"I find it almost inconceivable that "business as usual" climate
change will not result in a rise in sea level measured in metres
within a century. Am I the only scientist who thinks so?"
I would think so, yes.
Well, maybe not the only one. But if a significant proportion of the
IPCC authors agreed, it is surely even more inconceivable that they
would have brushed such a critical aspect of climate change under the
carpet in the way they did (by presenting figures which excluded any
assessment of ice sheet collapse).
Hansen's "measured in metres" means two or more meters.
That is at least 4 times what the IPCC is predicting.
To say that ice responds nonlinearly and that things
are accelerating is one thing, but one needs to put
numbers to those statements. I have a very hard time
believing Hansen's statement about sea levels at the
end of this century.
The IPCC stated that they were being conservative and that nonlinear
effects could not be ruled out. You might also want to keep in mind
the fact that Hansen rarely makes a bet without being pretty sure he
is going to win. I also believe that he would be very good at darts.
One more point: earlier this year the IPCC projected that the Arctic
sea would not see an ice-free summer until at least 2050, and quite
well into the 2100s. Now after having examined things more closely,
one team has said 2040, and another Nasa was more recently saying
2020. It appears that the IPCC tends to be somewhat conservative in
It seems he might have been quoted fairly in that alarmism ad absurdum
What probability is "almost inconceivable" I wonder? >99%
You have not publicly offered to take his bet yet? You much be
You might even get him to bet that sea level rise doubles in the next
decade, eliminating the need for cryonic preservation to collect on
You might get him to throw in a cup of Starbucks in 2100, thereby
almost doubling the size of the bet.
1) The ice sheets are so far out of equilibrium that an exponential
increase in their net decay is a reasonable approximation.
2) The doubling time for the rate is about a decade.
I suspect point 1 fails to hold once we get into significant fractions
of a meter per decade, and I have no idea how he calibrated point 2,
but I think both points are quite plausible and worth discussing.
I agree that the "almost inconceivable" is disturbingly excessive.
However, as noted in the current IPCC discussion, even if the streams
accelerate substantially, the limited width of the outflows
originating from Greenland may still limit the impact. Even a very
large increase in flow rate may only contribute some tens of
centimeters of additional sea level rise, rather than meters, during
So, even though the knowledge of ice flows is quite limited, some
geographical constraints suggest that the impact of ice flows from
Greenland could remain moderate rather than severe. By contrast, ice
discharge could play a major role in the evolution of the West
Antarctic ice sheet, but the IPCC continues to predict that increased
precipitation will keep the Antarctic stable during this century (i.e.
no net mass loss).
-Robert A. Rohde
That said, there are some interesting hints that ice sheet melting may
be occurring considerably faster than most models to date have
projected. I expect we will hear a lot more about this in the next few
years, especially as a result of Hansen's outspokenness.
Oh, and the quote in the documentary implied a complete melting of the
Greenland icesheet in the next half century, which is quite a
different thing than a few meters of sea level rise by 2100. No one
would claim that there is any reasonable probability of a complete
loss of the Greenland (or West Antarctic) ice sheets in the timeframe
of a century.
Greenland, perhaps not. But West Antarctica? Plenty of people worried
But as you imply, "complete" is something of a straw man in this
Whatever the underlying reality of the behaviour of our big ice sheets
in these literally unprecedented circumstances, Hansen is acting
sensibly, as I've blogged elsewhere: http://hot-topic.co.nz/2007/07/27/knee-deep-and-still-digging/
Well, I think this section from Hansen's article linked by Michael
Tobis comes awfully close:
"However, the primary issue is whether global warming will reach a
level such that ice sheets begin to disintegrate in a rapid, non-
linear fashion on West Antarctica, Greenland or both. Once well under
way, such a collapse might be impossible to stop, because there are
multiple positive feedbacks. In that event, a sea level rise of
several metres at least would be expected.
As an example, let us say that ice sheet melting adds 1 centimetre to
sea level for the decade 2005 to 2015, and that this doubles each
decade until the West Antarctic ice sheet is largely depleted. This
would yield a rise in sea level of more than 5 metres by 2095."
