The new hockeystick

Andre · Sep 7, 2008

The new hockeystick is here

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the abstract:

Following the suggestions of a recent National Research Council report [NRC (National Research Council) (2006) Surface Temperature reconstructions for the Last 2,000 Years (Natl Acad Press, Washington, DC).], we reconstruct surface temperature at hemispheric and global scale for much of the last 2,000 years using a greatly expanded set of proxy data for decadal-to-centennial climate changes, recently updated instrumental data, and complementary methods that have been thoroughly tested and validated with model simulation experiments. Our results extend previous conclusions that recent Northern Hemisphere surface temperature increases are likely anomalous in a long-term context. Recent warmth appears anomalous for at least the past 1,300 years whether or not tree-ring data are used. If tree-ring data are used, the conclusion can be extended to at least the past 1,700 years, but with additional strong caveats. The reconstructed amplitude of change over past centuries is greater than hitherto reported, with somewhat greater Medieval warmth in the Northern Hemisphere, albeit still not reaching recent levels.
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They use two methods of reconstruction:

...Most attempts to reconstruct hemispheric temperatures have used some variant on the ‘‘composite plus scale’’ (CPS) methodology (10), in which proxy data (such as tree rings, ice cores, or corals) considered to be sensitive to past surface temperature variations are standardized and centered, potentially weighted, and then composited to form a regional or hemispheric series,...

recently, Hegerl et al. (13) use a weighted composite of proxy temperature series, but scaling is accomplished by a so-called ‘‘error-in-variables’’ (EIV) regression method (‘‘total least squares’’) to allow for errors in both predictors (i.e., proxy composite) and predictand (i.e., the instrumental hemispheric mean temperature series)....
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About those methods they observe:

The skill diagnostics (Fig. 2; see also Dataset S4) for the validation experiments indicate that both the CPS reconstructions (with the screened network) and EIV reconstruction (with the full network) produce skillful NH land reconstructions back to A.D. 400. When tree-ring data are eliminated from the proxy data network, a skillful reconstruction is possible only back to A.D. 1500 by using the CPS approach but is possible considerably further back, to A.D. 1000, by using the EIV approach. We interpret this result as a limitation of the CPS method in requiring local proxy temperature information, which becomes quite sparse in earlier centuries. This situation poses less of a challenge to the EIV approach, which makes use of nonlocal statistical relationships, allowing temperature changes over distant regions to be effectively represented through their covariance with climatic changes recorded by the network.
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So looking at the reconstruction we see the EIV showing a distinct medieval warm period and what also bugs a bit is the addition of the instrumental records, showing a increase of about 1.3 degrees over the last century:

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The normal temperature increase used to be about 0.6 degrees per century (for instance Folland et al.

What also seems to be strange is the increasing deviation between those temperature records and the reconstruction, staying clearly behind in the last part of the graph.

Now see what happens if we remove those "recorded" temperatures:

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See that the temperature range of the reconstructions are indeed close to 0.6 degrees in the last century. So it would be interesting to see where that 1.3 range comes from.

Note also that the reconstructions without those instrumental records do not support the claim:

...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...
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The new hockeystick

Andre Registered Senior Member

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The new hockeystick

Andre Registered Senior Member

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