from: Keith Briffa <???@uea.ac.uk>
subject: Re: EOS text
to: "Michael E. Mann" <???@virginia.edu>
Mike
there is often no benefit in bandying fine points of emphasis and implication- Hence , I
think that what you have already drafted is fine. Do not start to dilute or confuse the
issue with too much additional detail. The job , as you state , is to place on record the
statement of disagreement with the "science(!)" and spin. To this end , it may also be
worth stating in less couched terms that merely eyeballing the relative magnitudes of
recent versus prior period(s) of large scale warmth, is in itself very limited as a basis
for claiming the reality OR OTHERWISE of anthropogenic forcing of the recent warming , if
this is done without reference to the uncertainty and causes of these differences.
The points you make to Tom are of course very valid , but do not be tempted to guild the
lily too much here - stick with your current content
Keith
At 09:15 AM 6/6/03 -0400, you wrote:
Thanks for the comments Tom,
I'm working on having a revised version by early this afternoon (in time for Phil to
look at before nightfall in the UK).
Phil has kindly agreed to take over the lead role on this if we're not ready to submit
by the time I have to leave (Jun 11th). Will update on this when necessary.
More soon,
mike
At 10:03 PM 6/5/2003 -0600, Tom Wigley wrote:
Mike,
Well put! By chance SB03 may have got some of these precip things right, but we don't
want to give them any way to claim credit.
Also, stationarity is the key. Let me tell you a story. A few years back, my son Eirik
did a tree ring science fair project using trees behind NCAR. He found that widths
correlated with both temp and precip. However, temp and precip also correlate. There is
much other evidence that it is precip that is the driver, and that the temp/width
correlation arises via the temp/precip correlation. Interestingly, the temp correlations
are much more ephemeral, so the complexities conspire to make this linkage
nonstationary.
I have not seen any papers in the literature demonstrating this -- but, as you point out
Mike, it is a crucial issue.
Tom.
++++++++++++++++++++++++++
Michael E. Mann wrote:
Hi Tom et al,
Wanted to comment briefly on some of the specifics below, just to make sure we don't get
too sidetracked. This is a very interesting and worthwhile discussion. In fact, these
are precisely the kinds of issues that Phil and I are trying to sort out w/ the review
paper we're writing for ROG [we'll probably be soliciting comments from many of you on
different sections of that paper in the near future]. But I think its useful at this
juncture to make a make a distinction between these sorts of scientifically interesting
issues, and the nonsensical arguments that SB03 are actually making.
We can quibble, for example, over the nature of the relationship between past
variations in the surface temperature field, the atmospheric circulation, and the types
of proxies that might inform our knowledge of each of these. I agree with Tom's point
that in many case precipitation indicators don't tell us much at all about the surface
temperature field, certainly in the 'local' sense. In a sort of 'state space' sense,
however, they may in some instances be quite helpful.
Winter drought-sensitive tree-ring chronologies provide us some of our best proxy
information with regard to winter synoptic-scale variability in semi-arid regions like
the desert southwest or the mediterannean. There appears to been some success (i.e.,
demonstrated statistical skill) in reconstructing patterns of anomalous atmospheric
circulation related to the usual suspect sorts of indices (PNA, NAO, etc.) from those
sorts of proxies. To the extent that much of the regional winter season variability in
the extratropical surface temperature field is related to these sorts of atmospheric
circulation anomalies, one expects some skill in using these predictors to reconstruct
features of the cold-season atmospheric circulation and, thus, regional temperature
anomalies related to those features. I think a good case has been made that we can,
perhaps, understand a good detail of the structure of the extratropical winter
temperature anomalies during parts of the 'LIA' in terms of, e.g., the behavior of the
NAO--a lot of evidence now seems to be pointing in that direction. A similar argument
can be made, for example, that a precipitation proxy in the western tropical Pacific may
be an excellent predictor of SST variability in the eastern and central tropical
Pacific, for the obvious reasons. So, in this larger-scale sense, there are some
potentially useful relationships, and I agree with what Kevin says in this regard. Of
course, it is also true that there are some obvious stationarity assumptions implicit in
this sort of reasoning, and in the use of any proxy precip/drought/atmospheric
circulation information to infer or help reconstruction features in the surface
temperature field. There are, however, similar stationarity assumptions implicit in the
idea that a modest network (say, of a dozen) proxy surface temperatures over, say, the
Northern Hemisphere, can be used to reconstruct hemispheric mean temperature. The
implicit assumption is that the relative importance of each of a small number of
locations in estimating the large-scale temperature field remains constant over time. As
the number of regions sampled approaches the number of degrees of freedom in the surface
temperature field, this because a better and better assumption. If were only talking
about a handful of locations, it may be a pretty bad assumption. This sort of
stationarity assumption is potentially just as, or even more (depending on the size of
the network used) suspect than the former stationarity assumption, but is much more
rarely discussed or acknowledge. Of course, there are ways to test these sorts of
assumptions in a modeling context, and there are several studies now published, and
others in the works, , that suggest the situation probably isn't as bad as we might have
feared (again, something Phil and I will touch on in our ROG paper). See for example,
these:
Mann, M.E., Rutherford, S., Climate Reconstruction Using
'Pseudoproxies, Geophysical Research Letters, 29 (10), 1501, doi:
10.1029/2001GL014554, 2002. Rutherford, S., Mann, M.E., Delworth,
T.L., Stouffer, R., Climate Field Reconstruction Under Stationary
and Nonstationary Forcing, Journal of Climate, 16, 462-479, 2003.
