WUWT reader Tom O’Hara writes in a question that seemed worthwhile to discuss. Paleo specialists can weigh in on this. It seems to me that he has a point, but like him, I don’t know all the nuances of calibrating a proxy. (Graphic at right by Willis Eschenbach, from another discussion.)
[This] is a puzzle to me.
Everything we know about past climate is based on “proxies.” As I understand the concept, science looks at “stuff” and finds something that tends to mirror the changes in temperature, or whatever, and uses that as a means to determine what the likely temperature would have been at an earlier time. This is, I am sure, an oversimplified explanation.
So what we have, in essence, is a 150 year or so record of temperature readings to use to determine our proxy’s hopeful accuracy.
Now my question would be, if we are continuously adjusting the “readings” of that record, how does that affect the usefulness of the proxy information?
If I have correlated my proxy to a moving target, doesn’t that effect the likelihood that the proxy will yield useful information?
It would seem to me that this constant massaging of the database used to define and tune my proxy, would, in the end, destroy the utility of my proxy to deliver useful information. Or have I got it all wrong?
A few primers for discussion:
1.Detecting instabilities in tree-ring proxy calibration
Abstract. Evidence has been found for reduced sensitivity of tree growth to temperature in a number of forests at high northern latitudes and alpine locations. Furthermore, at some of these sites, emergent subpopulations of trees show negative growth trends with rising temperature. These findings are typically referred to as the “Divergence Problem” (DP). Given the high relevance of paleoclimatic reconstructions for policy-related studies, it is important for dendrochronologists to address this issue of potential model uncertainties associated with the DP. Here we address this issue by proposing a calibration technique, termed “stochastic response function” (SRF), which allows the presence or absence of any instabilities in growth response of trees (or any other climate proxy) to their calibration target to be visualized and detected. Since this framework estimates confidence limits and subsequently provides statistical significance tests, the approach is also very well suited for proxy screening prior to the generation of a climate-reconstruction network.
Two examples of tree growth/climate relationships are provided, one from the North American Arctic treeline and the other from the upper treeline in the European Alps. Instabilities were found to be present where stabilities were reported in the literature, and vice versa, stabilities were found where instabilities were reported. We advise to apply SRFs in future proxy-screening schemes, next to the use of correlations and RE/CE statistics. It will improve the strength of reconstruction hindcasts.
Citation: Visser, H., Büntgen, U., D’Arrigo, R., and Petersen, A. C.: Detecting instabilities in tree-ring proxy calibration, Clim. Past, 6, 367-377, doi:10.5194/cp-6-367-2010, 2010.
From WUWT August 16, 2013 A new paper now in open review in the journal Climate of the Past suggests that “modern sample bias “has “seriously compromised” tree-ring temperature reconstructions, producing an “artificial positive signal [e.g. 'hockey stick'] in the final chronology.”
Basically, older trees grow slower, and that mimics the temperature signal paleo researchers like Mann look for. Unless you correct for this issue, you end up with a false temperature signal, like a hockey stick in modern times. Separating a valid temperature signal from the natural growth pattern of the tree becomes a larger challenge with this correction. More here
Calibration trails using very long instrumental and proxy data
Esper et al. 2008
The European Alps are one of the few places that allow comparisons of natural climate proxies, such as tree-rings, with instrumental and documentary data over multiple centuries. Evidence from local and regional tree-ring analyses in the Alps clearly showed that tree-ring width (TRW) data from high elevation, near treeline environments contain substantial temperature signals (e.g., Büntgen et al. 2005, 2006, Carrer et al. 2007, Frank and Esper 2005a, 2005b, Frank et al. 2005). This sensitivity can be evaluated over longer timescales by comparison with instrumental temperature data recorded in higher elevation (>1,500 m asl) environments back to the early 19th century, and, due to the spatially homogenous temperature field, back to the mid 18th century using observational data from stations surrounding the Alps (Auer et al. 2007, Böhm et al. 2001, Casty et al. 2005, Frank et al. 2007a, Luterbacher et al. 2004). Further, the combination of such instrumental data with even older documentary evidence (Pfister 1999, Brázdil et al. 2005) allows an assessment of temporal coherence changes between tree-rings and combined instrumental and documentary data back to AD 1660. Such analyses are outlined here using TRW data from a set of Pinus cembra L. sampling sites from the Swiss Engadin, and calibrating these data against a gridded surface air temperature reconstruction integrating long-term instrumental and multi-proxy data (Luterbacher et al. 2004).
paper here: Esper_et_al_TraceVol_6 (PDF)