Myth vs. Fact Regarding the "Hockey Stick"

Numerous myths regarding the so-called "hockey stick" reconstruction of past temperatures, can be found on various non-peer reviewed websites, internet newsgroups and other non-scientific venues. The most widespread of these myths are debunked below:

MYTH #0: Evidence for modern human influence on climate rests entirely upon the "Hockey Stick" Reconstruction of Northern Hemisphere mean temperatures indicating anomalous late 20th century warmth.

This peculiar suggestion is sometimes found in op-ed pieces and other dubious propaganda, despite its transparant absurdity. Paleoclimate evidence is simply one in a number of independent lines of evidence indicating the strong likelihood that human influences on climate play a dominant role in the observed 20th century warming of the earth’s surface. Perhaps the strongest piece of evidence in support of this conclusion is the evidence from so-called “Detection and Attribution Studies”. Such studies demonstrate that the pattern of 20th century climate change closely matches that predicted by state-of-the-art models of the climate system in response to 20th century anthropogenic forcing (due to the combined influence of anthropogenic greenhouse gas concentrations and industrial aerosol increases).

MYTH #1: The "Hockey Stick" Reconstruction is based solely on two publications by climate scientist Michael Mann and colleagues (Mann et al, 1998;1999).

This is patently false. Nearly a dozen model-based and proxy-based reconstructions of Northern Hemisphere mean temperature by different groups all suggest that late 20th century warmth is anomalous in a long-term (multi-century to millennial) context (see Figures 1 and 2 in “Temperature Variations in Past Centuries and The So-Called ‘Hockey Stick'”).

Some proxy-based reconstructions suggest greater variability than others. This greater variability may be attributable to different emphases in seasonal and spatial emphasis (see Jones and Mann, 2004; Rutherford et al, 2004; Cook et al, 2004). However, even for those reconstructions which suggest a colder “Little Ice Age” and greater variability in general in past centuries, such as that of Esper et al (2002), late 20th century hemispheric warmth is still found to be anomalous in the context of the reconstruction (see Cook et al, 2004).

MYTH #2: Regional proxy evidence of warm or anomalous (wet or dry) conditions in past centuries contradicts the conclusion that late 20th century hemispheric mean warmth is anomalous in a long-term (multi-century to millennial) context.

Such claims reflect a lack of awareness of the distinction between regional and large-scale climate change. Similar such claims were recently made in two articles by astronomer Willie Soon and co-authors (Soon and Baliunas, 2003; Soon et al, 2003). These claims were subsequently rebutted by a group of more than a dozen leading climate scientists in an article in the journal “Eosof the American Geophysical Union (Mann et al, ‘Eos‘, 2003). The rebuttal raised, among other points, the following two key points:

(1) In drawing conclusions regarding past regional temperature changes from proxy records, it is essential to assess proxy data for actual sensitivity to past temperature variability. In some cases (Soon and Baliunas, 2003, Soon et al, 2003) a global ‘warm anomaly’ has been defined for any period during which various regions appear to indicate climate anomalies that can be classified as being either ‘warm’, ‘wet’, or ‘dry’ relative to ’20th century’ conditions. Such a criterion could be used to define any period of climate as ‘warm’ or ‘cold’, and thus cannot meaningfully characterize past large-scale surface temperature changes.

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