Brand New Hockey Sticks

There’s a new hockey stick in town, in Mann et al. 2008, Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia, PNAS, 105, no. 6. A great deal of supporting information is also available, including all the data and programs used for the reconstructions.

These reconstructions provide estimates of past temperature for the northern hemisphere, the southern hemisphere, and the globe as a whole, as well as for land only and for land+ocean. The research uses 1,209 proxy data series, a much larger network of proxy data than any which came before it.

All 1,209 series were available back to at least A.D. 1800, 460 extend back to A.D. 1600, 177 back to A.D. 1400, 59 back to A.D. 1000, 36 back to A.D. 500, and 25 back to year “0″ (i.e., 1 B.C.).

As a result, it’s possible to reduce the probable errors, further back in time than before. The reconstructions are also done by two different statistical methods: “cps” and “eiv.” The “cps” method is “composite-plus-scaling,” in which the available proxies are combined, then the composite is scaled to find the best match to the calibration data. In this method, data don’t have to match local temperature change, they can still be applied as long as they give information about global or hemispheric temperature change. The “eiv” method is “error-in-variables,” in which proxies are fit to calibration data allowing for errors in both the predictor and the predictand. For this method, proxies are fit to temperature data for the locale from which they derive. Both methods are in excellent agreement, but the authors find better verification statistics for the eiv method, and generally express higher confidence in that method.

Mann et al. also derived past temperature using subsets of the proxies. One such analysis omits individual proxies which don’t show statistically significant correlation with local instrumental data:

Reconstructions were performed based on both the “full” proxy data network and on a “screened” network (Table S1) consisting of only those proxies that pass a screening process for a local surface temperature signal. The screening process requires a statistically significant (P < 0.10) correlation with local instrumental surface temperature data during the calibration interval. Where the sign of the correlation could a priori be specified (positive for tree-ring data, ice-core oxygen isotopes, lake sediments, and historical documents, and negative for coral oxygen-isotope records), a one-sided significance criterion was used. Otherwise, a two-sided significance criterion was used. Further details of the screening procedure are provided in SI Text.

They also created reconstructions without using any tree-ring data at all, in response to objections that tree ring data aren’t reliable indicators of temperature change. But for each subset of data used, for northern hemisphere, southern hemisphere, and globe, for land-only and for land+ocean, for the cps and the eiv analysis methods, all results were qualitatively similar: a hockey stick.

Nominally, the recent observed decadal warmth recorded in the instrumental observations exceeds the uncertainty range of the reconstructions over at least the past 1,600 years for NH land temperatures as reconstructed by CPS (Fig. S5) and the past 1,700 years for NH land plus ocean temperatures as reconstructed by EIV (Fig. S6). Because this conclusion extends to the past 1,300 years for EIV reconstructions withholding all tree-ring data, and because non-tree-ring proxy records are generally treated in the literature as being free of limitations in recording millennial scale variability (11), the conclusion that recent NH warmth likely** exceeds that of at least the past 1,300 years thus appears reasonably robust. For the CPS (EIV) reconstructions, the instrumental warmth breaches the upper 95% confidence limits of the reconstructions beginning with the decade centered at 1997 (2001). It is intriguing to note that the removal of tree-ring data from the proxy dataset yields less, rather than greater, peak cooling during the 16th–19th centuries for both CPS and EIV methods (see Figs. S5a and S6b, respectively), contradicting the claim (33) that tree-ring data are prone to yielding a warm-biased “Little Ice Age” relative to reconstructions using other high-resolution climate proxy indicators.

For different methods, regions, and proxy subsets, there are differences in how far back in time on can draw reliable conclusions about comparisons of modern temperatures with the past:

We find that the hemispheric-scale warmth of the past decade for the NH is likely anomalous in the context of not just the past 1,000 years, as suggested in previous work, but longer. This conclusion appears to hold for at least the past 1,300 years (consistent with the recent assessment by ref. 2) from reconstructions that do not use tree-ring proxies, and are therefore not subject to the associated additional caveats. This conclusion can be extended back to at least the past 1,700 years if tree-ring data are used, but with the additional strong caveats noted. When differences in scaling between previous studies are accounted for, the various current and previous estimates of NH mean surface temperature are largely consistent within uncertainties, despite the differences in methodology and mix of proxy data back to approximately A.D. 1000. The reconstructions appear increasingly more sensitive to method and data quality and quantity before A.D. 1600 and, particularly, before approximately A.D. 1000. Conclusions are less definitive for the SH and globe, which we attribute to larger uncertainties arising from the sparser available proxy data in the SH. Given the uncertainties, the SH and global reconstructions are compatible with the possibility of warmth similar to the most recent decade during brief intervals of the past 1,500 years.

The reconstructions, as well as previous published reconstructions, are displayed in a classic “spaghetti graph”:

For a simpler view, the first graph in this post shows just the composited global land+ocean temperature reconstructions using the eiv method. Here’s the composited global land-only reconstructions by the same method:

But I won’t bother to display individual graphs for all regions for all methods. Those who are keenly interested can get the data and look for themselves.