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?
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.
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.
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.
ask Harvard scientists Willie
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.
into the maw of the Mann-loving machine marched Canadians Steve McIntyre
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.
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
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
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
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.,
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.
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
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.
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.”
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.
J., D.C. Frank, and J.S. Wilson, 2004. Climate reconstructions: Low-frequency
ambition and high-frequency ratification. Eos, 85, 133,120.
J., E.R. Cook, and F.H. Schweingruber, 2002. Low frequency signals
in long tree-ring chronologies for reconstructing past temperature
M.E., R.S. Bradley, and M.K. Hughes, 1999. Northern Hemisphere temperatures
during the past millennium: inferences, uncertainties, and limitations.
Geophysical Research Letters,
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.
W., and S. Baliunas, 2003. Proxy climatic and environmental changes
of the past 1,000 years. Climate
Research, 23, 89–110.