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The Medieval Warm Period and Little Ice Age in Western Equatorial Africa
Reference
Ngomanda, A., Jolly, D., Bentaleb, I., Chepstow-Lusty, A., Makaya, M., Maley, J., Fontugne, M., Oslisly, R. and Rabenkogo, N. 2007. Lowland rainforest response to hydrological changes during the last 1500 years in Gabon, Western Equatorial Africa. Quaternary Research 67: 411-425.

What was done
The authors derived high-resolution (<40 years) paleoenvironmental reconstructions for the last 1500 years based on pollen and carbon isotope data obtained from sediment cores retrieved from Lakes Kamalete (043'S, 1146'E) and Nguene (012'S, 1028'E), which are located about 200 km apart from each other in the lowland rainforest of Gabon.

What was learned
"After a sharp rise at ~1200 cal yr BP," in the words of Ngomanda et al., "the A/H [aquatic/hygrophytic] pollen ratios showed intermediate values and varied strongly from 1150 to 870 cal yr BP, suggesting decadal-scale fluctuations in the water balance during the 'Medieval Warm Period'." Thereafter, lower A/H pollen ratios "characterized the interval from ~500 to 300 cal yr BP, indicating lower water levels during the 'Little Ice Age'." In addition, the nine researchers report that "all inferred lake-level low stands, notably between 500 and 300 cal yr BP, are associated with decreases in the score of the TRFO [Tropical Rainforest] biome."

What it means
Ngomanda et al. state that "the positive co-variation between lake level and rainforest cover changes may indicate a direct vegetational response to regional precipitation variability," noting that, "indeed, evergreen rainforest expansion occurs during wet intervals, with contraction during periods of drought." Hence, it would appear that in this part of Western Equatorial Africa, the Little Ice Age was a time of low precipitation, low lake levels, and low evergreen rainforest presence, while much the opposite was the case during the Medieval Warm Period, when fluctuating wet-dry conditions led to fluctuating lake levels and a greater evergreen rainforest presence.

Placing these findings within a broader temporal context, Ngomanda et al. additionally note that "rainforest environments during the late Holocene in western equatorial Africa are characterized by successive millennial-scale changes [our italics] according to pollen (Elenga et al., 1994, 1996; Reynaud-Farrera et al., 1996; Maley and Brenac, 1998; Vincens et al., 1998), diatom (Nguetsop et al., 2004), geochemical (Delegue et al., 2001; Giresse et al., 1994) and sedimentological data (Giresse et al., 2005; Wirrmann et al., 2001)," and that "these changes were essentially driven by natural [our italics] climatic variability (Vincens et al., 1999; Elenga et al., 2004)," all of which observations suggest there is nothing unusual, unnatural or unprecedented about the African continent's Current Warm Period status.

References
Delegue, A.M., Fuhr, M., Schwartz, D., Mariotti, A. and Nasi, R. 2001. Recent origin of large part of the forest cover in the Gabon coastal area based on stable carbon isotope data. Oecologia 129: 106-113.

Elenga, H., Schwartz, D. and Vincens, A. 1994. Pollen evidence of Late Quaternary vegetation and inferred climate changes in Congo. Palaeogeography, Palaeoclimatology, Palaeoecology 109: 345-356.

Elenga, H., Schwartz, D., Vincens, A., Bertraux, J., de Namur, C., Martin, L., Wirrmann, D. and Servant, M. 1996. Diagramme pollinique holocene du Lac Kitina (Congo): mise en evidence de changements paleobotaniques et paleoclimatiques dans le massif forestier du Mayombe. Compte-Rendu de l'Academie des Sciences, Paris, serie 2a: 345-356.

Elenga, H., Maley, J., Vincens, A. and Farrera, I. 2004. Palaeoenvironments, palaeoclimates and landscape development in Central Equatorial Africa: A review of major terrestrial key sites covering the last 25 kyrs. In: Battarbee, R.W., Gasse, F. and Stickley, C.E. (Eds.), Past Climate Variability through Europe and Africa. Springer, pp. 181-196.

Giresse, P., Maley, J. and Brenac, P. 1994. Late Quaternary palaeoenvironments in Lake Barombi Mbo (West Cameroon) deduced from pollen and carbon isotopes of organic matter. Palaeogeography, Palaeoclimatology, Palaeoecology 107: 65-78.

Giresse, P., Maley, J. and Kossoni, A. 2005. Sedimentary environmental changes and millennial climatic variability in a tropical shallow lake (Lake Ossa, Cameroon) during the Holocene. Palaeogeography, Palaeoclimatology, Palaeoecology 218: 257-285.

Maley, J. and Brenac, P. 1998. Vegetation dynamics, paleoenvironments and climatic changes in the forests of western Cameroon during the last 28,000 years B.P. Review of Palaeobotany and Palynology 99: 157-187.

Nguetsop, V.F., Servant-Vildary, S. and Servant, M. 2004. Late Holocene climatic changes in west Africa, a high resolution diatom record from equatorial Cameroon. Quaternary Science Reviews 23: 591-609.

Reynaud-Farrera, I., Maley, J. and Wirrmann, D. 1996. Vegetation et climat dans les forets du Sud-Ouest Cameroun depuis 4770 ans B.P.: analyse pollinique des sediments du Lac Ossa. Compte-Rendu de l'Academie des Sciences, Paris, serie 2a 322: 749-755.

Vincens, A., Schwartz, D., Bertaux, J., Elenga, H. and de Namur, C. 1998. Late Holocene climatic changes in Western Equatorial Africa inferred from pollen from Lake Sinnda, Southern Congo. Quaternary Research 50: 34-45.

Vincens, A., Schwartz, D., Elenga, H., Reynaud-Farrera, I., Alexandre, A., Bertauz, J., Mariotti, A., Martin, L., Meunier, J.-D., Nguetsop, F., Servant, M., Servant-Vildary, S. and Wirrmann, D. 1999. Forest response to climate changes in Atlantic Equatorial Africa during the last 4000 years BP and inheritance on the modern landscapes. Journal of Biogeography 26: 879-885.

Wirrmann, D., Bertaux, J. and Kossoni, A. 2001. Late Holocene paleoclimatic changes in Western Central Africa inferred from mineral abundance in dated sediments from Lake Ossa (Southwest Cameroon). Quaternary Research 56: 275-287.

Reviewed 29 August 2007