Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 190

this expanded polar vortex highly influences the transport of all chemical
species to Greenland, thus the very high level of variance explained. Without
the prevalence of the expanded polar vortex during the Holocene, a greater
variety of air masses and sources of chemical species were able to reach the
Summit region of Greenland. A greater variety of prevalent circulation
patterns in the NH undoubtedly would lead to a much greater degree of
regionalization during post-glacial times.
Superimposed on the periodicity of Holocene climate is an overall general
trend that is observed not only in the GISP2 glaciochemical record, but is seen
in various records from the smaller ice caps of the Canadian Arctic.
Interestingly, these records, and others we discuss later, provide even more
evidence for the regionalization of NH climate once the influence of the large
ice sheets is removed from the climate system. The GISP2 record essentially
homogenizes NH conditions, and in doing so, a general straight line fit of the
data suggests an overall warming of climate through the Holocene as noted by
the overall decreasing trend in concentration values for most glaciochemical
species and in EOF1 (Figure 6.3 ) . Not all species display this trend, probably
reflecting the multiple controls on the glaciochemical record in combination
with the regionalization of Holocene climate in the NH.
On the other hand, the records from the smaller ice caps show an overall
general cooling through the Holocene (Figure 6.3 , lower right). The small ice
caps in the Canadian Arctic contain more permanent ice than any place but
the Greenland and Antarctica Ice Sheets, thus they also provide an excellent
source of past environmental conditions. Evidence for the overall decrease in
temperatures in the Canadian Arctic comes from the summer melt record for
the Agassiz Ice Cap. The melt record reflects summer temperatures with an
overall decrease in summer temperatures from the early Holocene to the
present, as reflected in the decreasing number of melt layers in the core
with time. Oxygen isotope records from the Penny and Devon Ice Caps,
also in the eastern Canadian Arctic, show an extensive period of early
Holocene warmth with the possibility of maximum warmth being attained
more toward the middle of the Holocene. Mid-Holocene warmth, that is the
Climatic Optimum, was the generally accepted norm in the early work on
Holocene climatic conditions (Figure 6.2 ). Nevertheless, oxygen isotopes
noticeably trend toward more negative values in the latest part of the
Holocene in these same cores, that is the coolest period of the Holocene
according to these records (Figure 6.3 ) .
An additional piece of evidence for identifying the regionalization of post-
glacial climate and the influence of regionalized climate on the record of
post-glacial climatic change comes from comparing the sea salt Na þ record
between the GISP2 and Penny Ice Cap ice cores. The sea salt Na þ time series
in the Penny ice core shows a general decrease in concentrations going from