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Millennial Cycling of Climate in West Africa During the Holocene

Reference
DeMenocal, P., Ortiz, J., Guilderson, T. and Sarnthein, M. 2000. Coherent high- and low-latitude climate variability during the Holocene warm period. Science 288: 2198-2202.
At the end of their paper, DeMenocal et al. (2000) state there is a need "to understand anthropogenic warming within the context of rates and amplitudes of natural late Holocene climate change."

Working with the vertical sediment profile of Ocean Drilling Program Hole 658C -- which was cored off Cap Blanc, Mauritania (20°45'N, 18°35'W) at a water depth of 2,263 meters -- the authors analyzed samples of two centimeters' length (equivalent to 50 to 100 years resolution) for various parameters, including planktonic foraminiferal assemblage census counts, from which they calculated warm- and cold-season sea surface temperatures throughout the entire Holocene, based on transfer functions derived from faunal analyses of 191 other Atlantic core tops.

De Menocal et al. report finding a series of abrupt millennial-scale cooling events that were followed by compensatory warming events that "appear to have involved the entire North Atlantic basin (O'Brien et al., 1995; Keigwin, 1996; Bond et al., 1997; Bianchi and McCave, 1999; Bond et al., 1999), recurred with a ~1500 ± 500 year period throughout glacial and interglacial intervals (O'Brien et al., 1995; Bond et al., 1997; Bianchi and McCave, 1999; Bond et al., 1999), were accompanied by terrestrial climate changes (COHMAP Members, 1988; Gasse and Van Campo, 1999), and involved large-scale ocean and atmosphere reorganizations that were completed within decades or centuries (Alley et al., 1993)."

In considering their findings, the four researchers remark that "these climate perturbations continue to persist during 'our time'," noting that "the most recent of these, the Little Ice Age, ended in the late 19th century," and stating that "some of the warming since that time may be related to the present warming phase of this millennial-scale oscillation," although they add that "the warming in recent decades is unprecedented relative to the past millennium." In viewing the graphical representation of their results, however, it can be seen that the peak warmth of the Medieval Warm Period -- which occurred just a tad before a thousand years ago -- was approximately 1.2°C higher than what their data show for the end of the 20th century. Consequently, instead of saying that "some" of the warming since the end of the Little Ice Age can be attributed to the natural warming that would have been expected to follow on the heels of that particularly cold period, we feel it may be more appropriate to say that the majority of that warming may have been natural and not anthropogenic-induced.

Additional References
Alley, R.B., Meese, D.A., Shuman, C.A., Gow, A.J., Taylor, K.C., Grootes, P.M., White, J.C.W., Ram, M., Waddington, E.D., Mayewski, P.A. and Zielinski, G.A. 1993. Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event. Nature 362: 527-529.

Bianchi, G.G. and McCave, I.N. 1999. Holocene periodicity in North Atlantic climate and deep-ocean flow south of Iceland. Nature 397: 515-517.

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and Glacial climate. Science 278: 1257-1266.

Bond, G., Showers, W., Elliot, M., Evans, M., Lotti, R., Hajdas, I., Bonani, G. and Johnson, S. 1999. The North Atlantic's 1-2 kyr Climate Rhythm: Relation to Heinrich Events, Dansgaard/Oeschger Cycles, and the Little Ice Age. In: Clark, P.U., Webb, R.S. and Keigwin, L.D. (Eds.), Mechanisms of Global Climate Change at Millennial Scales, American Geophysical Union, Washington, DC, USA, pp.35-58.

COHMAP Members. 1988. Climatic changes of the last 18,000 years: Observations and model simulations. Science 241: 1043-1052.

Gasse, F. and Van Campo, E. 1994. Abrupt post-glacial climate events in West Asia and North Africa monsoon domains. Earth and Planetary Science Letters 126: 435-456.

Keigwin, L.D. 1996. The Little Ice Age and Medieval Warm Period in the Sargasso Sea. Science 274: 1504-1507.

O'Brien, S.R., Mayewski, P.A., Meeker, L.D., Meese, D.A., Twickler, M.S. and Whitlow, S.E. 1995. Complexity of Holocene climate as reconstructed from a Greenland ice core. Science 270: 1962-1964.

Archived 5 August 2010