Three quick points.
First, Andy (if I may), I was responding to the following remark you made in
your NYT blog after Richard Alley's comment on the Dahl-Jensen article:
"I do think [the new work] closes the case that Greenland, despite all of
its drama (moulins, for example) - drama that focused my attention for a few
years too - is a sideshow in the sea level question."
I'm having trouble squaring the above remark with the one immediately below
in this thread ("Don't get me wrong. I'm not endorsing that the probable
Greenland contribution to sea level doesn't matter to policy").
Second, you cite Pfeffer 2008, which argues kinematic constraints will keep
Greenland's contribution to SLR to less than 20 cm by 2100. But Pfeffer 2008
almost certainly isn't the last word on this matter. As Hansen notes, recent
data suggest Greenland ice sheet mass loss may be exponential, and he and
others have challenged the assumptions in Pfeffer 2008. So a flat
declaration that "Greenland is a sideshow" seems a tad premature.
Third, I'd like to challenge the notion that the "policy-relevant" time
scale operative in this case ends at 2100. The relevant horizon is actually
both farther away from us than 2100 or nearer to us, depending on our policy
interests. The horizon should probably be pushed further away if we're
focusing on climate or SLR impacts on our societies. Nearly half of the
children under 10 today will still be alive in 2100 (assuming current
incremental increases in longevity continue). Also, 2100 is easily within
the planning horizon of major infrastructural projects, including storm
sewers, sea walls, coastal roads and bridges, etc.
The relevant time horizon should be brought much closer to us, given the
inertia in climate and ice systems, if we are interested in taking
mitigative action to prevent severe climate and SLR impacts in the future.
Mike MacCracken has already made a similar point. Even if we project that
Greenland's melting won't raise sea level by a meter till (say) 2150, if our
emission commit us to that meter by 2050, then 2050 is the relevant policy
horizon for avoiding that meter through mitigation.
THD
From: Andrew Revkin [mailto:rev...@gmail.com]
Sent: Monday, January 28, 2013 9:23 PM
To: Mike MacCracken
Cc: Thomas Homer-Dixon; Geoengineering
Subject: Re: [geo] Re: Why Greenland's melting could be the biggest climate
disaster of all
Don't get me wrong. I'm not endorsing that the probable Greenland
contribution to sea level doesn't matter to policy.
I'm just stating a fact related to how humans - as individuals and groups -
have responded to risks that require big changes in the status quo. Thomas,
I'd be eager to see any data you have showing otherwise.
On Mon, Jan 28, 2013 at 9:15 PM, Mike MacCracken <mmacc...@comcast.net>
wrote:
Hi Andy-Your agreement with the dismissive statement on Greenland seems
terribly short-sighted. Over the coming decade (if not already), we'll be
setting a course for Greenland that will lead to much higher sea level in
the future (and the contributions from Greenland and Antarctica will end up
being far more than from thermal expansion and melting glaciers). A key
issue at present among politicians is the impacts we are imposing on future
generations (national debt, etc.)--well, dealing with Greenland melting is
quite the predicament we would be posing to future generations (so the
children and grandchildren of today's politicians).
Mike
On 1/28/13 9:56 AM, "Andy Revkin" <rev...@gmail.com> wrote:
A sideshow to sea-level questions on policy-relevant time scales. (2100-ish
at best)..
You're talking geological scale here.
Tad Pfeffer's 2008 analysis of worst-case discharge rate still a keystone to
clear thinking on this.
On Mon, Jan 28, 2013 at 2:38 PM, Thomas Homer-Dixon <t...@homerdixon.com>
wrote:
"Greenland . . . is a sideshow in the sea level question."
I see nothing in the Dahl-Jensen article that could possible justify such a
sweeping and dismissive claim. Alley himself says: "We have high confidence
that warming will shrink Greenland, by enough to matter a lot to coastal
planners."
Thomas Homer-Dixon
University of Waterloo
On Jan 28, 2013 5:12 PM, "Andrew Revkin" <rev...@gmail.com> wrote:
There's also fresh input from Richard A. (and Waleed Abdalati) on Greenland
and sea level in this new dot earth post:
Eyes Turn to Antarctica as Study Shows Greenland's Ice Has Endured Warmer
Climates http://nyti.ms/Yq7uhA
I turned to Richard Alley
<http://dotearth.blogs.nytimes.com/2009/02/06/richard-alleys-orbital-and-cli
mate-dance/> , who's become a vital touchstone for me on such research, for
some insights. Here's his comment, followed by my closing thoughts:
I have three immediate responses: Satisfaction in the great success of the
collaboration, concern that this slightly increases worries about future
sea-level rise from human-caused warming, but technical questions that may
leave us more-or-less where we were before on the biggest picture.
Taken in turn:
Having watched colleagues go to the immense effort of learning what
information is desired by policymakers and other citizens, assemble the
logistical and scientific abilities to supply that information, and actually
do it over a lot of years, and knowing just how many of their kids' soccer
games and recitals some of the scientist-parents missed, I have to smile
when the team succeeds so well.
As to the big picture, there is strong evidence from the history of sea
level on coasts from the Eemian that both Greenland and Antarctic ice sheets
shrank notably, contributing to a globally averaged sea-level rise of very
roughly 20 feet. This occurred primarily in response to a rearrangement of
where sunshine reached the planet and when during the year, with more summer
sunshine in the north but very little total change. And, some uncertainty
has remained on the exact balance between Greenland and Antarctic
contributions. The new paper suggests that the contribution from Greenland
was on the low end of the prior estimates, but has little effect on the
estimated total sea-level change, which points to a larger Antarctic source
than the previous best estimate.
