The hockey stick chart
was a biased global
average temperature
reconstruction chart
for the past 2,000 years,
made famous by Al Gore.
It's shows almost no
climate change for
2,000 years, and then
a sharp rise in the
past 100 years,
shaped like
a hockey stick
on the ground,
with the blade
facing up.
Michael Mann used one
small and controversial
subset of tree rings
records.
They were bristlecone
pine cores from high
and arid mountains
in the US Southwest.
The trees are very long-lived,
but grow in highly contorted
shapes, as bark dies back
to a single twisted strip.
The scientists who
published the cone data
(Graybill and Idso 1993)
specifically warned
that tree ring widths
should NOT be used
for global average
temperature
reconstructions.
Mann ignored their advice.
The 20th century portion
of cone data were
unlike the climatic history
of the region, and
were probably biased
by other factors.
They also did not show
global warming.
Mann’s method exaggerated
the significance of the
bristlecones.
He presented them
as representing
the dominant global
climate pattern,
rather than a minor
regional climate pattern.
Mann understated
uncertainties
of his final climate
reconstruction.
He claimed1998 was the
warmest year of the
last millennium, a claim
not supported by data.
Mann also tried hard
to block other researchers
from obtaining his data,
and trying to replicate
his results.
This FOIA issue
had to be resolved
by US Congressional
investigators and
the editors of the
Nature magazine.
They both demanded
full release of his data
and methodologies,
six years after publication
of his original Nature paper.
The bristle cone data
didn't show warming
in the 20th century,
so Mann truncated the
that portion of the
bristlecone data, and
substituted surface
thermometer data
showing a lot of
global warming
in that period.
Viewers of the chart
were not informed that
two completely different
types of temperatures
were used -- a climate
proxy (cones) that showed
little change for about
1,900 years, and then
surface temperature
measurements used
for the last 100 years.
Bristlecones are
sensitive to higher
atmospheric CO2
concentrations
(Graybill and Idso 1993),
possibly because of
greater water-use
efficiency,
(Knapp et al. 2001,
Bunn et al. 2003)
or different carbon
partitioning among
tree parts
(Tang et al. 1999).
Bristlecone
records
are sensitive
to a variety of
environmental
conditions,
other than
temperature,
and should
be avoided
for global
temperature
reconstructions.
But Mann’s results
strongly depend on
bristlecone records.
So his results
are not trustworthy.