Zorita, E., Gonzalez-Rouco, F., and Legutke, S., Testing the Mann et
al. (1998) Approach to Paleoclimate Reconstructions in the Context
of a 1000-Yr Control Simulation with the ECHO-G Coupled Climate
Model, J. Climate, 16, 1378-1390, 2003.
But these are all legitimate caveats, and interesting points, that would be great to
discuss over some beers sometime, and which will be given more than adequate treatment
in e.g. the review paper mentioned above.
Unfortunately, that's not the task at hand. SB03 have no appreciation whatsoever for
these sorts of subtle, legitimate considerations, which involve thinking in a much
higher sphere than the one they are thinking in, and certainly, the one that they are
playing to. Their logic is much more basic, and immensely less reasonable, than anything
we're talking about here.
Their logic, in essence, literally EQUATES hydroclimatic and temperature anomalies,
since they hold that the existence of a large extreme in precipitation/drought in a
particular region is as good as evidence of anomalous warmth, in support of the
proposition of e.g. a "medieval warm period". So, in a very roundabout way, what I'm
saying is, lets definitely not give these bozos more credit than they deserve!
Unfortunately, we have precious little space in this Eos piece. Phil and I have a lot
more space in our ROG article, and this sort of discussion will help us in making sure
that these issues are adequately addressed there. I suspect that this longer review, and
others that Ray and folks are working on, will be helpful in e.g. the next IPCC report.
But for the time being, we have to keep things simple and to the point here. What we say
of course needs to be rigorously defensible and we would like to educate the readers as
much as we can in the short space available, but most of all we really have to do, in as
simple terms as possible, is explain why the SB03 stuff is so fundmentally flawed. And,
to boot, we have to do so in such a way that it seems more a casual consequence of what
we say, than (as it is in fact) the central motivation of the article.
So there is a real balancing act here, and thats what we're coming up against. Let me
do my best to strike this balance, and see if I can come up with a revised version that
strikes the right balance between everyones concerns here. Again, I still need comments
from several more people before I can attempt a revised draft. So responses (e.g. in the
next day or so) would be greatly appreciated from those I haven't heard back from...
thanks in advance,
mike
At 05:08 PM 6/5/2003 -0600, Tom Wigley wrote:
Dear all,
Re AGU's position, this is something I must have overlooked if it was in an earlier
email. One way around this is to make the scientific error points and quote SB as an
example of how not to do it (which one would have to do for at least three specific
points).
Re Kevin's suggestion, his text could be misinterpreted. It implies that one might be
able to use wet/dry as a T proxy if the right statistical analysis were done first. I
agree with what Kevin says, but I have looked at these sort of physically meaningful
relationships and they are invariably too weak to use in a paleo context. For example,
if the paleo indicator explains 50% of the precip (seasonal) variance (and such a high,
independently validated value is rare), and if the r**2 for precip vs temp were similar,
then we are left with 25% (at most -- the above assumptions are very optimistic). This
is weak. Worse still, this assumes no paleo atmos circulation changes, also doubtful.
The bottom line is that proxy precip data *cannot* be used as a T indicator except in
the rarest of circumstances. Even in high latitudes there are problems -- see, e.g.,
Bradley and England, late 1970s report (Ray, I'm sure you will remember this about the
rareness of precip events).
