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Millennial-Scale Climate Variation at Chesapeake Bay, USA
Reference
Willard, D.A., Bernhardt, C.E., Korejwo, D.A. and Meyers, S.R.  2005.  Impact of millennial-scale Holocene climate variability on eastern North American terrestrial ecosystems: pollen-based climatic reconstruction.  Global and Planetary Change 47: 17-35.

What was done
As scientists seek to determine the climatic impacts of rising anthropogenic greenhouse gas concentrations, it is critical that they first obtain a firm understanding of the forcings and feedback factors that contribute to earth's natural climate variability.  Only then can they honestly assess the likelihood that an anthropogenic influence may be evident in historical temperature data.  Consequently, and in an attempt to advance our knowledge of natural climate variability, Willard et al. utilized pollen assemblages identified in four sediment cores extracted from the mainstem of the USA's Chesapeake Bay as a proxy for the winter temperature of this region over the past 10,000 years.

What was learned
Multi-taper harmonic and power spectral analyses of the pollen data revealed five highly significant centennial- to millennial-scale oscillations with periods of 148, 177, 282, 521 and 1429 years, the troughs of the latter of which oscillations are temporally correlated with relatively prolonged minima in Pinus abundance and represent winter temperature declines of up to 2C.  The most recent such minimum was associated with the Little Ice Age and represented a two-stage event.  The first and more severe low-temperature stage occurred between 650 and 550 years BP, while the second occurred between 450 and 350 years BP.

With respect to the cause of the 1429-year millennial-scale oscillation, Willard et al. note that the climate cycle correlates well with a similar-scale cycle of solar activity evident in cosmogenic isotope records.  In addition, they say it is well correlated with proxy climate cycles found in records from Greenland, the North Atlantic and Alaska, which have also been shown to be correlated with cyclical changes in solar activity.

What it means
Evidence for an approximate 1500-year solar-driven climate cycle continues to grow, suggesting to us that the warming that has occurred since the end of the last cold stage of the Little Ice Age is most likely natural in origin.

Reviewed 12 October 2005