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:
[…]
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