My understanding is that there are a fair number of places where the
ice sheet extends from "mainland" Greenland out to some "islands"
which have not been seen for tons of millenia. These areas could go
"... Our principal results are that (1) marine ice sheets do not
exhibit neutral equilibrium but have well-defined, discrete
equilibrium profiles; (2) steady grounding lines cannot be stable on
reverse bed slopes; and (3) marine ice sheets with overdeepened beds
can undergo hysteresis under variations in sea level, accumulation
rate, basal slipperiness, and ice viscosity. This hysteretic behavior
can in principle explain the retreat of the West Antarctic ice sheet
following the Last Glacial Maximum and may play a role in the dynamics
of Heinrich events."
When a seafloor ice sheet retreats beyond certain undersea features,
it is unstable until it reaches another configuration. This retreat
can be relatively rapid. For the near future, understanding the
hysteresis is sort of secondary. The point is that a mechanism for
rapid retreat is identified.
An animation of a related simulation was shown; the abrupt retreat of
a large piece of the Antarctic during the last glacial retreat was
striking. I'm still having trouble getting the ice people to be
specific about time scales, though.
On Jul 30, 5:53 pm, "Michael Tobis" <mto...@gmail.com> wrote:
> We just discussed this paper in journal club last week. It's a lot
> easier to understand these ideas in conversation than in reading a
This is as close as I'm getting to such conversation. Much
"Recent observations suggest that major changes in parts of the ice
sheets are taking place over timescales of a few years to decades, not
thousands of years as traditionally believed" explains Jonathan Bamber
at the University of Bristol, who has recently published a paper with
colleagues examining the growing evidence that there is still much we
don't understand about ice sheets. "These changes were not predicted
by numerical models and the underlying causes are uncertain."
"Contra to Jim Hansen's concern that we may see significant ice sheet
changes on a scale of less than a century, I note that a 2000 paper by
UCLA's Glen MacDonald concluded that the eurasian arctic was 4-12 deg
F warmer in summer for the period from 7000 to 9000 ybp -
www.sscnet.ucla.edu/geog/downloads/634/269.pdf. Presumably it was also
much warmer in Greenland at that time, but it seems that we did not
"The only discussion I could find of this paper is by Pat Michaels.
Does anyone have any thoughts on how this and other evidence for a
warmer early Holocene fits into Hansen's concern? Is there a dispute
over the degree, location or duration of the warmth of that period, or
some other explanation?"
Jonathan Overpeck et al. (2006) try to compare present and future
conditions with the last interglacial period (LIG), a warm era between
129,000 and 118,000 years ago. That time, sea levels were between 4
and more than 6 m higher than today. For Greenland, the team concludes
that with 3.5°C global and hence about 5°C local warming, "the high
northern latitudes around Greenland will be as warm as or warmer than
they were 130,000 years ago and hence warm enough to melt at least the
large portions of the [Greenland Ice Sheet] that apparently melted
during the LIG" (Overpeck et al. 2006: 1748). For the Antarctic, the
situation could also worsen substantially, leading the authors to
conclude that "a threshold triggering many meters of sea-level rise
could be crossed well before the end of this century" (ibid.: 1750).
Source: Overpeck, Jonathan T., Bette L. Otto-Bliesner, Gifford H.
Miller, Daniel R. Muhs, Richard B. Alley, and Jeffrey T. Kiehl (2006):
Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-
Level Rise, in: Science, Vol. 311, 24 March, pp. 1747-1750, see under
Before I started to look into sea level rise more intensively a few
weeks ago, I was rather wary of anything more than 1m this century.
But now, with the Arctic sea ice shrinking faster than anyone had
predicted before, glaciers reacting dynamically like no one had
predicted before, and more than a quarter of Greenland's surface area
now subject to snow melting, I think Hansen's point is supported by
If 5°C global warming from the last ice age to the holocene resulted
in 120m sea level rising, I think that only 1m due to the likely 3-X°C
we can expect this century seems pretty low. Whether it's going to be
as rapid as during meltwater pulse 1A of course I can't tell. Back
then, sea levels rose by 1m every 20 years. But given the sheer size
of global warming, I would rather like to see one who can rule out
this possibility instead of asking for someone to prove that it could
Regards, '1.5M' Brown...
