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 “Eos” of 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.

(2) It is essential to distinguish (e.g. by compositing or otherwise assimilating different proxy information in a consistent manner—e.g., Jones et al., 1998; Mann et al., 1998, 1999; Briffa et al., 2001) between regional temperature changes and changes in global or hemispheric mean temperature. Specific periods of cold and warmth differ from region to region over the globe (see Jones and Mann, 2004), as changes in atmospheric circulation over time exhibit a wave-like character, ensuring that certain regions tend to warm (due, for example, to a southerly flow in the Northern Hemisphere winter mid-latitudes) when other regions cool (due to the corresponding northerly flow that must occur elsewhere). Truly representative estimates of global or hemispheric average temperature must therefore average temperature changes over a sufficiently large number of distinct regions to average out such offsetting regional changes. The specification of a warm period, therefore requires that warm anomalies in different regions should be truly synchronous and not merely required to occur within a very broad interval in time, such as AD 800-1300 (as in Soon et al, 2003; Soon and Baliunas, 2003).

MYTH #3: The "Hockey Stick" studies claim that the 20th century on the whole is the warmest period of the past 1000 years.

This is a mis-characterization of the actual scientific conclusions. Numerous studies suggest that hemispheric mean warmth for the late 20th century (that is, the past few decades) appears to exceed the warmth of any comparable length period over the past thousand years or longer, taking into account the uncertainties in the estimates (see Figure 1 in “Temperature Variations in Past Centuries and The So-Called ‘Hockey Stick'”). On the other hand, in the context of the long-term reconstructions, the early 20th century appears to have been a relatively cold period while the mid 20th century was comparable in warmth, by most estimates, to peak Medieval warmth (i.e., the so-called “Medieval Warm Period”). It is not the average 20th century warmth, but the magnitude of warming during the 20th century, and the level of warmth observed during the past few decades, which appear to be anomalous in a long-term context. Studies such as those of Soon and associates (Soon and Baliunas, 2003; Soon et al, 2003) that consider only ‘20th century’ conditions, or interpret past temperature changes using evidence incapable of resolving trends in recent decades , cannot meaningfully address the question of whether late 20th century warmth is anomalous in a long-term and large-scale context.

MYTH #4: Errors in the "Hockey Stick" undermine the conclusion that late 20th century hemispheric warmth is anomalous.

This statement embraces at least two distinct falsehoods. The first falsehood holds that the “Hockey Stick” is the result of one analysis or the analysis of one group of researchers (i.e., that of Mann et al, 1998 and Mann et al, 1999). However, as discussed in the response to Myth #1 above, the basic conclusions of Mann et al (1998,1999) are affirmed in multiple independent studies. Thus, even if there were errors in the Mann et al (1998) reconstruction, numerous other studies independently support the conclusion of anomalous late 20th century hemispheric-scale warmth.

The second falsehood holds that there are errors in the Mann et al (1998, 1999) analyses, and that these putative errors compromise the “hockey stick” shape of hemispheric surface temperature reconstructions. Such claims seem to be based in part on the misunderstanding or misrepresentation by some individuals of a corrigendum that was published by Mann and colleagues in Nature. This corrigendum simply corrected the descriptions of supplementary information that accompanied the Mann et al article detailing precisely what data were used. As clearly stated in the corrigendum, these corrections have no influence at all on the actual analysis or any of the results shown in Mann et al (1998). Claims that the corrigendum reflects any errors at all in the Mann et al (1998) reconstruction are entirely false.

False claims of the existence of errors in the Mann et al (1998) reconstruction can also be traced to spurious allegations made by two individuals, McIntyre and McKitrick (McIntyre works in the mining industry, while McKitrick is an economist). The false claims were first made in an article (McIntyre and McKitrick, 2003) published in a non-scientific (social science) journal “Energy and Environment” and later, in a separate “Communications Arising” comment that was rejected by Nature based on negative appraisals by reviewers and editor [as a side note, we find it peculiar that the authors have argued elsewhere that their submission was rejected due to ‘lack of space’. Nature makes their policy on such submissions quite clear: “The Brief Communications editor will decide how to proceed on the basis of whether the central conclusion of the earlier paper is brought into question; of the length of time since the original publication; and of whether a comment or exchange of views is likely to seem of interest to nonspecialist readers. Because Nature receives so many comments, those that do not meet these criteria are referred to the specialist literature.” Since Nature chose to send the comment out for review in the first place, the “time since the original publication” was clearly not deemed a problematic factor. One is logically left to conclude that the grounds for rejection were the deficiencies in the authors’ arguments explicitly noted by the reviewers]. The rejected criticism has nonetheless been posted on the internet by the authors, and promoted in certain other non-peer-reviewed venues (see this nice discussion by science journalist David Appell of a scurrilous parroting of their claims by Richard Muller in an on-line opinion piece).

