Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 201

comparisons with controlled runs from a coupled model, Crowley found
that natural multiple forcing factors such as changes in solar irradiance and
volcanic eruptions (e.g. Zielinski 1995 ) account for 41 to 64% of the decadal-
scale variability in climate over the last 1000 years prior to anthropogenic
time (pre-1850). Recent warming above the suspected natural variability
within the system suggests that greenhouse gas forcing is responsible for
that recent warming (Jones and Mann 2004 ). Clearly, the regionalization of
Holocene climate in the NH is a function of many forcing factors operating on
different time scales. These factors become even more prevalent once the
impact of the large ice sheets is removed with deglaciation.
6.3.5 Conclusions
Post-glacial climate (i.e. the Holocene or the last 11 500 years) in the NH has
undergone a series of fluctuations between generally cool to cold conditions
and warm or mild conditions. Although these fluctuations are not of the
same magnitude as those that occurred during the last glacial (i.e.
Dansgaard-Oeschger Events), they are significant in light of the overall
warmer climate of the Holocene. These cyclical changes occur on the order
of every 1500 to about 2500 years depending on the record used for recon-
struction. Nevertheless, there does appear to be a millennium-scale compo-
nent to Holocene climatic change. Periods when colder conditions were
prevalent are 8400-8000, 5900-5100, 4200-3800, 3100-2400 and 600 BP.
However, superimposed on these cycles of Holocene climatic change is an
overall general trend to climatic conditions that varies depending on the
records observed. Greenland ice-core records suggest a slight warming
trend through the Holocene, whereas records such as those from ice caps in
the Canadian Arctic suggest an overall general cooling through the Holocene.
These differences are probably a reflection of the regionalization of post-
glacial climate in the NH once the influence of the large ice sheets is gone.
Greenland records reflect general conditions in the NH while other proxy
records may reflect more regional conditions. Similarly, other highly
resolved records do not show the same magnitude of change in a particular
cool event as the ice-core or marine records may show, or the timing of the
environmental change may be different, again reflecting the temporal and
spatial variability in environmental conditions with regionalization of NH
climate during the Holocene.
Specific time periods during the Holocene further reflect the complex
nature of the post-glacial climate. The 8200-year cooling event is found in
many proxy records from the NH, although it is especially prevalent in the
Greenland ice-core records. Its significance comes from the high magnitude
of the signals that characterize it, a magnitude that is about half that of the