Climate Audit

Jean S has written me suggesting that I review the NAS panel claim that Mann was the “first systematic, statistically based synthesis of multiple climate proxies”. This claim is true only in the sense that Al Gore invented the Internet and I will discuss this in the next few days.

While I was reviewing the earlier literature, I stumbled across a previous NAS report, which dealt with some of the topics of the present study: Natural Climate Variability on Decade-to-Century Time Scales (1995) available here and here. It reports on a 1992 workshop involving many leading figures with two interesting essays on proxies – one by Jones and Briffa and one by Bradley and Diaz. NAS panelists Gerald North, John Wallace and Robert Dickinson are mentioned as participants.

There are many interesting perspectives on pre-MBH views of what you could do with proxies.

I couldn’t help but notice the following recommendation:

Climate data must be made freely available to researchers worldwide; data from many sources contribute to the solution of research problems.

It’s nice to know that the present NAS panel also more or less endorses this position. It is disquieting that problems still persist and the matter needs to be dealt with over and over again, seemingly with minimal impact on the Hockey Team.

The Diaz and Bradley essay is entitled “Documenting Natural Climatic Variations: How Different is the Climate of the Twentieth Century from That of Previous Centuries?” and well worth a read. The list of sites here includes many familiar names (most of which recur in MBH98) – a point which I will return to on another occasion. Diaz and Bradley state:

With the exception of the data from tropical ice cores, the proxies indicate that the recent decades were not very unusual, either in regard to the mean or in terms of increased variability. While seasonal and annual temperature changes in the last two decades have been rather large in most areas of the Northern Hemisphere, the available paleoclimate evidence suggests that in many areas there have been decadal periods during the past several centuries in which reconstructed temperatures were comparable to those of the 1970s and 1980s, with climatic variability as large as any recorded in recent decades. …

Figures 6 through 8 show decadal-mean values of various tree-ring reconstructions, generally representing summer (or growing-season) mean temperature. For the most part, these tree-ring reconstructions do not sample the 1980s; nevertheless, it is clear from the degree of interdecadal variability that it would be hard to point to any particular recent period as being unique or exceptional.

From NAS 1995.

With the exception of the tropical ice cores, the recent decades were not generally unique, in terms of either the average values or increased variability….

We can only say that, for most areas of the coterminous United States, the climate of the most recent decades cannot be considered unique, even in the context of the last century.

The Jones and Briffa essay here has some interesting nuggets. They say:

We fail to see any consistent trends in interdecadal variability associated with these tree-ring temperature reconstructions (see indices 5-9, 12-13, and 16-18 in Table 2). Interdecadal variability is typically about half the interannual values. This implies that substantial low-frequency variance is present in the paleotemperature record, so that recent high values of reconstructed decadal-mean summer temperature may yet represent an oscillation within the range of natural variability….

In only one (the Northern Urals, Briffa et al., 1995) of the thousand-year dendroclimatic reconstructions is the twentieth century clearly the warmest century. [SM note: this is the problematic Polar Urals reconstruction of Briffa et al 1995, that I’ve written many posts on. The updated version has a very elevated MWP] In all of the other reconstructions, however, this century is one of the warmest. Warmer (summer) conditions occurred in previous centuries, but never at the same time at all locations. Clearly, a greater geographical spread of long paleoclimatic reconstructions is required; types of proxy evidence that cover seasons other than summer must be employed before the apparently unprecedented warmth shown in instrumental records of the twentieth century can be placed in a longer-term context. …

In the discussion afterwards, Thomas Karl observed:

Dr. Jones also showed that there is little coherence among the several 1000-year tree-ring time series that have been assembled. That is very interesting; if it is true, putting together a global-scale multi-century time history of temperature change will be very challenging indeed!

Here’s another interesting question and answer:

MYSAK: What’s the state of the art in extracting, say, precipitation or runoff from tree-ring data?

JONES: Most of the tree-ring work we’ve dealt with is from Scandinavia, where the response is clearly to temperature. In southern Europe or the southern part of the United States you can get a good reconstruction of river flow or precipitation from trees, because there the trees are responding to moisture. In between, as in England or northern Germany, the trees are responding to a mix of the two, and it’s hard to unravel the climate signal.

So Bradley and the older members of the Hockey Team seem to have got stuck trying to extract the desired signal from the dog’s breakfast of proxies. This is the milieu leading up to MBH98. It’s not that Mann was the “first” to do a multiproxy study. Even Mann didn’t claim that. It’s not even that Mann had new proxies to work with that were unavailable to Bradley and Jones in 1993. I’ll analyse proxy usage in another post, as the selection differences between MBH98 and Bradley and Jones 1993 (or the list in NAS 1995) are illuminating.

What they perceived theie originality (and this was the contemporary view) was that they used a “new statistical approach”, because “conventional” methods had proved “relatively ineffective”. Bradley said last year that the disctinctive contribution was that Mann had “originated new mathematical approaches that were crucial to identifying strong trends” [Goldscheider, 2005]. NAS 1995 is interesting because it shows the pre-MBH mentality quite clearly. In case you’re wondering, it isn’t mentioned in the NAS bibliography.