Recently, it has been posited that the 1990s were the warmest decade of the past two millenniaa direct result of an anthropogenic influence on the earths climate. This view, which has been adopted by global-warming activists and some scientists and loudly touted by many in the media, first appeared in the late 1990s when Dr. Michael Mann, of the University of Virginia, and his colleagues began to challenge the longstanding scientific evidence that global air temperatures have been quite variable. Manns claim is that the annual temperature for the Northern Hemisphere was nearly constant until 1900; then human activity caused a dramatic warming, particularly during the late 1900s, to produce a curve with a shape resembling an upturned hockey stick.
Then, in a chapter he co-authored in the UN Intergovernmental Panel on Climate Change (IPCC) report, Mann inserted his claim, thus giving the appearance that it is widely accepted in the scientific community. More recently, Mann and his colleagues have extended their hockey stick back to A.D. 200 and to cover both Hemispheres, suggesting now that humans were the sole cause of any variability in the earths climate over the last two millennia.
But is Manns hockey stick consistent with the data? Along with my colleagues, Harvard astrophysicists Dr. Willie Soon and Dr. Sallie Baliunas, I contend that it is not.
After examining more than 240 individual proxy records analyzed by nearly 1000 researchers, we concluded that taken individually, proxy records offer strong support for the widespread existence of both a Medieval Warm Period (~A.D. 800 to 1300) and a Little Ice Age (~A.D. 1400 to 1850), undermining Manns conclusion that the climate of the 20th Century was unusual when compared to the variability over the last millennium. (Proxy records are secondary or inferred sources from which assumptions about air temperature may be drawn).
So why does Manns hockey stick representation fail to retain the fidelity of individual proxy records? Many reasons involve detailed issues of statistical climatology, although some are fairly obvious and fundamental. Here are a few:
- While Mann contends that his curve represents Northern Hemisphere temperature trends, four of the twelve proxy sources used for the A.D. 1000 to 1400 analysis are from the Southern Hemisphere. In a paper published last month (in Geophysical Research Letters), we took a closer look at Manns attachment of thermometer-based estimates to the end of his proxy averages, which produces the characteristic hockey stick shape, and found we could not reproduce his values for the latter end of the 20th Century, although we could easily reproduce the results produced by others.
- More importantly, Manns curve shows a 0.95ºC warming during the 20th Centurya figure that grossly exceeds the IPCC assessment of a 0.6ºC warming during the same time period. Manns claim that the 1990s were the warmest decade becomes completely unfounded if the IPCC figure is used instead (as Mann had used on some of his earlier graphs). Manns claim is also unfounded if his own proxy reconstruction or the satellite-based temperature record (which shows only a slight warming during the late 1900s) is used.
- The range of uncertainty for the pre-A.D. 1400 analysis depends on only one proxy sourcefrom western North Americaand Mann admits that his entire millennial reconstruction hinges on that single source.
- Other researchers, including Dr. Jan Esper of the Swiss Federal Research Institute and colleagues, have demonstrated that a careful analysis of tree-ring chronologies yields an annual temperature curve for a large portion of the Northern Hemisphere that, unlike Manns 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.
Our analysis, as well as independent research by Canadian scientists Mr. Steven McIntyre and Dr. Ross McKitrick, highlights many statistical flaws present in Manns hockey stick reconstruction, which is why it is being challenged on scientific grounds by a number of serious scientists. We must take a closer look at the science of the hockey stick 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?