got that wrong.
I am just curious... how satellites which orbit the Earth in 800-36000 km
height can measure a length of a few millimeters. Are these global data?
Have been taken into account that the wind force may press the water mass
to one edge of the ocean and that this effect can result in a change of
a magnitude (sorry, but that's what I learned at university, any corrections
are welcome) higher than the mentioned 2 mm!?
One should be careful to draw quick conclusions from these data.
Regards,
Dennis
--
Dennis Schulze Free University of Berlin
Fax/Voice: (+49 30) 793 49 51 Department of Meteorology
Email: den...@bibo.met.fu-berlin.de
http://www.met.fu-berlin.de/~dennis/ #include <standard.disclaimer>
Bruce Hamilton
>In article <...> Simen...@math.uio.no "Simen Gaure" writes:
>> In article <...>, bsh...@efn.org (Ben Sharvy) wrote:
>> <> iv...@erich.triumf.ca (Ivan D. Reid) writes:
>> >In article <Simen.Gaure-19...@ma-mac17.uio.no>,
>> > Simen...@math.uio.no (Simen Gaure) writes...
>> >>Is there any evidence that the ice has actually melted?
>> >>It has broken up, yes, but is that due to large scale melting?
>> >>It might also be the case that there's now more ice in
>> >>Antarctica (e.g. due to increased precipitation).
....
> Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
>> ice masses are retreating. 2+2=4.
>> Glaciers in Norway are advancing, rapidly. To the extent that
>> nearby houses are threatened. So are glaciers in
>> New Zealand.
>Bruce Hamilton mentioned this some weeks back in another thread on
>sci.environment. From the responses to his posting it emerged that this
>is actually an indication that the glacier is melting - it results from
>increased lubrication from meltwater underneath the glacier, and loss of
>mechanical strength in the ice making up the glacier.
As I noted at the time, this is silly. Anything "glaciers"
do is claimed by some to indicate global warming.
Some proponents of global warming want to claim the retreat of
glaciers _is_ evidence of global warming - without bothering to
qualify which glaciers or why. Now some others, or perhaps
even the same proponents :-), also want to claim the advance of
glaciers is also evidence of global warning, once again without
qualification.
It would seem that there are many factors at work in a large
glacier, including water lubrication, pressure and snow burden,
and perhaps there are several types of glacier. The responses
to my post ( that I saw ) postulated possible mechanisms for
the advance, they were not verified, definitive explanations.
There are 360 glaciers in the NZ Southern Alps, and two of the
largest are advancing, I don't know about the others.
I haven't been looking at any literature or reports, but it would seem
to me that the use of "glacier advance or retreat" as "evidence" of
anything to do with global warming is extremely suspect.
The claims need to be qualified.
[ rest deleted - sea level increase due to thermal expansion,
mantle land mass movement, Antartic water sliding into ocean,
more water from combustion of fossil CO2, or increased human
peeing, is being debated by others more qualified than I. ]
Bruce Hamilton
John Hellstrom
in recent winters, not colder temperatures. The increased precipitation
is said to be caused by the extended El Nino phase of the Southern
Oscillation during the 1990's (how or if El Nino is related to global
warming is anyone's guess).
The glaciers, BTW, are extremely impressive at the moment, moving up to
1m/day, pushing boulders, trees etc before them (even at 1m/day, they are
several years from regaining their positions of late last century).
John Hellstrom.
Robert Grumbine
Dennis Schulze <den...@hamrun.met.fu-berlin.de> wrote:
>l...@schur.math.nwu.edu (Len Evens) writes:
>
>>My memory of the satellite data was that it showed an increase of about
>>2 mm/year, but I could have got that wrong.
>
>I am just curious... how satellites which orbit the Earth in 800-36000 km
>height can measure a length of a few millimeters. Are these global data?
>Have been taken into account that the wind force may press the water mass
>to one edge of the ocean and that this effect can result in a change of
>a magnitude (sorry, but that's what I learned at university, any corrections
>are welcome) higher than the mentioned 2 mm!?
>
>One should be careful to draw quick conclusions from these data.
