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Medieval Warm Period (Regional - Africa) -- Summary
Was there a Medieval Warm Period anywhere other than in countries surrounding the North Atlantic Ocean? This question is of great importance to the ongoing global warming debate, since if there was, and if that period was as least as warm as it is currently, there is no need to consider the temperature increase of the past century as anything other than the natural phase of a millennial-scale oscillation of climate that regularly brings the earth several-hundred-year periods of modestly higher and lower temperatures that are totally independent of variations in atmospheric CO2 concentration. Hence, we here consider this question as it applies to Africa.

Based on the temperature and water needs of the crops that were cultivated by the first agropastoralists of southern Africa, Huffman (1996) constructed a climate history of the region based on archaeological evidence acquired from various Iron Age settlements. In the course of completing this project, dated relic evidence of the presence of cultivated sorghum and millets was considered by Huffman to be so strong as to essentially prove that the climate of the subcontinent-wide region must have been warmer and wetter than it is today from approximately AD 900-1300, for these crops cannot be grown in this part of southern Africa under current climatic conditions, which are much too cool and dry.

Other evidence for this conclusion comes from Tyson et al. (2000), who obtained a quasi-decadal record of oxygen and carbon-stable isotope data from a well-dated stalagmite of Cold Air Cave in the Makapansgat Valley (30 km southwest of Pietersburg, South Africa), which they augmented with 5-year-resolution temperature data that they reconstructed from color variations in banded growth-layer laminations of the stalagmite that were derived from a relationship calibrated against actual air temperatures obtained from a surrounding 49-station climatological network over the period 1981-1995, which had a correlation of +0.78 that was significant at the 99% confidence level. This record revealed the existence of a significantly warmer-than-present period that began prior to AD 1000 and lasted to about AD 1300. In fact, Tyson et al. report that the "maximum warming at Makapansgat at around 1250 produced conditions up to 3-4C hotter than those of the present."

In a similar study, Holmgren et al. (2001) derived a 3000-year temperature record for South Africa that revealed several multi-century warm and cold periods. Of particular interest with respect to the Medieval Warm Period was their finding of a dramatic warming at approximately AD 900, when temperatures reached a level that was 2.5C higher than that prevailing at the time of their analysis of the data.

Lamb et al. (2003) provided strong evidence for the hydrologic fingerprint of the Medieval Warm Period in Central Kenya via a study of pollen data obtained from a sediment core taken from Crescent Island Crater, which is a sub-basin of Lake Naivasha. Of particular interest in this regard is the strong similarity between their results and those of Verschuren et al. (2000). The most striking of these correspondences occurred over the period AD 980 to 1200, when lake-level was at an 1100-year low and woody taxa were significantly underrepresented in the pollen assemblage. We also note, with respect to this finding, that when two independent data sets tell essentially the same story -- in this particular case, a tale of a two-century-long depression of precipitation and elevated temperature (which leads to lower lake levels) -- one can have increased confidence that the story they tell is true.

Expanding on the earlier work of Holmgren et al. (2001), Holmgren et al. (2003) developed a 25,000-year temperature history from a stalagmite retrieved from Makapansgat Valley's Cold Air Cave based on δ18O and δ13C measurements dated by 14C and high-precision thermal ionization mass spectrometry using the 230Th/234U method. This work revealed, in the words of the nine researchers [together with our interspersed notes], that "cooling is evident from ~6 to 2.5ka [thousand years before present, during the long interval of coolness that preceded the Roman Warm Period], followed by warming between 1.5 and 2.5 ka [the Roman Warm Period] and briefly at ~AD 1200 [the Medieval Warm Period, which followed the Dark Ages Cold Period]," after which "maximum Holocene cooling occurred at AD 1700 [the depth of the Little Ice Age]." They also note that "the Little Ice Age covered the four centuries between AD 1500 and 1800 and at its maximum at AD 1700 represents the most pronounced negative δ18O deviation in the entire record."

This new temperature record from far below the equator (24S) reveals the existence of all of the major millennial-scale oscillations of climate that are evident in data collected from regions surrounding the North Atlantic Ocean. Hence, it attests to the global extent of these significant intervals of relative warmth and coolness; and it suggests that after the coldest such period of the entire Holocene, it was only to be expected that there would be a significant increase in mean global air temperature when the next scheduled warming occurred.