In my opinion (and I believe the opinions of many colleagues), we have
greater understanding of Greenland's ice than Antarctica's, and we have
greater confidence that Greenland will be "well-behaved" - we will more
easily project changes in Greenland's ice, with greater confidence that
changes begun now will take centuries or longer to be mostly completed.
By shifting more of the sea-level rise into the less-understood ice, and
thus into the ice with greater chance of doing something rapidly, I believe
the new paper at least slightly increases the concerns for coastal planners,
even if the chance of a rapid change from Antarctic ice remains small.
As to the technical parts, as described in many sources, we have lots of
paleothermometers for the central Greenland ice cores over the last 100,000
years, providing multiple validation and high confidence that temperatures
have been estimated accurately. The very changes in the ice sheet that are
of greatest interest here also make the effort quite difficult. The melting
of the Eemian interferes with gas-based paleothermometry, and with the
total-gas technique that provides constraints on changes in surface
elevation.
A U.S. government CCSP report on Arctic paleoclimates a few years ago (to
which I contributed) [link
<http://dotearth.blogs.nytimes.com/2013/01/24/eyes-turn-to-antarctica-as-stu
dy-shows-greenlands-ice-has-endured-warmer-climates/%3Ehttp:/www.climatescie
nce.gov/Library/sap/sap1-2/final-report/default.htm> ] estimated changes in
temperature and ice volume for this interval. The new estimates overlap with
the older ones. Were I working on that report now, I would recommend
expanding the uncertainties a little to include the new results. However,
considering that ice shrinkage on Greenland has a feedback effect (exposing
rocks allows more sun to be absorbed, causing more warming), considering the
evidence of Eemian warmth from marine records around Greenland, considering
climate model runs for that time, considering other studies of Greenland,
and recalling the notable uncertainties associated with untangling the
changes in total gas and in the ice sheet itself, I suspect that the
estimates in that CCSP report will stand up pretty well, with the new work
primarily confirming the prior understanding of climate changes and
ice-sheet and sea-level response in the Eemian.
If anyone is thinking that this paper means we can crank up the temperature
without worrying about sea level, they should seriously re-think. Overall, a
great and successful scientific effort leaves us with the knowledge that
warming does tend to melt ice, and that contributes to sea-level rise.
In a followup note to him, I said:
Beautifully articulated. but I do think [the new work] closes the case that
Greenland, despite all of its drama (moulins, for example
<http://dotearth.blogs.nytimes.com/2008/07/03/a-tempered-view-of-greenlands-
gushing-drainpipes/> ) - drama that focused my attention for a few years too
- is a sideshow in the sea level question.
That's not how it's been cast. There's been talk of regional geo-engineering
to "save" the ice sheet
<http://iopscience.iop.org/1755-1315/6/45/452009/pdf/1755-1315_6_45_452009.p
df> . The dramatic surface melting
<http://www.washingtonpost.com/blogs/capital-weather-gang/post/greenland-ice
-sheet-surface-melt-huge-deal-or-overblown/2012/07/25/gJQAlfcT9W_blog.html>
, while important to track and understand (as is being done by Jason Box and
others) has little policy significance.
Alley replied:
I do think it has been clear for a while that interactions with the ocean
provide the greatest potential for surprises and rapid changes, and that
Greenland's ice sheet would mostly pull out of the ocean before it lost most
of its mass. The discussion in the attached, as well as in Ian Joughin's and
my [West Antarctic Ice Sheet] review in 2011
<http://www.nature.com/ngeo/journal/v4/n8/box/ngeo1194_BX1.html> , were
pointing in that direction. The lack of huge danger from the lake drainages
probably was argued (possibly for the first time) by Byron Parizek and I in
Quaternary Science Reviews
<http://www.journals.elsevier.com/quaternary-science-reviews/> in 2004.
There are dynamics issues, but the biggest ones go away once shrinkage pulls
the ice out of the ocean. Then, a serious focused research effort should be
able to produce (and indeed, is producing) quantified projections with
useful uncertainties that can be narrowed by continuing effort on the
established research path. We are still thinking about one or two
interesting and possibly surprising things, but Greenland looks like it is
mostly the known-unknown ice sheet.
On Mon, Jan 28, 2013 at 11:56 AM, David Lewis <jrandomwin...@gmail.com>
wrote:
Richard Alley discussed the potential Greenland and Antarctic contribution
to sea level rise in a talk at Stanford in late October 2012 which is
available on Youtube <http://www.youtube.com/watch?feature=player_embedded
<http://www.youtube.com/watch?feature=player_embedded&v=o4oMsfa_30Q&noredire
ct=1> &v=o4oMsfa_30Q&noredirect=1>
On Monday, January 28, 2013 2:45:00 AM UTC-8, Oliver Tickell wrote:
http://grist.org/climate-energy/why-greenlands-melting-could-be-the-biggest-
climate-disaster-of-all/
--
_
ANDREW C. REVKIN
Dot Earth blogger, The New York Times
http://www.nytimes.com/dotearth
Senior Fellow, Pace Acad. for Applied Env. Studies
Cell: 914-441-5556 Fax: 914-989-8009
Twitter: @revkin Skype: Andrew.Revkin
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