The claims of McIntyre and McKitrick, which hold that the “Hockey-Stick” shape of the MBH98 reconstruction is an artifact of the use of series with infilled data and the convention by which certain networks of proxy data were represented in a Principal Components Analysis (“PCA”), are readily seen to be false , as detailed in a response by Mann and colleagues to their rejected Nature criticism demonstrating that (1) the Mann et al (1998) reconstruction is robust with respect to the elimination of any data that were infilled in the original analysis, (2) the main features of the Mann et al (1998) reconstruction are entirely insensitive to whether or not proxy data networks are represented by PCA, (3) the putative ‘correction’ by McIntyre and McKitrick, which argues for anomalous 15th century warmth (in contradiction to all other known reconstructions), is an artifact of the censoring by the authors of key proxy data in the original Mann et al (1998) dataset, and finally, (4) Unlike the original Mann et al (1998) reconstruction, the so-called ‘correction’ by McIntyre and McKitrick fails statistical verification exercises, rendering it statistically meaningless and unworthy of discussion in the legitimate scientific literature.

The claims of McIntyre and McKitrick have now been further discredited in the peer-reviewed scientific literature, in a paper to appear in the American Meteorological Society journal, “Journal of Climate” by Rutherford and colleagues (2004) [and by yet another paper by an independent set of authors that is currently “under review” and thus cannot yet be cited–more on this soon!]. Rutherford et al (2004) demonstrate nearly identical results to those of MBH98, using the same proxy dataset as Mann et al (1998) but addressing the issues of infilled/missing data raised by Mcintyre and McKitrick, and using an alternative climate field reconstruction (CFR) methodology that does not represent any proxy data networks by PCA at all.

References:

Cook, E.R., J. Esper, and R.D. D’Arrigo, Extra-tropical Northern Hemisphere land temperature variability over the past 1000 years, Quat. Sci. Rev., 23, 2063-2074, 2004.

Crowley, T.J., and T. Lowery, How Warm Was the Medieval Warm Period?, Ambio, 29, 51-54, 2000.

Esper, J., E.R. Cook and F.H. Schweingruber, Low-frequency signals in long tree-line chronologies for reconstructing past temperature variability, Science, 295, 2250-2253, 2002.

Jones, P.D., K.R. Briffa, T.P. Barnett and S.F.B. Tett, High-resolution palaeoclimatic records for the last millennium: Integration, interpretation and comparison with General Circulation Model control run temperatures, Holocene, 8, 455-471, 1998.

Jones, P.D., Mann, M.E., Climate Over Past Millennia, Reviews of Geophysics, 42, RG2002, doi: 10.1029/2003RG000143, 2004.

Mann, M.E., R.S. Bradley, and M.K. Hughes, Global-scale temperature patterns and climate forcing over the past six centuries, Nature, 392, 779-787, 1998.

Mann, M.E., R.S. Bradley, and M.K. Hughes, Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations, Geophysical Research Letters, 26, 759-762,
1999.

Mann, M.E., Ammann, C.M., Bradley, R.S., Briffa, K.R., Crowley, T.J., Hughes, M.K., Jones, P.D., Oppenheimer, M., Osborn, T.J., Overpeck, J.T., Rutherford, S., Trenberth, K.E., Wigley, T.M.L., On Past Temperatures and Anomalous Late 20th Century Warmth, Eos, 84, 256-258, 2003.

Rutherford, S., Mann, M.E., Osborn, T.J., Bradley, R.S., Briffa, K.R., Hughes, M.K., Jones, P.D., Proxy-based Northern Hemisphere Surface Temperature Reconstructions: Sensitivity to Methodology, Predictor Network, Target Season and Target Domain, Journal of Climate, in press, 2004.

Soon, W., and S. Baliunas, Proxy climatic and environmental changes over the past 1000 years, Climate Research, 23, 89-110, 2003.

Soon, W., S. Baliunas, C, Idso, S. Idso and D.R. Legates, Reconstructing climatic and environmental changes of the past 1000 years, Energy and Environment, 14, 233-296, 2003.