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average temperature.
That single aspect of our work got all the attention. In truth, it did also represent an advance.
Compared to previous Northern Hemisphere temperature reconstructions, ours went further back, it
resolved individual years rather than just decades, and it had error bars associated with it. Thus, we
were able to draw the specific conclusion that, even taking into account the margin for error, three
recent years—1990, 1995, and 1997—all appeared warmer than any other in the past six centuries.
Our study, which has come to be known as “MBH98” for the authors—Mann, Bradley, Hughes—
was published in Nature on April 22, 1998—Earth Day. 17 I was caught completely off-guard by the
amount of media attention the article received. Generally, one is lucky to get a nibble or two from the
local media in response to press releases on a published scientific paper. This time was different. No
sooner had the press releases gone out (one from U. Mass, another from Nature , and a third from the
National Science Foundation) than the phone calls began coming in nonstop. Our study was written up
in the New York Times , USA Today , Boston Globe , and a host of other major U.S. newspapers.
Articles soon appeared in Time magazine and U.S. News and World Report . We even made it into
Rolling Stone (though not the cover). I was asked in one afternoon to do television interviews with
CNN, CBS, and NBC. In the CBS interview, John Roberts put it to me bluntly: “So does this prove
humans are responsible for global warming?” He repeated the question at least three times during the
interview, clearly not having gotten the money quote he was fishing for. I wouldn't take the bait. I
repeated that our results were “highly suggestive” of that conclusion, but I wouldn't go further than
that. I well knew that establishing that recent warming is anomalous in a long-term context alone did
not establish that human factors were responsible for it. Any conclusion about causality required the
use of climate models to estimate the relative contributions of the various factors, including human
increases in greenhouse gas concentrations, hypothesized to be responsible for the observed changes.
There are several reasons that our paper might have received more than the usual expected
attention. The globe had experienced record-breaking warmth that winter. The first three months of
1998 were the warmest on record (in western Massachusetts, where I was living, it barely felt like
we'd had a winter). Temperatures had likely been spiked, to some extent, by a fluke of nature, a
particularly large El Niño event. It was in part due to this fluke that the 1998 record for the hottest
year in the instrumental record had still not unambiguously been broken by 2011 (one group has 2005
and 2010—in a statistical tie—narrowly beating it out, but another group has 1998 still holding the
title). 18 In any case, 1998 was as of that date the warmest year in the instrumental record; with the
advent of our study, “warmest on record” meant not just in 150 years but in at least 600!
That our paper coincidentally happened to be published on Earth Day no doubt gave journalists
an extra news hook to cover the study. Some commentators attached an almost diabolical significance
to the timing, as if Nature was somehow conspiring with the world's environmental activists. The
truth is much less interesting; the publication date at Nature is determined by the date of a paper's
final acceptance and placement in the journal's publication queue.
Extending the Handle of the Stick
The original MBH98 hockey stick had a comparatively short “handle,” 19 extending back six centuries,
and did not reach back far enough to establish whether a medieval warm period actually existed, let
 
 
 
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