Dark Ages Cold Period

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Noting that "knowledge of natural climatic variability is essential for evaluating possible human impacts on recent and future climate changes," Hu et al. (2001) "conducted multiproxy geochemical analyses of a sediment core from Farewell Lake in the northwestern foothills of the Alaska Range," obtaining what they describe as "the first high-resolution quantitative record of Alaskan climate variations that spans the last two millennia." The team of five scientists say their results "suggest that at Farewell Lake SWT [surface water temperature] was as warm as the present at AD 0-300 [during the Roman Warm Period], after which it decreased steadily by ~3.5°C to reach a minimum at AD 600 [during the depths of the Dark Ages Cold Period]." From that point in time, they say "SWT increased by ~3.0°C during the period AD 600-850 and then [during the Medieval Warm Period] exhibited fluctuations of 0.5-1.0°C until AD 1200." Completing their narrative, they say that "between AD 1200-1700, SWT decreased gradually by 1.25°C [as the world descended into the depths of the Little Ice Age], and from AD 1700 to the present, SWT increased by 1.75C," the latter portion of which warming initiated the Modern Warm Period.

Dropping down to the central part of the conterminous United States, Follett et al. (2004) "used 13C:12C ratio (δ13C) and 14C dating to evaluate relationships and changes in warm (C4) versus cool season (C3) plant signatures with age of soil organic carbon" across "three soil temperature regimes and three soil moisture regimes within the historic grasslands [of the] US Great Plains and Western Corn Belt" for the entire period of the current interglacial or Holocene. They report that their data "indicate a change from C4 plants to increasing C3 plant dominance (as a surrogate of cooler temperature) at ~1,500 yr BP." More specifically, they say that "the yr BP when δ13C was least negative was 1560," and that "δ13C was more negative before or after that time," which puts this significantly colder interval of time right in the middle of the Dark Ages Cold Period, as delineated by the results of the 30-plus other reports of this cold phase of earth's millennial-scale oscillation of climate that we have reviewed and archived in the various Dark Ages Cold Period subsections of the Subject Index.

Moving to the east coast of the United States, Willard et al. (2003) "examine[d] the late Holocene (2300 yr BP to present) record of Chesapeake Bay and the adjacent terrestrial ecosystem in its watershed through the study of fossil dinoflagellate cysts and pollen from sediment cores." They report that "several dry periods ranging from decades to centuries in duration are evident in Chesapeake Bay records." The first of these periods of lower-than-average precipitation, which spanned the period 200 BC-AD 300, occurred during the Roman Warm Period. The next such period (~AD 800-1200) "corresponds to the 'Medieval Warm Period', which has been documented as drier than average by tree-ring (Stahle and Cleaveland, 1994) and pollen (Willard et al., 2001) records from the southeastern USA." In addition, they note that "mid-Atlantic dry periods generally correspond to central and southwestern USA 'megadroughts', described by Woodhouse and Overpeck (1998). Hence, it would appear that the intervening Dark Ages Cold Period was a time of relatively greater wetness throughout much of the United States.

A similar pattern of alternating multi-century wet and dry regimes was found by Campbell (2002), who analyzed the grain sizes of sediment cores obtained from Pine Lake, Alberta, Canada to provide a high-resolution record of climate variability for this part of North America over the past 4000 years. Periods of both increasing and decreasing grain size (moisture availability) were noted throughout the 4000-year record at decadal, centennial and millennial time scales. The most prominent of the major dry periods once again occurred during the Roman Warm Period (abut 900-100 BC) and Medieval Warm Period (about AD 700 to 1300), while the major wet periods occurred during the Dark Ages Cold Period (about 100 BC to AD 700) and Little Ice Age (about AD 1500 to 1900).

In conclusion, throughout many parts of the United States and some of Canada, it would appear that the Dark Ages Cold Period was a time of both relative coolness and wetness, much like the Little Ice Age was in this part of the world.

References

Campbell, C. 2002. Late Holocene lake sedimentology and climate change in southern Alberta, Canada. Quaternary Research 49: 96-101.

Follett, R.F., Kimble, J., Leavitt, S.W. and Pruessner, E. 2004. Potential use of soil C isotope analyses to evaluate paleoclimate. Soil Science 169: 471-488.

Hu, F.S., Ito, E., Brown, T.A., Curry, B.B. and Engstrom, D.R. 2001. Pronounced climatic variations in Alaska during the last two millennia. Proceedings of the National Academy of Sciences, USA 98: 10,552-10,556.

Stahle, D.W. and Cleaveland, M.K. 1994. Tree-ring reconstructed rainfall over the southeastern U.S.A. during the Medieval Warm Period and Little Ice Age. Climatic Change 26: 199-212.

Willard, D.A., Cronin, T.M. and Verardo, S. 2003. Late-Holocene climate and ecosystem history from Chesapeake Bay sediment cores, USA. The Holocene 13: 201-214.

Willard, D.A., Weimer, L.M. and Holmes, C.W. 2001. The Florida Everglades ecosystem, climatic and anthropogenic impacts over the last two millennia. In: Wardlaw, B.R. (Ed.). Paleoecology of South Florida. Bulletins of American Paleontology 361: 41-55.

Woodhouse, C.A. and Overpeck, J.T. 1998. 2000 years of drought variability in the Central United States. Bulletin of the American Meteorological Society 79: 2693-2714.

External Links

CO2Science.org

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