Scientific Stick Check
David R. Legates
08 April 2004
A recent report from the Independent Institute questions whether the climate of the 20th Century was unusual when compared to the variability
over the last millennium.
it has been posited that the 1990s were the warmest decade of the past two
millennia—a direct result of an anthropogenic influence on the earth’s 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. Mann’s 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 earth’s climate over the last two millennia.
But is Mann’s “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
Mann’s 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 Mann’s “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
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 Mann’s 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.
importantly, Mann’s curve shows a 0.95ºC warming during the 20th Century—a
figure that grossly exceeds the IPCC assessment of a 0.6ºC warming during
the same time period. Mann’s 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). Mann’s 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.
range of uncertainty for the pre-A.D. 1400 analysis depends on only one proxy
source—from western North America—and Mann admits that his entire millennial
reconstruction hinges on that single source.
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 Mann’s ‘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.
as well as independent research by Canadian scientists Mr. Steven McIntyre
and Dr. Ross McKitrick, highlights many statistical flaws present in Mann’s
‘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’?”
R. Legates is an Associate Professor and Director of the Center for Climatic
Research at the University of Delaware and a Research Fellow at The Independent Institute. Republished with permission of The Independent Institute.
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