I think it is extremely dangerous to leave SB any loopholes here. In my view, what Kevin
says does just this.
Tom.
_________________-
Michael E. Mann wrote:
Thanks Kevin,
I've already made some revisions in response to your earlier comment about explicitly
discussing the spatial variability issue with regard to the LIA/MWP. The prospective
Figure 2 should help in this regard--looking forward to hearing back from Ray/Phil on
that...
I'll do my best to come up w/ a revised version that reflects everyones suggestions and
wishes once all the comments are in,
mike
At 02:53 PM 6/5/2003 -0600, Kevin Trenberth wrote:
Tom
I agree with Mike that it is not possible to directly confront their methods in this
way. It can be confronted by stating clearly that cold periods that are not
contemporaneous at different locations do not make for a cold hemispheric value:
currently the article already makes this point to some extent but it can be made more
directly relevant to SB. In fact it may be worthwhile pointing out that the LIA is
defined by different authors to be in different periods precisely because they were
looking at a different part of the world (like blind men exploring the elephant).
And we can also say that it makes no sense to equate wet or dry period with cold or warm
universally (ref SB).
In fact what is found generally in mid lats is that warm in winter goes with wet
(through moist and warm advection) and with dry in summer (drought and heat waves). So
seasonality matters a lot.
Maybe we can say womething like this:
It is well established in current climate studies that warm conditions tend to accompany
wet conditions in the extratropics in winter owing to the dominant role of the
atmospheric circulation so that southerlies are warm and moist in the northern
hemisphere while northerlies are cold and dry. But in summer, the weaker atmospheric
circulation means that moist thermodynamics is more important so that dry conditions
favor warm spells and heat waves, as heat from the sun no longer evaporates moisture and
instead increase temperatures. In the Tropics, during El Nino events, droughts occur
in one part of the world (e.g. Australia) while wet conditions and floods occur in other
parts (e.g. Peru), and the wet spots tend to switch with the dry spots during La Nina.
Accordingly, there is no unique link between wet or dry with warm or cold conditions
(such as erroneously assumed by SB).
Not sure if this is useful but I offer it anyway.
Kevin
Tom Wigley wrote:
Mike et al.,
I will send tracked editorial suggestions later. In the meantime, what is lacking in my
view is a clear statement at the start of the SB method. At present, the context of your
later comments is a bit unclear to those who have not read the papers -- which will be
the case for most readers. I suggest adding the attached before your point (1). What I
say here overlaps with some things you say later, so minor changes are needed (which I
will send later) to avoid clear duplication.
We are using this to educate people about the good paleo work, but a key motivation is
to demolish the bad stuff. I think, therefore, that the criticism of SB must be more
focussed and specific -- which is why a statement of their work is essential. This
suggested new material also provides a balance, and makes what we now have appear less
self serving (which I know you are not trying to do, but there is still a hint of this).
Tom.
--
****************
Kevin E. Trenberth e-mail: ???@ucar.edu
Climate Analysis Section, NCAR
[1]www.cgd.ucar.edu/cas/ <[2]http://www.cgd.ucar.edu/cas/>
<[3]http://www.cgd.ucar.edu/cas/>
P. O. Box 3000, (303) 497 1318
Boulder, CO 80307 (303) 497 1333 (fax)
Street address: 3080 Center Green Drive, Boulder, CO 80301
________________________________
Professor Michael E. Mann
Department of Environmental Sciences, Clark Hall
University of Virginia
Charlottesville, VA 22903
_________________________________________
e-mail: ???@virginia.edu Phone: (434)??? FAX: (434)???
[4]http://www.evsc.virginia.edu/faculty/people/mann.shtml
________________________________
Professor Michael E. Mann
Department of Environmental Sciences, Clark Hall
University of Virginia
Charlottesville, VA 22903
_________________________________________
e-mail: ???@virginia.edu Phone: (434)??? FAX: (434)???
[5]http://www.evsc.virginia.edu/faculty/people/mann.shtml
________________________________
Professor Michael E. Mann
Department of Environmental Sciences, Clark Hall
University of Virginia
Charlottesville, VA 22903
_________________________________________
e-mail: ???@virginia.edu Phone: (434)??? FAX: (434)???
[6]http://www.evsc.virginia.edu/faculty/people/mann.shtml
--
Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.
Phone: +4 ???-1603-593909
Fax: +4 ???-1603-507784
[7]http://www.cru.uea.ac.uk/cru/people/briffa[8]/
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