>
sea level had risen globally at a rate of about 3 mm per year for the
two years. It was noted that this is consistent with global warming
scenarios, but that many other things also cause such changes (including
having a couple ordinary warm years).
The satellite orbits near the low end of the range given. More like
700 km, I think. It is a radar based satellite, measuring return time
for an emitted pulse. Features which get measured include:
Waves
Tides
Ocean Current Systems
Sea Level
The gravity field of the earth
All of these give characteristic signatures to the returns. The
tides, for instance, have a well-known structure and extremely accurately
known periods. Ocean current systems have predictable length scales and
strengths. The gravity field has different length scales and is nearly
constant. Sea level (_global_) shows up as a uniform increase in elevation
at global scale. It is because of the differences in space and time
scales that the various signals can be disentangled.
Keep in mind, too, that in estimating global sea level, the measurement
is one of combining thousands of observations (the footprint of the beam,
I think, is about 50 km) to estimate the mean (on a daily basis). Then
you further reduce those daily measurements to a handful of long term
averages.
This is not to say that the problem is easy, or that it has necessarily
been solved in the best possible way. Just an outline of some things
which are considered. What I've seen of the Topex/Poseidon work has
been very good, so I'm inclined to accept their figure as accurate (to
the limit they gave, I think it was +- 1 mm/yr).
Followups directed to sci.geo.meteorology. I apologize to the spectroscopy
readers, but this should be the last note you see.
--
Bob Grumbine rm...@access.digex.net
Sagredo (Galileo Galilei) "You present these recondite matters with too much
evidence and ease; this great facility makes them less appreciated than they
would be had they been presented in a more abstruse manner." Two New Sciences
Richard I Cullather
Richard I Cullather <rcul...@magnus.acs.ohio-state.edu> wrote:
>As I understand this,
>
I defer to Dr. Grumbine.
~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~
Richard Cullather
Polar Meteorology Group
Byrd Polar Research Center
Richard I Cullather
Dennis Schulze <den...@hamrun.met.fu-berlin.de> wrote:
>l...@schur.math.nwu.edu (Len Evens) writes:
>
>>My memory of the satellite data was that it showed an increase of about
>>2 mm/year, but I could have got that wrong.
>
>I am just curious... how satellites which orbit the Earth in 800-36000 km
>height can measure a length of a few millimeters. Are these global data?
>Have been taken into account that the wind force may press the water mass
>to one edge of the ocean and that this effect can result in a change of
>a magnitude (sorry, but that's what I learned at university, any corrections
>are welcome) higher than the mentioned 2 mm!?
>
As I understand this, the accuracy of an individual point measurement _can_
be made to within a few centimeters, which is achieved by knowing
the satellite's orbit to a high degree of accuracy using laser ranging.
The measurements are absolute with respect to the Earth's geocenter, so
the corrections you refer to are not necessary.
The interannual change in the global sea level is inferred at a higher
resolution "...given the observed variations and the known error sources."
The TOPEX-POSEIDON figure of three mm/yr over the first two years of
observations was widely quoted from R.S. Nerem (GSFC) at the AGU fall meeting.
The figure does not appear in the abstract, however. Would anyone know of
any interannual global sea level change value published from TOPEX data,
this study or otherwise?
For what it's worth, V. Gornitz has an excellent review of estimates based on
tide gauge data (in Climate and Sea Level Change, RA Warrick et al.,
Eds, 1993), most of which are in the 1 to 2 mm/yr range.
~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~
Richard Cullather
Polar Meteorology Group
Byrd Polar Research Center
Grant W. Petty
Len Evens <l...@schur.math.nwu.edu> wrote:
>
>My memory of the satellite data was that it showed an increase of about
>be a bit careful about drawing conclusions from just two or three
>years of data. More to the point, since greenhouse gas concentrations
While I agree completely on the need to be cautious about looking for
trends in short time series, it seems to me that, unlike (say) direct
measurements of atmospheric temperature, an observation of sea level
rise must already represent an integrated (i.e., smoothed) response to
something happening over a much longer time period.
