Africa's Medieval Warm Period

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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, DeMenocal et al. (2000) 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. This work revealed a series of abrupt millennial-scale cooling events 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, 1994), and involved large-scale ocean and atmosphere reorganizations that were completed within decades or centuries (Alley et al., 1993).” The four researchers remark, “these climate perturbations continue to persist during ‘our time’.” With respect to the MWP, they state it was “marginally warmer than present.”

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 Mechanisms of Global Climate Change at Millennial Scales, edited by P.U. Clark, R.S. Webb, and L.D. Keigwin, 35–58. Washington, DC: American Geophysical Union.

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

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.

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.

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