Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 196

Figure 6.5 Greenland ice-
core records that show the
pronounced cooling episode
between 8000 and 8400
years ago (i.e. 8200-year
event). All plots are shown
with an approximately
50-year running mean.
Timing of the Medieval
Warm Period (MWP) and
Little Ice Age (LIA) is also
noted for these parameters.
All records are from the
GISP2 ice core except the
methane record (dashed
line), which is from the GRIP
ice core. (Modified from
Alley et al. 1997a )
increased aridity at this time. These proxy data indicate that the 8200-year
event, like other cold events in the Greenland ice-core records, was character-
ized by cold, dry, and dusty conditions in the NH.
The significance of this event is further enhanced by evidence of distinct
climatic change from other proxy records in the NH, thus further supporting
the idea of a synchronous NH to global event. The dating and sampling
resolution of these other records is not as high as that for the Greenland ice
cores, as the timing of the peak of the event spans a millennium with a
duration of 100 to 1000 years or more. Nevertheless, the temporal relation-
ship is supportive of an overall hemispheric synchroneity for this event. For
instance, cold conditions are suggested for northern Sweden and the eastern
Canadian Arctic together with fresh, cool sea-surface conditions in the North
Atlantic. Dry, windy conditions are suggested for the Laurentian Great Lakes
region of North America, as well as in the monsoonal regions of Africa and
Asia. These are the same general conditions indicated by the ice-core records
from Greenland and by other proxy records shown in Figure 6.4 .
Given the abundant evidence for this event within the cyclicity of
Holocene climate, the more elusive problem associated with the 8200-year
event is along the lines of why or what forced this event to occur? This is
especially problematic because none of the other cool periods of the
Holocene appear to be of an equivalent magnitude, at least in the Greenland
ice-core records. One possible explanation is that the cause of this event is the