In short, I (a non-specialist on this topic) would personally tend to
view the observed sea level rise over 2-3 years as stronger evidence
for a disturbing long-term trend than anything in the available record
of observed global tropospheric temperature, despite the much greater
length of the latter.
Has anyone yet been able to determine whether the observed sea level
rise is associated with (1) a large temperature increase in the
near-surface layers of the ocean or (2) a smaller increase over a much
greater depth?
- Grant
--
Grant W. Petty gpe...@rain.atms.purdue.edu
Asst. Prof. of Atmospheric Science
Dept. of Earth & Atmospheric Sciences (317) 494-2544
Purdue University, West Lafayette IN 47907-1397 FAX:(317) 496-1210
Robert Grumbine
Grant W. Petty <gpe...@rain.atms.purdue.edu> wrote:
>In article <3nekre$r...@news.acns.nwu.edu>,
>Len Evens <l...@schur.math.nwu.edu> wrote:
>>
>>My memory of the satellite data was that it showed an increase of about
>>2 mm/year, but I could have got that wrong. In any case, one should
>>be a bit careful about drawing conclusions from just two or three
>>years of data. More to the point, since greenhouse gas concentrations
>
>In short, I (a non-specialist on this topic) would personally tend to
>view the observed sea level rise over 2-3 years as stronger evidence
>for a disturbing long-term trend than anything in the available record
>of observed global tropospheric temperature, despite the much greater
>length of the latter.
I'd agree that the ocean is a better integrator than the atmosphere.
But, seat of the pants, I don't think it is enough better to justify
preferring a 2-3 year ocean record to a 100 year atmospheric record.
Certainly better than 2-3 years of atmospheric information.
>Has anyone yet been able to determine whether the observed sea level
>rise is associated with (1) a large temperature increase in the
>near-surface layers of the ocean or (2) a smaller increase over a much
>greater depth?
Based on a talk by Ted Fujita on El Nino, which included a video of
Topex/Poseidon sea level data as well as SST, my impression is that
much of the signal is surface/upper ocean derived. Further, potentially
a significant portion of the 3? mm/year signal is due to the presence
of an El Nino, rather than (necessarily) a global climate signal.
This latter effect was mentioned in the NASA press release that I
base the 3? mm/year estimate on.
I was just down to the reading room looking for something else, and
ran across the December, 1994 issue of J. Geophysical Research. This
is a special issue on the Topex/Poseidon mission. Several hundred
pages on how the thing works and all the wondrous information which
can be derived from it. Global sea level change, unfortunately, was
not one of the papers (to casual glance).
Aside to Rich Cullather: No need for deference. Aside from the
laser versus radar issue, everything you said squares with my understanding.
And I could have been wrong on the radar.
Followups to sci.geo.meteorology.
Jan Schloerer
I won't reply to individual articles, there are too many, they do not
arrive well ordered, and so on. Apologies for any repetitions.
Some inappropriate groups removed, followup set to sci.environment.
Regarding glaciers, the most recent major piece of work I happened
to come across suggests that, on global average, glaciers have indeed
retreated during the past hundred years. The observed glacier retreat
is consistent with (although no proof for) a surface warming trend of
0.66 degrees C per century. This may not be the last word. There are
uncertainties, factors other than air temperature affect the mass
balance of glaciers. Anyway, about a year ago this was the most
plausible explanation Oerlemans could think of.
Johannes Oerlemans, Quantifying global warming from the retreat
of glaciers. Science 264 (8 April 1994), 243-245.
Discussion: Science 265 (23 Sept 1994), 1790-1791
Last I heard, it was unclear whether the Antarctic and Greenland ice
sheets are presently gaining or losing mass. I can dig out some 1992/3
references in case anyone is interested. I do not know, whether the
state of affairs resp. knowledge has changed since. Anyone ?
Assessing and predicting sea level change may be even more difficult
than thought previously. A presumably minor quirk is that humans may
literally put some water into the oceans by extracting it from aquifers
and other water reservoirs. Sahagian et al. estimated that this may
currently account for a sea level rise of about 0.5 mm per year. A long
discussion ensued, which to my amateurish eye showed that this quantity
is difficult to estimate and open to revision.
