A New Take on an Old Millennium

The subject of reconstructions of temperature variations of the past millennium has been discussed many times before on this site (see e.g. here, here, here, and here). Despite the apparent controversy, the basic conclusion–that the global and hemispheric-scale warmth of the past few decades appears anomalous in a very long-term context–has stood up remarkably well in many independent studies (see Figure 1).

Temperatures over the Past 1000 Years

Figure 1. Reconstructions of Northern Hemisphere temperatures for the last 1000 years (various colored curves) compared with instrumental record (black curve).

[source: Wikipedia] (click to enlarge)

This is not to say that all estimates agree in their details. Indeed, there is a fair scatter among the various published estimates. Some of these differences are believed to reflect differences in seasonality and spatial emphasis. Past summer, extratropical temperature changes appear, for example, to have have differed significantly from annual temperature changes over the entire (tropical and extratropical) Northern Hemisphere, and tropical Pacific Sea Surface Temperatures appear to have varied oppositely with temperatures in the extratropical regions of the globe. See for example the review paper by Jones and Mann (2004), in particular the discussion and references in section 5.3 therein. Some differences appear to be related to the particular method used to “calibrate” the proxy data against modern instrumental records. While the various methods can be tested with climate model simulations , it would arguably be more satisfying if inferences could be obtained in a manner which bypasses the difficult issue of calibration entirely, and also eliminates any need to establish the precise seasonality of information reflected by the various available proxy records.

This is what Osborn and Briffa have done in their article “The Spatial Extent of 20th Century Warmth in the Context of the Past 1200 Years”, which appears in the Feb 10 issue of the journal Science. The article uses a rigorous statistical methodology to re-examine the question of whether late 20th century warmth is anomalous in the context of the past 1200 years. This is done in a manner that does not require the explicit calibration of the proxy records. In essence, the authors have revisited a question posed earlier in a paper by Willie Soon and Sallie Baliunas (2003: see our previous discussion here), investigating whether or not evidence from past proxy records of temperature support the existence of past intervals of warmth with the widespread global scale of 20th century warming. The Soon and Baliunas (2003) paper was heavily criticized in the scientific literature (e.g. Mann et al, 2003) for failing to distinguish between proxy evidence of temperature and drought or precipitation, and for not accounting for whether temperature anomalies in different regions were contemporaneous or not.

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