Geoscience Reference
In-Depth Information
Figure 4.2: Competing Estimates of Millennial Temperature Trends
The graph compares different reconstructions of Northern Hemisphere temperature trends over the past thousand years as shown in the
2001 IPCC
Third Assessment Report
. Shown for comparison is the instrumental record from the mid-nineteenth to late twentieth
centuries. The data have been smoothed to emphasize multidecadal and longer timescale variations.
The Implications
Though we didn't quite realize it at the time, the gauntlet had been laid down with the initial
publication of the extended MBH99 millennial hockey stick and, especially, its subsequent
prominence in the 2001 IPCC report. We had taken on a sacred cow of climate change contrarianism:
the medieval warm period (MWP). Our reconstruction did not “eliminate the MWP,” as our
detractors liked to claim.
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It did in fact include a period of relative warmth during the medieval era
of the eleventh to fourteenth centuries. While medieval conditions were relatively warm, however,
the modern tip of the blade—as can be seen in the graphic featured in the prologue—was warmer than
the peak reconstructed medieval warmth.
One pillar of the contrarian case against human-caused climate change was that the mere
existence of a warmer period centuries earlier somehow disproved any human influence on modern
warmth. In reality, this was not true. Scientists had known for some time, for example, that there were
periods in the deep geological past during which temperatures were warmer than today, such as in the
mid-Cretaceous period 100 million years ago. The reason was atmospheric CO
2
concentrations that
were several times higher than today owing to the slow geological processes that modify atmospheric
composition on very long timescales. It was indeed possible that other natural factors, be they
changes in solar output or volcanic activity, could have led to conditions that were as warm as today.
Whether conditions in past centuries might have been warmer than today, then, would not have a
scientific bearing on the case for the reality of human-caused climate change. That case, as we've
seen, rests on multiple independent lines of evidence: the basic physics and chemistry of the
greenhouse effect, for example; the relationship between greenhouse gas concentrations and
temperatures over geological time; and a pattern of observed climate change that can only be
explained by climate models when human-produced greenhouse gases are included in the
calculations. While the presence of a medieval warm period warmer than today would not negate the
reality of modern human-caused climate change, evidence of its absence nonetheless would take away
an important (if misguided) talking point of contrarians—something of which they were well aware.
Our finding that recent warming was anomalous in a long-term (now, apparently, millennial)
context was suggestive of the possibility that human activity was implicated in the warming. I was
always very careful not to claim that our work could firmly establish a human role in the warming. To
draw such a conclusion based on our work alone would necessarily buy into the classic logical
fallacy of “correlation without causation.” We had established correlation—the anomalous warming
that we documented coincided with the human-caused ramp-up in greenhouse gas concentrations—but
we hadn't established causality.
A little more than a year after we had published our millennial hockey stick reconstruction,
paleoclimatologist Thomas Crowley of Texas A&M University (and coauthor of the Crowley and
