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Climate Oscillations (Millennial Variability: Africa) -- Summary
Earth's climate oscillates on many different time scales, all without any respect to the activities of man.  Hence, we here review evidences for such a millennial-scale oscillation of climate that have been discovered in Africa, for the purpose of providing an historical perspective on natural climate change, which is an important prerequisite for determining whether any of the global warming of the past couple of centuries may have had an anthropogenic origin.

Tyson et al. (2000) obtained a quasi-decadal-resolution record of oxygen- and carbon-stable isotope data from a well-dated stalagmite recovered from Cold Air Cave in the Makapansgat Valley of South Africa, which they augmented with 5-year-resolution temperature data 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 relatively colder temperatures prior to the Medieval Warm Period that prevailed from before AD 1000 to around AD 1300, the subsequent establishment of the Little Ice Age that prevailed from about AD 1300 to 1800, and the ultimate development of the Modern Warm Period over the past couple of centuries.

Speaking of the most recent cold and warm nodes of this millennial-scale cycling of climate, Tyson et al. remark that relative to the period 1961-1990, the Little Ice Age, which they say "was a widespread event in South Africa specifically and southern Africa generally," was characterized by a mean annual temperature depression of about 1C at its coolest point.  The Medieval Warm Period, on the other hand, was as much as 3-4C warmer than the recent past at its warmest point.  They also note that the coolest period of the Little Ice Age occurred during the Maunder Minimum of sunspot activity and that the Medieval Warm Period occurred during the Medieval Maximum of solar activity.

In a companion study of the past three thousand years of stalagmite-derived South African climate, Holmgren et al. (2001) again report that the most pronounced cold interval of this period occurred during the Little Ice Age between AD 1500 and 1800, when temperatures were estimated to have been about 1C colder than they were during the last half of the 20th century.  Their data also reveal that southern Africa had experienced a dramatic warming at the start of the Medieval Warm Period around AD 900, when temperatures reached a level 2.5C higher than the mean of the past half-century in the initial stage of their recovery from the global chill of the Dark Ages Cold Period.

This millennial-scale oscillation of the climate of South Africa was also evident in the study of Holmgren et al. (2003), wherein nine climate scientists developed a 25,000-year temperature history from a Cold Air Cave stalagmite that was based on 18O and 13C measurements dated by 14C and high-precision thermal ionization mass spectrometry using the 230Th/234U method.  In this case they report [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 tail-end of the Medieval Warm Period, which followed the Dark Ages Cold Period that followed the Roman Warm 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 18O deviation in the entire record."

In a vastly different approach to unraveling the nature of climate change over the prior millennium-and-then-some, Huffman (1996) constructed a 2000-year climate history for southern Africa based on archaeological evidence related to the locations and sizes of various Iron Age settlements uncovered there.  By this means it was determined that much of southern Africa is presently neither as warm nor as wet as it was from approximately AD 900-1300.  It was also determined that this "medieval warm period," as it was described, was followed by a cold period that corresponds to the Little Ice Age, which continued to approximately 1780, whereupon it gradually began to warm again.  With respect to these millennial-scale climatic meanderings, Huffman additionally notes that the archaeological data "show the subcontinent-wide extent of the climatic changes."

Further reinforcing the reality of cyclical climate change of a millennial nature in Africa, Stager et al. (2003) studied changes in diatom assemblages preserved in a sediment core extracted from Pilkington Bay, Lake Victoria, East Africa, together with diatom and pollen data acquired from two nearby sites.  These three coherent data sets, in their words, revealed a "roughly 1400- to 1500-year spacing of century-scale P:E [precipitation:evaporation] fluctuations at Lake Victoria," which they say "may be related to a ca. 1470-year periodicity in northern marine and ice core records that has been linked to solar variability (Bond et al., 1997; Mayewski et al., 1997)."

In a somewhat similar study that was conducted on the other side of the continent, Nguetsop et al. (2004) developed a high-resolution proxy record of West African precipitation based on analyses of diatoms recovered from a sediment core retrieved from Lake Ossa, West Cameroon, which rainfall history they describe as "the first paleohydrological record for the last 5500 years in the equatorial near-coastal area, east of the Guinean Gulf."  In reporting their results, Nguetsop et al. say that the Lake Ossa record provides evidence for alternating periods of increasing and decreasing precipitation "at a millennial time scale for the last 5500 years," which oscillatory behavior they interpret as being "a result of south/northward shifts of the ITCZ [Intertropical Convergence Zone]."  Specifically, they report that "a southward shift of the ITCZ, combined with strengthened northern trade winds, was marked by low and high precipitation at the northern subtropics and the subequatorial zone, respectively," and that "these events occurred in coincidence with cold spells in the northern Atlantic."  They thus conclude that "the climatic evolution in the tropical zone of Africa [is] essentially driven by interactions between the northern and southern hemispheres," indicating the global nature of the forces involved in the millennial-scale oscillation of earth's climate.

Last of all, in a more spatially- and temporally-focused study, Lamb et al. (2003) provided complementary evidence for the hydrologic fingerprint of the Medieval Warm Period in central Kenya (Verschuren et al., 2000) via a study of pollen data derived from a sediment core taken from Crescent Island Crater, a sub-basin of Lake Naivasha.  Their analysis indicated there were good correlations between the lake-level and salinity histories of Verschuren et al. and the authors' pollen-derived histories of both aquatic and surrounding terrestrial vegetation.  The most striking of these correspondences occurred during the Medieval Warm Period, when from AD 980 to 1200 lake-level was at a uniform 1100-year low and woody taxa were significantly underrepresented in the pollen assemblage.

In light of these several findings, it would appear that a millennial-scale oscillation of climate has reverberated throughout Africa for thousands of years.  Over the past two and a half millennia, for example, this likely solar-driven phenomenon has brought the continent its own unique versions of the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age, and Modern Warm Period, demonstrating that temperature increases of the type that produced the Modern Warm Period have occurred many times in the past without any help from increases in the air's CO2 concentration.  Hence, there is no compelling reason to attribute the most recent of these climatic transitions to the concomitant anthropogenic-induced increase in this trace constituent of the atmosphere.  Mother Nature, by means not yet fully understood, has apparently been the major -- and possibly only source -- of our deliverance from the cold and debilitating conditions of the Little Ice Age.

References
Bond, G., Showers, W., Chezebiet, 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 climates.  Science 278: 1257-1266.

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.

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.

Mayewski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S., Yang, Q., Lyons, W.B. and Prentice, M.  1997.  Major features and forcing of high-latitude northern hemisphere atmospheric circulation using a 110,000-year-long glaciochemical series.  Journal of Geophysical Research 102: 26,345-26,366.

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.

Stager, J.C., Cumming, B.F. and Meeker, L.D.  2003.  A 10,000-year high-resolution diatom record from Pilkington Bay, Lake Victoria, East Africa.  Quaternary Research 59: 172-181.

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.