TCS Daily

Same Old Story

By David R. Legates - November 19, 2003 12:00 AM

The conventional wisdom has been that temperatures during the early years of the last millennium (~A.D. 800 to 1300) were relatively warmer -- in what was known as the Medieval Warm Period -- while temperatures decreased during the middle years of the millennium (~A.D. 1400 to 1850) -- during what was known as the Little Ice Age. During the 1900s, temperatures increased as a result of a number of factors, including the demise of the Little Ice Age. Both introductory scientific texts as well as extensive scientific literature confirm these facts.


But in 1999, Dr. Michael Mann of the University of Virginia and his colleagues produced what has now become known as the 'hockey stick' curve -- a representation of the annual temperature for the Northern Hemisphere over the last millennium. This curve, compiled by averaging a number of proxy records (secondary or inferred sources from which assumptions about temperature can be drawn), shows a very slight cooling trend from A.D. 1000 to 1900 with a dramatic warming during the 1900s. This led Dr. Mann, the Intergovernmental Panel on Climate Change (IPCC), and the US National Assessment of Climate Change to assert that the 1990s were the warmest decade of the last millennium with 1998 being the warmest year.


But is the "hockey stick" assumption consistent with the observations? Harvard astrophysicists Dr. Willie Soon and Dr. Sallie Baliunas and their colleagues contend that it isn't. After examining more than 240 individual proxy records analyzed by nearly 1000 researchers, they concluded that taken individually, proxy records offer strong support for the widespread existence of both the Medieval Warm Period and the Little Ice Age and that they do not support the claim that the climate of the 20th Century is unusual when compared to the variability over the last millennium.


So why does Mann's "hockey stick" representation of average Northern Hemisphere temperature fail to retain the fidelity of individual proxy records? Many reasons involve detailed statistical issues, although some are rather obvious and fundamental. For example, Mann contends that the curve represents Northern Hemisphere temperature trends. So why is it that four of the twelve proxy sources used for the pre-A.D. 1400 analysis are from the Southern Hemisphere? Mann also simply affixed thermometer-based estimates for the 1900s to the end of his proxy averages -- a classic apples-versus-oranges comparison -- thereby producing the characteristic 'hockey stick' shape. But the thermometer-based record shows more variability than the proxy records during the 1900s and Mann represents it without the assignment of uncertainty. If the thermometer-based record was not included or if a satellite-based temperature record (where only a small warming trend exists for the late 1900s) were used instead, the claim that the 1990s were the warmest decade becomes unfounded. Even if a reasonable estimate of the error in the thermometer-based record were provided, the claim becomes questionable. Moreover, the range of uncertainty for the pre-A.D. 1400 analysis depends on a single proxy source for western North America; and Mann admits that his entire millennial reconstruction hinges on that single source.


But do proxy records really represent air temperature fluctuations? Most of the analyses on which the "hockey stick" relies are taken from tree-ring cores. Trees, however, respond not only to temperature fluctuations but also to species competition, fire episodes, pest infestations, and droughts. For example, if rainfall is limited, as often is the case in western North America (where the preponderance of data for Mann's pre-A.D. 1400 analysis is located), tree growth is severely restricted, regardless of the temperature conditions. It is impossible under such conditions to discriminate between a cold period and a dry period -- which is why Soon and Baliunas correctly characterized their assessments as "climate anomalies" rather than boldly assert they reflect air temperature fluctuations, as Mann does. Moreover, Dr. Jan Esper of the Swiss Federal Research Institute and colleagues demonstrated that their careful analysis of tree-ring chronologies yields an annual temperature curve for a large portion of the Northern Hemisphere that, unlike the "hockey stick," clearly shows the existence of the Medieval Warm Period and that temperatures during the early years of the millennium were commensurate with those of the 1900s.


These and other more complex issues are fundamental reasons the "hockey stick" is being challenged on scientific grounds by a number of serious scientists. But the IPCC and the US National Assessment of Climate Change continue to demand that policy be based on this flawed and biased research. We must take a closer look at the "science" behind the IPCC and, in this case, ask the question, "How much of the warming of the 20th Century was 'man-induced' and how much of it is 'Mann-induced'?"

David R. Legates is Associate Professor and Director of the Center for Climatic Research at the University of Delaware and Research Fellow at the Independent Institute in Oakland, Calif., publisher of the new report, New Perspectives in Climate Change: What the EPA Isn't Telling Us (


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