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Climate Change in the Great Lakes Region of North America
Reference
Sharma, S., Mora, G., Johnston, J.W. and Thompson, T.A.  2005.  Stable isotope ratios in swale sequences of Lake Superior as indicators of climate and lake level fluctuations during the Late Holocene.  Quaternary Science Reviews 24: 1941-1951.

What was done
The authors used δ13C values of Sphagnum remains from peat deposits located along a sequence of beach ridges of Lake Superior to reconstruct changes in regional water balance from about 1000 to 3500 years BP (where elevated δ13C values correspond to wetter conditions), after which they compared their findings with reconstructed water levels of Lake Michigan derived by Baedke and Thompson (2000) from sedimentological studies covering the past 4000 years.

What was learned
Sharma et al. report they "found two maxima of Sphagnum δ13C values in peat deposits developed from 3400 to 2400 years BP and from 1900 to 1400 years BP," which closely match two periods of Lake Michigan high-water stands evident in the lake level record of Baedke and Thompson.  These two periods coincide with the cooler climatic conditions that prevailed on either side of the Roman Warm Period, the earlier of which is unnamed but the most recent of which is the well known Dark Ages Cold Period.  This latter cold high-water period was then followed by a period of low water and declining δ13C values, which coincide with the well known Medieval Warm Period that ultimately gave way to the Little Ice Age.  Thereafter, there are no more δ13C data; but the lake level data reveal a third low-level stand of Lake Michigan from about 600 to 500 years BP that coincides with the Little Medieval Warm Period we have identified in many paleoclimate records.

What it means
The close correspondence between the Sphagnum δ13C values of Sharma et al. and the lake level data of Baedke and Thompson bears strong witness to the reality of the millennial-scale oscillation of earth's climate that has brought the world alternating multi-century periods of relative cold and warmth, which in the Great Lakes region of North America are closely associated with concomitant relative wet and dry conditions, respectively.  It is our belief that this climatic oscillation is also responsible for the development of the Modern Warm Period, which would have occurred when it did even if man had never appeared on the scene.

Reference
Baedke, S.J. and Thompson, T.A.  2000.  A 4700-year record of lake level and isostasy for Lake Michigan.  Journal of Great Lakes Research 26: 416-426.

Reviewed 25 January 2006