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same process as that which forces the 1500-year cycles evidenced by the
marine sediment record (Figure 6.4 ), as described above. An alternative cause
may be the slowdown of the thermohaline circulation system from the influx
of fresh water to the North Atlantic. However, such a ''flooding'' event is not
know, thus there may have been an unknown fresh water event at this time or
the thermohaline circulation system responded in a very large way to a very
small fresh-water event. Ice-marginal lakes known to have existed at that
time would provide a very small amount of fresh water to the North Atlantic
system. Should this have been the case, the 8200-year event may be an
excellent example of the non-linearity of the climate system. As pointed
out by Alley et al.( 1995 ), if the amount of fresh water put into the North
Atlantic by those ice-marginal lakes was sufficient to slow down the thermo-
haline circulation, then the amount of meltwater that may be added to the
North Atlantic from melting of NH ice with anthropogenic warming, is also
of sufficient volume to slow down thermohaline circulation. Consequently,
the nature of the 8200-year event has serious ramifications for future climatic
change.
Medieval Warm Period
There are often suggestions of and reference to two prevalent climatic periods
of very contrasting characteristics over the last millennium, the MWP and the
LIA. Initially, the discussion will focus on the characteristics of the MWP and
why it is argued that a MWP does not exist. Discussion about the LIA then
follows.
The MWP was first labeled by Lamb ( 1995 ) as the period from about the
ninth century to the fourteenth century AD when many places in not only the
NH, but the world as a whole seemed to have felt renewed warmth
(Figure 6.6 ) . Parts of the period, such as during the eleventh and twelfth
centuries, were nearly equivalent to that of the warmest millennium in post-
glacial times (Hughes and Diaz 1994 ). Questions have arisen, however, that
address the synchroneity of the warmer periods and the actual magnitude of
the warmth as a whole. Evidence used to evaluate the existence of the MWP
follows with the major source being continuous paleoclimatic records, such
as those available in tree-rings and ice cores.
One of the most often cited lines of evidence for warm conditions, parti-
cularly in Europe and the northern Atlantic, is the establishment of Norse
colonies in Greenland and over to Newfoundland (see Section 8.2 ). This
evidence adds to an overall suggestion of warmer conditions, particularly in
the tenth to thirteenth centuries, for most of northern Europe, southern
Greenland and Iceland, and basically the North Atlantic, as a whole. More
specifically, there is a persistent warm anomaly in northwestern Europe
annual temperature reconstructions from 1190 to 1350. On the other hand,
 
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