Geoscience Reference
In-Depth Information
dioxide for photosynthesis, but immersed in the thinner atmosphere of high elevations they may be
somewhat starved of this resource. In a 1993 article, Graybill and Idso had shown that these very
trees might be expected to exhibit a positive growth response to increasing atmospheric carbon
the growth rates of the high-elevation western U.S. trees and the low-elevation boreal tree line
stands: The timing of the divergence was almost perfectly correlated with the exponential rise of
atmospheric carbon dioxide since the early nineteenth century associated with the Industrial
Revolution. The disappearance of the divergence in the twentieth century was consistent with warmth
once again returning as the key factor controlling the growth of the high-elevation trees in the
presence of adequate carbon dioxide.
By correcting for that carbon dioxide effect through comparing otherwise similar trends in low-
make use of the far-longer-term western U.S. data. Indeed, when we used the corrected version of the
western U.S. tree ring data in our analysis, our validation tests gave us the green light; we could
indeed now meaningfully reconstruct Northern Hemisphere average temperatures over the entire past
millennium.
Despite this success, we were rather guarded in our conclusions, recognizing that our ability to
obtain a reliable millennial Northern Hemisphere temperature reconstruction relied heavily on those
western U.S. tree ring data as well as our somewhat
ad hoc
correction for potential CO
2
fertilization
effects. The abstract of our article, entitled “Northern Hemisphere Temperatures During the Past
Millennium: Inferences, Uncertainties, and Limitations,” stated: “We focus not just on the
reconstructions, but the
uncertainties therein
, and
important caveats
,” and “Though
expanded
uncertainties
prevent decisive conclusions for the period prior to
A.D.
1400, our results
suggest
that
the latter 20th century is anomalous in the context of at least the past millennium” (emphasis added).
The expanded uncertainties were a result of additional cross-checks that indicated that the
reconstruction was less effective at capturing century-scale and longer variations in earlier centuries,
which led us to increase the error bars relative to MBH98. Even with these larger uncertainties taken
into consideration, the reconstruction indicated that the 1990s were likely the warmest decade and
1998 likely the warmest year of the millennium.
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article, once again garnered a fair amount of media attention, including a major spread in the Tuesday
science section of the
New York Times
. But perhaps because it represented a cautious and incremental
development relative to our earlier work and appeared in the lower-profile journal
Geophysical
Research Letters
(and wasn't published on Earth Day), it didn't make quite the same splash, at least
at the time. The article, however, would soon gain the attention of climate change contrarians, who
perceived that our findings undermined one of their primary arguments against human-caused global
warming: that a period of warmth comparable to that of the present day had existed in the relatively
recent past, prior to any appreciable increase in greenhouse gas concentrations.
The Third Assessment Report
In order to understand the rise to prominence of the hockey stick, it is necessary to delve into the