Dork L. Sahagian, Frank W. Schwartz & David K. Jacobs,
Direct anthropogenic contributions to sea level rise in the
twentieth century. Nature 367 (6 Jan 1994), 54-57.
Discussion: Nature 369 (1994), 615-616. Nature 370 (1994), 258.
Nature 371 (1994), 481.
Perhaps more important, the dominant role of air temperature and preci-
pitation for the mass balance of the major ice sheets has been called
into question. So far, it was often assumed that precipitation over
Greenland and Antarctica depends mainly on temperature. Warm air can
deliver more moisture than cool air, so warming should increase preci-
pitation and the mass of the ice sheets. This in turn should tend to
slow down sea level rise.
In the GISP2 Greenland ice core, Kapsner et al. compared temperature
and accumulation (which is closely correlated with precipitation) over
the past 18,000 years. Their data suggest that temperature probably was
not the dominant control for precipitation. They think that changes in
atmospheric circulation, specifically of the dominant storm track, look
like a better candidate. The results are broadly consistent with
previous shorter records from Greenland. Precipitation over Antarctica
may also strongly be influenced by changes of storm patterns.
How storm patterns and tracks may change under a changing climate, and
how this may influence the mass balance of the ice sheets, does not yet
seem very clear. Anyway, estimates are likely to get more difficult.
W.R.Kapsner, R.B.Alley, C.A.Shuman, S.Anandakrishnan & P.M.Grootes,
Dominant influence of atmospheric circulation
on snow accumulation in Greenland over the past 18,000 years.
Nature 373 (5 Jan 1995), 52-54
David Bromwich, Ice sheets and sea level, ibid. 18-19
Finally, the recent IPCC Report "Climate Change 1994" (Cambridge Univ.
Press 1995) addresses greenhouse gases, aerosols, radiative forcing
and related topics. It doesn't say anything about sea level change.
According to a preliminary outline of the contents, this will be treated
in the forthcoming Second IPCC Assessment, scheduled for late 1995.
Jan Schloerer schl...@rzmain.rz.uni-ulm.de
Uni Ulm Klinische Dokumentation D-89070 Ulm Germany
David P Norwood
: >In article <Simen.Gaure-19...@ma-mac17.uio.no>,
: > Simen...@math.uio.no (Simen Gaure) writes...
: >>Is there any evidence that the ice has actually melted?
: >>It has broken up, yes, but is that due to large scale melting?
: >>It might also be the case that there's now more ice in
: >>Antarctica (e.g. due to increased precipitation).
: >>I'd guess (although I haven't made any computations) that
: >>even a minor increase in ice thickness (say 1 metre) will
: >>result in a lot of ice floating outwards towards the sea.
: Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
: ice masses are retreating. 2+2=4.
Is this because there's more water or because the water is warmer (on
average)?
dave
: --
David P Norwood
: >
: >Global sea-levels are rising about 1.2 mm per year. Also, glaciers and
: >ice masses are retreating. 2+2=4.
: It is a bit more complicated than that. Warming produces sea level
: rise just by thermal expansion. See my recent previous posting.
Oops. I suffer from premature posting. Sorry.
dave
: Leonard Evens l...@math.nwu.edu 708-491-5537
Hugh Easton
Simen...@math.uio.no "Simen Gaure" writes:
> > My point was simply that one should have some evidence that ice
> > in Antarctica is actually melting, not just assume that it is.
> > After all, it's well below freezing most of the time in Antarctica,
> > ice simply doesn't melt then. (Under normal pressure).
> >
>
> It does start to lose its strength well below freezing point though. Also,
> the ice near the bottom of a glacier or an ice sheet is under considerable
> pressure, which lowers its freezing point.
>
> It does, but at that depth (assuming now land-ice) there's no
> way any climate change can have reached yet.
That assumption is wrong if an unconventional mode of long range heat
conduction (such as infrared heat transmission) does operate in ice. Did
you not see my posting that started this thread, which included physical
data and calculations clearly demonstrating this?
