date: Fri Jun 6 14:57:47 2003 from: Keith Briffa subject: Re: EOS text to: "Michael E. Mann" 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: trenbert@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: mann@virginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137 [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: mann@virginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137 [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: mann@virginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137 [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: +44-1603-593909 Fax: +44-1603-507784 [7]http://www.cru.uea.ac.uk/cru/people/briffa[8]/