Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 200

Given these general climatic conditions, it may be summarized that
coldest climatic conditions in the NH occurred 1590s-1610s, 1690s-1710s,
1800s-1810s and 1880s-1900s. Between these periods of cooler conditions
synchronous warm conditions during the LIA are found in the 1650s, 1730s,
1820s and more recently in the 1930s and 1940s. Nevertheless, the use of the
LIA to describe climatic conditions over most of the last six centuries is
generally valid.
6.3.4 Climate forcing
The variability in Holocene climate from both a temporal and spatial per-
spective is reflected in the factors that have caused these changes. As alluded
to in the discussion about the periodicity of climatic fluctuations in post-
glacial times, there is an unknown factor that produces these approximately
regular millennial-scale cycles. However, there are other forcing factors that
have been shown to contribute to the variability in NH climate. Mayewski
et al.( 1997 ) used a series of band-pass filters to construct a series of band-
pass components (i.e. a type of spectral analysis) on the complete 110 000-
year glaciochemcial record from the GISP2 ice core. Although there are
components of the record with periodicities beyond the 11 500-year
Holocene record, Mayewski et al. were able to identify components of the
climate system that force Holocene climate, such as the 6100-year periodicity
that maintains itself through the Holocene. This component of the climate
system appears to coincide with iceberg discharge and ocean surface water
cooling events including those found during the Holocene that demarcate the
LIA and other cooling events (Figures 6.3 and 6.4 ). It appears that this
periodicity, as well as the less prominent 4500- and 3200-year periodicities
in the record, could be a function of suborbital-scale changes in insolation,
thus the likelihood of an orbital component that forces Holocene climate.
In addition to an orbital forcing component, solar variability also plays a
role in Holocene climate variability, as indicated by the presence of a 2300-
year periodicity together with the 1450-year cycle. In fact, the 1450-year
cycle may be related both to cycles in orbital parameters and to the solar
cycle. However, feedback mechanisms, that is changes in atmospheric
circulation associated with changes in sea ice extent and thermohaline
circulation, may be produced with insolation and with solar variability
(Mayewski et al. 1997 ).
Evaluation of factors that may force Holocene climate was also done for
more recent time periods. Crowley ( 2000 ) modeled climatic conditions for
the last millennium, including the MWP and the LIA, to determine the role
that various climatic forcing mechanisms had in controlling changes over
that time frame. Using an energy balance climate model verified through