On 21 Aug, 18:41, Nils Simon <nils.si...@gmx.net> wrote:
> Tom, you may want to take a look at Overpeck et al. 2006 (Hansen
> relies on this study for example in "Climate Change and Trace Gases").
> I'm lazy and simply copy and paste something I'm writing at the
> Jonathan Overpeck et al. (2006) try to compare present and future
> conditions with the last interglacial period (LIG), a warm era between
> 129,000 and 118,000 years ago. That time, sea levels were between 4
> and more than 6 m higher than today. For Greenland, the team concludes
> that with 3.5°C global and hence about 5°C local warming, "the high
> northern latitudes around Greenland will be as warm as or warmer than
> they were 130,000 years ago and hence warm enough to melt at least the
> large portions of the [Greenland Ice Sheet] that apparently melted
> during the LIG" (Overpeck et al. 2006: 1748). For the Antarctic, the
> situation could also worsen substantially, leading the authors to
> conclude that "a threshold triggering many meters of sea-level rise
> could be crossed well before the end of this century" (ibid.: 1750).
> Source: Overpeck, Jonathan T., Bette L. Otto-Bliesner, Gifford H.
> Miller, Daniel R. Muhs, Richard B. Alley, and Jeffrey T. Kiehl (2006):
> Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-
> Level Rise, in: Science, Vol. 311, 24 March, pp.1747-1750, see underhttp://www.sciencemag.org/cgi/content/abstract/311/5768/1747
I asked the same thing over at Stoat a while back -- left some notes
and cites in the 'Why Study Antarctica' thread there, after coming
across a bunch of articles about water moving under the ice, liquid
water found in large voids below the ice when drilling through the
ice, rapid drumlin formation under the Antarctic ice, and then taking
another look at the Channeled Scablands.
All these lovely drawings of "meltwater lakes dammed up behind ice"
seem to have been done before people started looking at the way melt
water goes down below the ice.
Oh dear, I'd better answer then :-)
As far as I know, the laurentide ice dams were holding back vast lakes. There is
nothing analogous to that in Antarctica now.
As for the sub-ice flow now in Ant: I'm (fairly) sure that this is in areas
where the ice grounds below sea level and the overpressure is low. Anthing above
sea level with 1km of ice above it is going to be pretty water tight. I think.
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See Grey et al: "Evidence for subglacial water transport in the West
Antarctic Ice Sheet through three-dimensional satellite radar
GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L03501, 2005
This can potentially be spectacular, and is probably the mechanism for
the earlier meltwater pulse.
Re Jacobshavn: http://en.wikipedia.org/wiki/Jakobshavn_Isbr%C3%A6
I think that misses the point, which is that there is no obvious way for
surface water to percolate down through 1km of ice to reach the ice bed.
For that matter, where there is already water down there, it presumably
wouldn't matter if more joined it, from the POV of lubrication.
I do wonder how much the discovery of water sloshing around has
influenced beliefs about the (possibly) isolated Vostok ecosystem. If
water can move more-or-less freely under large areas of the ice sheet,
then it seems entirely plausible to me (but I'm guessing really) that it
is connected to the open ocean. Anyway, that's another matter entirely,
but not one that I've seen much discussion of. Maybe there are good
reasons why an ocean connection is implausible.
Liquid water and soft mud at the base of a moving glacier matters a lot.
For the bulk of an ice sheet I suppose it matters much less.
For West Antarctica, there is the question of what sets the maximum
width of a glacier, and the answer appears to be topography. Which
means that parts of the WAIS might disappear relatively quickly by
becoming very wide moving glaciers, which is probably what the 12ka
meltwater pulse was about.
This in turn means that the hydrology under the WAIS matters, which in
turn, if I understand correctly, makes a young fellow down the hall
from me busy on a doctoral thesis.