Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 195

this event is its high magnitude, as observed in many proxy records, and the
apparent lack of a definitive cause of the event. Following this discussion will
be a more specific evaluation of climatic conditions over the last millennium.
In particular, the discussion will focus on the MWP and the LIA, two
contrasting climatic periods over the last 1000 years. The significance of
these two climatic periods centers on whether or not they should actually be
called distinct climatic periods and if so, when did they occur and what
caused them to occur. It is especially critical to understand our climate system
over the last 1000 years if we are to predict what could happen in the future
and the magnitude of the human influence on climate.
The 8200-year BP event
As presented above, there are several periods of cool climate conditions
during the Holocene occurring roughly every 1500 years. However, the
period that, in general, may be greatest from the perspective of the magnitude
of the signal in proxy records, is the first of these events, that is, the 8200-year
event. In fact, in the Greenland ice cores, the signal of this event in many of
the individual parameters measured is about half that of the Younger Dryas.
None of the other cool events are of that relative magnitude. Adding to its
overall significance as a climatic event is the fact that evidence of the event is
found in other proxy records in the NH, as well as globally. A summary of the
key pieces of evidence that demarcate the event and its significance in
the overall scheme of Holocene climatic change is taken primarily from
the summary by Alley et al.( 1997b ).
Several key parameters in the Summit Greenland ice-core records provide
evidence of climatic conditions in the NH during the 8200-year event. The
overall signal in these parameters (Figure 6.5 ) generally covers the period
from about 8400 to 8000 years ago, but the peak is typically at about 8250
years ago. The characteristics of the event are the same as other cold events,
such as the Younger Dryas and stadials within the last glacial, as indicated by
the following characteristics. Accumulation during the peak of the 8200-year
event is about 20% lower than baseline conditions at 8400 and 8000 years ago
indicating dry conditions on a local to regional scale around Greenland.
Oxygen isotope ( 18 O) values show a decrease of about 2 per mil over the
same baseline, which translates into a cooling of 6 2 8C, based on the
calibration of Cuffey et al.( 1995 ). Concentrations in Na þ ,Cl and Ca 2 þ
are about 60% above this baseline indicating overall more vigorous circula-
tion systems and the enhanced transport of sea salt species (Na þ ,Cl ) and
dust from continental areas (Ca 2 þ ). Greater Ca 2 þ concentrations also reflect
more availability of dust for transport, and thus more aridity. Decreased
methane production, because lesser amounts of the NH were in wetlands,
and evidence for increased forest fire activity also support the contention of