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At the time, we couldn't really address that question. First, our proxy dataset only went back to
A.D. 1400, perhaps close to the transition point between the two intervals, but not firmly back into the
interval generally considered to constitute the medieval warm period. Second, we were not
reconstructing temperature patterns per se , but simply evaluating whether the various proxy records
were varying in a coherent oscillatory fashion. While most of the records were believed to reflect
local temperature changes, we had not explicitly calibrated them against an actual quantitative
temperature scale. To identify the relationship between the behavior in these proxy data and the long-
term changes they implied in surface temperature patterns—in other words, to reconstruct past
temperature patterns from the proxy data—we would need a different approach.
The Medieval Warm Period
Seemingly quite relevant to the issue of whether modern warming might be natural in cause, at least in
substantial part, was the purported existence of a period in our not-so-distant preindustrial past
characterized by warmth rivaling that of the present day. Evidence of such warmth would not, in and
of itself, necessarily contradict a human role in the current warming; after all, that proposition, as we
have seen, is based on multiple lines of evidence. However, if warmth less than a thousand years ago
rivaled modern warmth, it might seem to support a far larger role for natural climate variability, and
the possibility that a large fraction of the current warming could itself be natural.
Investigations into the evidence for a medieval warm period begin with the work of Hubert
Lamb (1913-1997), a prominent British scientist who founded the University of East Anglia's
Climatic Research Unit. Lamb's seminal work on the climate changes of past centuries has,
unfortunately, been among the most misunderstood and most misrepresented in all of
paleoclimatology.
In the 1960s, Lamb attempted to estimate past temperature changes in central England from
various forms of qualitative and, in some cases, rather anecdotal information (such as reports of
vineyards in southern Britain and Vikings in Greenland during medieval times, reports of frost fairs
on the Thames, and icebergs off the coast of Norway during later centuries). 10 From the 1960s to the
1980s, Lamb produced various versions of a curve depicting temperature variations over the past
thousand years. The curve was at best a rough approximation and was certainly not offered by Lamb
as a quantitative reconstruction of past temperature changes (with the exception of the portion of the
record since the seventeenth century, which was based on actual thermometer measurements from
central England). Nor was the curve ever supposed to represent temperature variations outside one
small subregion of Europe. All of that notwithstanding, the Lamb curve was the only one available
purporting to depict temperature changes over past centuries at the time of the IPCC First Assessment
Report in 1990, wherein it had been featured.
The Lamb curve appeared to suggest a warm period between roughly A.D. 1000 and 1400 during
which temperatures in central England were rather warm, followed by an interval of relatively cold
conditions from roughly 1500 to 1900, the Little Ice Age. The medieval warm period was depicted as
even warmer than the present by a few tenths of a degree in Lamb's estimate. This latter feature
would be celebrated for decades to come by climate change contrarians who would continue to point
to the Lamb curve as evidence that modern warming is not unusual, even as Lamb's work was
 
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