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such caveats in our own work, and indeed sought to use tree ring data that were processed to retain
long-term trends.
While we did agree with the primary conclusion of Pollack et al. that “to obtain a complete
picture of past warming, the differences between the approaches need to be investigated in detail,”
we were skeptical of the very large Little Ice Age cooling indicated by the bore-hole data. It was
inconsistent, we argued, with the few long instrumental records that were available, 5 and we showed
that certain influences, such as changes over time in the extent of insulating winter snow cover, could
adversely impact the borehole estimates. 6 In the years ahead, we would maintain a healthy debate in
the scientific literature with these researchers on the relative merits of the traditional proxy and
borehole estimates. 7 But once again, none of this called into question the basic hockey stick
conclusions regarding anomalous recent warmth.
Taking another tack, prominent paleoclimatologist Wallace Broecker of the Lamont Doherty
Earth Observatory contended that there was a more pronounced medieval warm period than was
indicated by either our work or the other studies cited in the IPCC Third Assessment Report. 8 He
argued that the type of data derived from recent multi-proxy studies (of tree rings, corals, ice cores,
etc.) lacked the precision needed to distinguish the small (roughly 0.5 °C) apparent difference
between medieval and modern hemispheric warmth. Moreover, he argued that proxy reconstructions
of Northern Hemisphere temperatures were contradicted by other regional climate evidence (e.g.,
changes in glacial extent or boreholes drilled in polar ice, and historical accounts of the Viking
colonization of Greenland during medieval times). However, we felt that Broecker hadn't backed up
his claims, in particular his claim that proxy reconstructions lacked the precision to distinguish
temperature differences of 0.5°C. 9 Much of the supposedly contradictory data he cited conflated
regional temperature changes—which often cancel out or differ in timing between regions—with truly
global or hemispheric-scale periods of warming and cooling. Indeed, later work of our own would
provide both evidence and a physical mechanism for why the Viking-colonized regions of southern
Greenland might have been warmer during medieval times than today, even while most other regions
were substantially cooler. 10
The larger context for Broecker's argument, however, was his belief that the medieval warm
period represented the warm phase of an internal millennial climate oscillation. Broecker and his
colleague Gerard Bond had articulated the view that the climate exhibits pronounced natural
oscillations with a periodicity of roughly 1,500 years. 11 Broecker believed that the oscillations were
intrinsic to the climate system, related to the so-called thermohaline or conveyor belt pattern of ocean
circulation that warms the North Atlantic ocean and parts of coastal Europe. Broecker argued that the
action of this climate system component was rather unpredictable and could either oscillate with a
characteristic millennial periodicity or react erratically and abruptly to external influences, such as
changes in solar output, or perhaps human-caused global warming.
Broecker had hinted at the belief that current warming could in substantial part be simply the
most recent warming period in the natural millennial cycle. Even so, human activity was, in
Broecker's view, “poking” an “angry beast” with “sticks.” We had no way of knowing when the beast
might react, and when it did, he suggested, the response might be both extreme and abrupt—the
conveyor belt ocean circulation pattern could suddenly collapse.
The premise appeared to have at least a grain of truth to it. The sinking of surface waters in the
high latitudes of the North Atlantic, by drawing in the warmer surface waters from the south to
 
 
 
 
 
 
 
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