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By David Wojick - August 5, 2003 12:00 AM

A central scientific issue in the debate over the sources of changes in the Earth's climate rose to the level of a Senate hearing last week. The hearing was called in an attempt to help determine the answers to some important questions: Was 20th Century warming unusual? Could it have been natural? Was it due to human-induced greenhouse gas emissions and if so how can we know?


These seemingly simple questions mask a very complex problem that is emerging as the most important issue in the global climate change debate.


The debate -- and the hearing -- were prompted by two academic studies that have come to differing conclusions about the nature of global climate change. One study, quite recent, was led by Harvard astrophysicist Willie Soon (Soon et. al.), who regularly writes for TCS. The other, from 1998, was by University of Virginia climate statistician Michael Mann (Mann et. al.). These two studies are the key flashpoints in a larger raging debate within the field called paleoclimatology.


Paleoclimatologists study the history of the Earth's climate and climate changes by using what are called proxy records. Since no thermometers or sensitive temperature-taking equipment existed for most of human history, paleoclimatologists use proxies, like tree rings or ice cores, to determine what the historical climate record was like.


Mann et. al.


The fight is over the so-called "Medieval Warm Period" (MWP) and "Little Ice Age" (LIA). The MWP is thought to have occurred from roughly the year 800 to1300, and the LIA from 1300 to 1900. The debate turns on whether or not the Medieval Warm Period and the Little Ice Age existed and, if so, if they were global phenomena.  If they were, then it's very likely that the 20th century warming may be simply the climate emerging from the Little Ice Age, a perfectly natural process -- not a human induced one.


Advocates of the man-made warming theory, such as Mann and his colleagues, are climate change skeptics -- that is, they believe the historical record says climate has been relatively stable compared to today, and that the 20th century warming is anomalous, due to human influence, and deeply troubling. Their argument is that the local periods of warming and cooling, while large, were not synchronous, so they do not represent true "climate change." In the last ten years they have developed a number of statistical techniques to support their claims.


Mann's 1998 paper was the high point in this scientific movement. Mann claimed to find no vestige of the either the Little Ice Age or the Medieval Warm Period in the paleoclimate record. His sample of that record was very small, just 10 locations. Despite that shortcoming, the Mann et. al. study was central to the 2001 Third Assessment Report of the United Nations Intergovernmental Panel on Climate Change (IPCC), when the IPCC concluded that most of the late 20th century warming was probably due to human influence. There is no overstating how influential the Mann et. al. paper has been within climate change science circles -- and in the public debate beyond.


Soon et. al.


Soon and colleagues published "Reconstructing Climatic and Environmental Changes of the Past 1000 Years: A Reappraisal" back in March in the British journal Energy and Environment. Soon et. al. reviewed about 250 paleoclimate studies and made some interesting conclusions.  They found that the Medieval Warm Period and Little Ice Age were both real and global, albeit with significant regional variations. The authors concluded that "the Medieval Warm Period of 800 to 1300 A.D. and the Little Ice Age of 1300 to 1900 A.D. were worldwide phenomena not limited to the European and North American continents. While 20th century temperatures are much higher than in the Little Ice Age period, many parts of the world show the medieval warmth to be greater than that of the 20th centuries." (Emphasis added.) In other words, the warming of the 20th century is not unprecedented.  There have been earlier periods of warming, taking place long before the industrial revolution and the large emissions of human-induced greenhouse gases into the atmosphere.


In addition to the Soon et. al. study, there is abundant evidence for the existence of both the Medieval Warm Period and Little Ice Age, from many parts of the world. The European Space Agency says that the MWP and LIA were global. It also says the MWP was around one degree C warmer than today.


Moreover, a recent book on the subject, "Geological Perspectives of Global Climate Change," by Gerhard et. al., says:


"That the current rate of temperature increase is not unusual, despite the human-induced addition of CO2, implies that it is not possible to detect a human imprint on earth temperatures."


Climate Debate Rages


As part of the Soon study, the researchers criticized the Mann et. al. study. The Soon group argues that Mann's statistical methods were flawed in such a way that they tended to hide variability, rather than to reveal it. The proxy records that paleoclimatologists use to recreate the climate record are termed "noisy" because the temperature signal is masked by a lot of other variables. Mann's method, according to Soon et. al., tends to amplify this noise.


Mann has his entire professional reputation riding on his study of climate history.  Assessments made by none other than the United Nations rely upon his work.  It was unlikely that he would fail to respond in some way to the Soon et. al. study.  Indeed, that's part of the nature of good scientific exchange and debate.


After the Soon et. al. study was published, Mann, along with twelve other prominent advocates of the theory of human-induced warming, published a pointed rejoinder in the July issue of the journal EOS, setting the stage for the Senate hearing.


At the hearing, and in the pages of EOS, Mann lays great stress on the claim that the 20th century warming is "unprecedented in the last 1000 years" and that this is the consensus view in the scientific community. Indeed, this claim has often appeared in the press since the IPCC voiced it in 2001, based on Mann's paper. Mann's criticism in EOS and at the hearing prompted several misleading press accounts of the ongoing debate.


If the Medieval Warm Period and Little Ice Age, and the cycles before them, did not actually exist, then the recent warming is indeed importantly unprecedented. But their non-existence is by no means settled, as the Soon et. al. study makes abundantly clear. As the Senate testimony from Soon's colleague, David Legates of the University of Delaware, puts it, "we chose... to... determine if the proxy records themselves indeed confirm the claim of the 1990s being the warmest decade of the last millennium.  That claim is not borne out by the individual proxy records."  So much for scientific consensus on the 20th century being the warmest.


There is a prominent group of climate modelers and statisticians who fervently believe that the recent observed warming is human induced. They simply assert that periods such as the Medieval Warm Period didn't happen. As the EOS group puts it, "modeling and statistical studies indicate that such anomalous warmth cannot be explained by natural factors but, instead, requires significant anthropogenic (that is, 'human') influences during the 20th century." The only conclusion to draw from statements such as that is that it matters not what the proxy record says - there can be no Medieval Warm Period.


But much of the broader scientific community, especially many geologists and paleoclimatologists, do not accept this argument. They argue that just because the modelers cannot explain natural climate variability, that does not mean it does not exist. They say they can see it clearly in the data. And they have a point.


David Wojick is an independent science journalist and policy analyst. He has a Ph.D in mathematical logic and the philosophy of science. He also does research on the logic of complex issues, especially the climate change debate. He may be reached at


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