Two years later, Kondrashov et al. (2005) applied advanced spectral methods to fill data gaps and locate interannual and interdecadal periodicities in historical records of annual low- and high-water levels on the Nile River over the 1300-year period AD 622-1922. In doing so, several statistically significant periodicities were noted, including cycles at 256, 64, 19, 12, 7, 4.2 and 2.2 years. With respect to the causes of these cycles, the three researchers say that the 4.2- and 2.2-year oscillations are likely due to El Nio-Southern Oscillation variations, that the 7-year cycle may be related to North Atlantic influences, and that the longer-period oscillations could be due to astronomical forcings. They also note that the annual-scale resolution of their results provides a "sharper and more reliable determination of climatic-regime transitions" in tropical east Africa, including the documentation of fairly abrupt shifts in river flow at the beginning and end of the Medieval Warm Period.

Skiping forward two more years, Ngomanda et al. (2007) 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 and Nguene in the lowland rainforest of Gabon. As for what they found, the nine researchers state that after a sharp rise at ~1200 cal yr BP, "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, they 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."

In discussing their findings, 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, "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.

Last of all, we come to the paper of Esper et al. (2007), who used Cedrus atlantica ring-width data "to reconstruct long-term changes in the Palmer Drought Severity Index (PDSI) over the past 953 years in Morocco, Northwest Africa." This work revealed, as they describe it, that "the long-term PDSI reconstruction indicates generally drier conditions before ~1350, a transition period until ~1450, and generally wetter conditions until the 1970s," after which there were "dry conditions since the 1980s." In addition, they determined that "the driest 20-year period reconstructed is 1237-1256 (PDSI = -4.2)," adding that "1981-2000 conditions are in line with this historical extreme (-3.9)." Also of significance, the six researchers note that "millennium-long temperature reconstructions from Europe (Buntgen et al., 2006) and the Northern Hemisphere (Esper et al., 2002) indicate that Moroccan drought changes are broadly coherent with well-documented temperature fluctuations including warmth during medieval times, cold in the Little Ice Age, and recent anthropogenic warming," which latter coherency would tend to suggest that the peak warmth of the Medieval Warm Period was at least as great as that of the last two decades of the 20th century throughout the entire Northern Hemisphere; and, if the coherency is strictly interpreted, it suggests that the warmth of the MWP was likely even greater than that of the late 20th century.

In light of these several research findings, which have been published in the standard peer-reviewed scientific literature, it would appear that (1) the Medieval Warm Period did indeed leave its mark over wide reaches of Africa, and that (2) the Medieval Warm Period was probably much more extreme in this part of the world than has been the Current Warm Period to this point in time.

References
Buntgen, U., Frank, D.C., Nievergelt, D. and Esper, J. 2006. Summer temperature variations in the European Alps, A.D. 755-2004. Journal of Climate 19: 5606-5623.

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., 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.

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.

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Esper, J., Frank, D., Buntgen, U., Verstege, A., Luterbacher, J. and Xoplaki, E. 2007. Long-term drought severity variations in Morocco. Geophysical Research Letters 34: 10.1029/2007GL030844.

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.

Holmgren, K., Lee-Thorp, J.A., Cooper, G.R.J., Lundblad, K., Partridge, T.C., Scott, L., Sithaldeen, R., Talma, A.S. and Tyson, P.D. 2003. Persistent millennial-scale climatic variability over the past 25,000 years in Southern Africa. Quaternary Science Reviews 22: 2311-2326.

Holmgren, K., Tyson, P.D., Moberg, A. and Svanered, O. 2001. A preliminary 3000-year regional temperature reconstruction for South Africa. South African Journal of Science 97: 49-51.

Huffman, T.N. 1996. Archaeological evidence for climatic change during the last 2000 years in southern Africa. Quaternary International 33: 55-60.

Kondrashov, D., Feliks, Y. and Ghil, M. 2005. Oscillatory modes of extended Nile River records (A.D. 622-1922). Geophysical Research Letters 32: doi:10.1029/2004GL022156.

Lamb, H., Darbyshire, I. and Verschuren, D. 2003. Vegetation response to rainfall variation and human impact in central Kenya during the past 1100 years. The Holocene 13: 285-292.

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.

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.

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.

Tyson, P.D., Karlen, W., Holmgren, K. and Heiss, G.A. 2000. The Little Ice Age and medieval warming in South Africa. South African Journal of Science 96: 121-126.

Verschuren, D., Laird, K.R. and Cumming, B.F. 2000. Rainfall and drought in equatorial east Africa during the past 1,100 years. Nature 403: 410-414.

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.

Last updated 29 October 2008