> However, the pressure
> may very well have increased due to increased precipitation.
How much precipitation are you talking about? My understanding is that,
although there is a lot of snow blowing around, the actual rate of
precipitation in Antarctica is very low - a few centimetres a year.
Even if the rate of precipitation doubled (and if it did I am sure
someone would notice) it would take centuries to increase the pressure
significantly.
> (Anyway, there seems to be a complete lack of facts about the
> precipitation in Antarctica in this group; so it's hard to speculate
> any further.)
>
> --
> Simen Gaure, Department of Mathematics, University of Oslo
>
--
Hugh Easton <Hu...@daflight.demon.co.uk>
Simen Gaure
How much precipitation are you talking about? My understanding is that,
although there is a lot of snow blowing around, the actual rate of
precipitation in Antarctica is very low - a few centimetres a year.
Even if the rate of precipitation doubled (and if it did I am sure
someone would notice) it would take centuries to increase the pressure
significantly.
I read your comments about infrared heat transmission, it may be
well worth looking at; however, to explain the current breakup
it may be wise to look at more conventional explanations first.
As I understand it, there's no evidence that the structure of the ice
has changed dramatically, I suppose your theory could be checked
by analyzing some samples.
Actually, I don't have the foggiest idea how much precipitation
there is in Antarctica each year, it just struck me as a plausible
reason for increased thickness and hence disintegration of
the ice at the rim. Btw, not only precipitation in the form
of snow should be taken into account, but also steam freezing
on the ground (perhaps not much due to extremely low humidity).
I'd guess on average some 4-5 cm each year (my guess is based
on which depth an expedition expected to find Amundsen's tent).
Note also for instance that "El Niño" has changed not long ago, this
may indicate local climate changes and hence change in precipitation.
Now, the regular cause for calving along the rim is that the ice
is pushed outwards by its own weight, in an almost stable system
this will be the major reason. (This happens all the time, even
if there's no "melting" going on).
If then the precipitation increases (even by a tiny amount)
it seems evident that the disintegration at the rim will increase,
probably some or many years after (depending on properties
of the ice) the increase in precipitation.
What one needs to check this, is some knowledge of the ice structure,
some knowledge of the level of precipitation and one or more
differential equations. (And probably a computer.)
It could even be as simple as looking at figures for outwards
ice transport, if they exist. (After all, there are some
research stations down there, they must have done something.)
Hugh Easton
Simen...@math.uio.no "Simen Gaure" writes:
>
> I read your comments about infrared heat transmission, it may be
> well worth looking at; however, to explain the current breakup
> it may be wise to look at more conventional explanations first.
What is the conventional explanation for the recent events in Antarctica?
Using the values for thermal conductivity I quoted earlier from the
Handbook of heat transfer fundamentals (2nd ed.), McGraw-Hill 1985, there
just hasn't been enough time for the small amount of warming detected so
far to have a significant effect.
> As I understand it, there's no evidence that the structure of the ice
> has changed dramatically,
Melting and disintegration are fairly dramatic structural changes.
For the benefit of those who don't know what we are discussing, allow me
to quote from the BAS press release which was issued shortly after
January's events:
<p>The disintegration of an ice shelf and the calving of a new giant
iceberg have dramatically changed the outline of Antarctica.
Satellite images relayed to Cambridge from the British Antarctic
Survey's (BAS) Rothera Research Station confirm that recent
warming of the Antarctic Peninsula is having a major impact on
the ice sheet covering this climatically sensitive region..
<p>The ice shelf which formerly occupied Prince Gustav Channel and
connected James Ross Island to the Antarctic Peninsula has
disintegrated. For the first time in recorded history, James
Ross Island is circumnavigable. The new iceberg calved from the
Larsen Ice Shelf and measures 78 km x 37 km, (roughly the size of
Oxfordshire), and is around 200 m thick. .
<p>Alerted by glaciologists 9000 miles away in Cambridge, scientists
on board a BAS Dash 7 aircraft confirmed