Bending the Hockey Stick

University of Virginia paleoclimatologist Michael Mann’s 1,000-year reconstruction of global temperature is pretty easy to visualize. There’s a 900-year-long handle across the bottom of the graph then, boom, a sharp spike in temperature during the last hundred years. It resembles the blade on the end of a hockey stick. A recent alternative reconstruction finds evidence of a series of warm and cold eras plus a period of high temperatures a thousand years ago rivaling today’s warmth. In between there are a gradual descent into and recovery from lower temperatures in intervening centuries. So, does a hockey stick or city skyline more accurately represent a thousand years of temperature history?
     After splicing “observed” temperature history onto the end of his “reconstructed” history, Mann’s graph culminates in 1998’s record high. What makes this controversial (besides his use of such a dubious splice) is the virtual absence of the Medieval Warm Period and the Little Ice Age along the “handle” — two widely-accepted climate phenomena that demonstrate the natural variability of earth’s climate. That debate is not confined to scientific literature. It echoes in Capitol Hill’s marbled halls and ornate hearing rooms, off the accordion walls of hotel ballrooms where scientific and professional conferences take place, in newspaper headlines, and in the classrooms, lunchrooms, boardrooms or anywhere climate science is discussed.
     What’s the big deal? The hockey stick’s “blade” bolsters claims that human industrial activity causes climate to act in “unnatural” ways. The controversy borders on “the personal” because it discredits voluminous research that clearly identifies a Medieval Warm Period a thousand years ago and a subsequent Little Ice Age.
     Because Mann’s reconstruction instantly was adopted as an article of faith by environmental advocacy groups and within the United Nations climate community, and was used by scientists who produced the U.S. National Assessment of global warming during the Clinton/Gore Administration, any challenge to it receives swift retaliation.
     Just ask Harvard scientists Willie Soon and Sallie Baliunas.
     Their peer-reviewed article in Climate Research concluded the climate of the 20th century is not unique within the context of the last 1,000 years. There were howls of protest. Several of the journal’s editors resigned. Rumors circulated that there would be an organized boycott of Climate Research. The vicious attack and petulance of its execution shocked the normally staid scientific community, but Climate Research remains popular with climate researchers and its readers, editors, and authors.
     Then into the maw of the Mann-loving machine marched Canadians Steve McIntyre and Ross McKitrick.
     Their paper presented data quality issues and methodological errors within some of Mann’s analyses, leading them to preliminarily conclude that the temperatures of the 20th century are not at all unique. McIntyre and McKitrick continue to explore these issues and promise more results in the near future.
     “Well,” sniff the paleoclimatologists, “these people don’t have our training or experience and aren’t qualified to undertake such examinations and make conclusions such as these,” an attitude so apparent in Mann’s testimony before the Senate Environment and Public Works Committee, last summer that its chairman, Sen. James Inhofe (R-OK), publicly took exception.
     If Mann and his partisans really believe this to be the case, then how do they the work of well-published paleoclimatologists Jan Esper, David Frank, and Robert Wilson in the March 23, 2004, edition of Eos—Transactions of the American Geophysical Union.
     Esper was lead author of a paper in Science in 2002 that presented a 1,000-year temperature history of the Northern Hemisphere quite different from Mann’s: It featured both a pronounced Medieval Warm Period and a Little Ice Age. In fact, Esper’s 2002 reconstruction showed a temperature change that is about twice that in Mann’s record. Figure 1 provides a comparison of the two. The temperature variations apparent in the Esper’s reconstruction (blue line) are absent in Mann’s (red line).

Figure 1. A comparison of 1,000-year temperature reconstructions. The red line is the temperature history of the Northern Hemisphere as developed by Mann and colleagues, a.k.a. “the Hockey Stick.” The blue line represents the Northern Hemispheric temperature history as constructed by Esper’s research team (source: Esper et al., 2002).

     Both Esper and Mann give reasons for the difference in their reconstructions. Those difference now are fully described in Esper’s Eos paper replete with data and analysis issues. Esper goes through each and performs tests to assess their influence on the reconstructions. He basically eliminates all the possibilities except the technique used to process tree-ring data sets — the primary information relied on to construct early portions of the temperature reconstructions.
     The problem with tree rings appears to be that their variations reflect more than year-to-year climate differences (temperature and/or precipitation). As the trees age, tree-ring production changes and introduces a spurious trend in the tree-ring series. This aging effect differs among tree species, as well as within species, depending on the trees’ growing conditions (soil type, elevation, slope aspect, etc.). It becomes difficult to separate trends due to aging from those due to climate.
     Although various research groups use different techniques to account for this problem, the absence of ground truth (true temperature) makes impossible to ascertain whose technique is best. Esper uses a method aimed at retaining long-period (greater than a century or so) variations in the tree-ring records, whereas Mann uses a method that virtually eliminates all long-term variation.
     Esper shows how the differences in the two reconstructions are very well explained by the difference in the way the tree-ring data are handled, but stops short of proclaiming which methodology is preferable (although it seems obvious by his choice). He prefers to succinctly say, “Higher-frequency [decadal] climate variations are generally better understood than lower-frequency variations.”
     Because Esper’s analysis reproduces the Medieval Warm Period and Little Ice Age, it renders 20th-century temperatures much less unusual than does Mann’s analysis. This latest development demonstrates that “the science” is far from settled on this issue. Those who deny this fact and deride the work of Soon, Baliunas, McIntyre and McKitrick are selectively ignoring a growing body of evidence. One can only hope in light of this development that the unseemly fuss over mere publication of such papers can end.

References
Esper J., D.C. Frank, and J.S. Wilson, 2004. Climate reconstructions: Low-frequency ambition and high-frequency ratification. Eos, 85, 133,120.

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

Mann, M.E., R.S. Bradley, and M.K. Hughes, 1999. Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations. Geophysical Research Letters, 26, 759–762.

McIntyre, S., and R. McKitrick, 2003. Corrections to the Mann et. al. (1998) Proxy database and Northern Hemispheric average temperature series. Energy & Environment,14, 751-771.

Soon, W., and S. Baliunas, 2003. Proxy climatic and environmental changes of the past 1,000 years. Climate Research, 23, 89–110.