Climate alarmists hotly contend that the degree of global warmth experienced over the latter part of the 20th century was greater than that experienced at any other time over the past two millennia (Mann and Jones, 2003). Why? Because this contention bolsters their claim that the "unprecedented" temperatures of the past few decades were caused by the cumulative anthropogenic CO2 emissions of the Industrial Revolution. As a result, CO2-bashing global warmers are loath to admit that temperatures of the Medieval Warm Period of a thousand years ago and the Roman Warm Period of two thousand years ago may have rivaled, or exceeded, those of the recent past, since the atmospheric CO2 concentrations of those two earlier eras were much lower than those of today. They also refuse to consider the possibility that these prior warm periods were global in extent, claiming they were local phenomena restricted to lands that border the North Atlantic Ocean. In this Summary we thus examine these contentions with respect to the Roman Warm Period as it has been manifest in South America.
We begin with the study of Jenny et al. (2002), who developed a 2200-year history of hydrologic climate based on geochemical, sedimentological and diatom-assemblage data derived from sediment cores extracted from one of the largest natural lakes in Central Chile (Laguna Aculeo). From 200 BC, when the record began, until AD 200, which coincides with the latter part of the Roman Warm Period, conditions there were primarily dry. Subsequently, however, from AD 200-700 (with a slight respite in the central hundred years of that period), there was a high frequency of flood events, during the Dark Ages Cold Period. Then came a several-hundred-year period of less flooding that was coeval with the Medieval Warm Period, which was followed by another period of frequent flooding from 1300-1700 that picked up again about 1850 and was coeval with the Little Ice Age.
Similar observations had earlier been made by Chepstow-Lusty et al. (1998), who analyzed the types and amounts of pollen found in sediment cores obtained from the in-filled basin of the ancient Inca's sacred lake of Marcacocha, which is located high in the Central Andean region of Peru some 45 km northwest of the legendary city of Cuzco. Specifically, they observed an overall decline in pollen content for several centuries subsequent to about AD 100, which they attributed to the development of increasingly colder conditions relative to the prior (Roman Warm) period. This period is strikingly evident in the pollen records of Chepstow-Lusty et al. (2003) and straddles the BC/AD calendar break with one to two hundred years of relative warmth and dryness located on either side of it. Also, there was a relative rise in sedge pollen around AD 100 that the researchers felt reflected "a shift to wetter conditions."
After the passing of the wetter Dark Ages Cold Period, a "more optimum climate," as Chepstow-Lusty et al. describe it, with warmer temperatures and drier conditions prevailed for several centuries after about AD 900, as indicated by the establishment and rapid growth of the Alnus acuminata (Aliso) tree during what Chepstow-Lusty and Winfield (2000) describe as "the warm global climatic interval frequently referred to as the Medieval Warm Epoch." Between AD 1700 and 1800, however, during what they call the "most intense episode of the Little Ice Age," a major decline of Alnus occurred, indicating once again a return to colder conditions.
In pursuing the aspect of millennial-scale moisture variability still further, Moy et al. (2002) retrieved four lake-bottom cores from the center of Laguna Pallcacocha in the southern Ecuadorian Andes; and from careful analyses of their sediments, they derived a 12,000-year history of El Niņo/Southern Oscillation (ENSO) events. In coming out of the Dark Ages Cold Period, which was one of the coldest intervals of the entire Holocene (McDermott et al., 2001), the frequency of occurrence of ENSO events dropped by an order of magnitude, from a high of about 33 events per 100 years to a low of about 3 events per 100 years, centered on the year AD 1000, right in the middle of the Medieval Warm Period as delineated by Esper et al. (2002). Then, at about AD 1250, the frequency of ENSO events exhibited a new peak of approximately 27 events per 100 years in the midst of the longest sustained cold period of the Little Ice Age, again as delineated by the work of Esper et al. Thereafter, ENSO event frequency declined in zigzag fashion to a low on the order of 4 to 5 events per 100 years at the start of the Modern Warm Period, which according to the temperature history of Esper et al. began about 1940.
Going back in time from 1200 years ago, Moy et al.'s data reveal that at 2000 cal. yr BP, the Roman Warm Period as delineated by McDermott et al. was near its peak warmth, and ENSO event frequency was again at a very low level. By 3000 cal. yr BP, however, ENSO frequency was once again significantly elevated in response to another millennial-scale cold period that appears in the climatic reconstruction of McDermott et al. Hence, it would appear that global warmth is closely associated with a significant decline in ENSO activity, which brings some parts of the world an abundance of moisture and other parts a dearth of the precious liquid.
Continuing to investigate the hydrological aspects of the millennial-scale cycling of climate in South America, as well as its anthropological implications, Haug et al. (2003) used the study of Haug et al. (2001) as a springboard from which to launch a high-resolution analysis of titanium concentrations in an ocean sediment core extracted from the Cariaco Basin on the Northern Shelf of Venezuela, from which the earlier of the two studies developed a hydrologic history of the entire Holocene for Mesoamerica and northern tropical South America. With respect to the implications of this record for the prior inhabitants of these lands, Haug et al. (2003) remark that the Pre-Classic period of Maya civilization flourished "before about 150 A.D.," which corresponds to the latter portion of the Roman Warm Period. However, during the transition to the Dark Ages Cold Period, which was accompanied by a slow but long decline in precipitation, Haug et al. report that "the first documented historical crisis hit the lowlands, which led to the 'Pre-Classic abandonment' (Webster, 2002) of major cities."
This crisis occurred during the first intense multi-year drought of the Roman Warm Period-to-Dark Ages Cold Period transition, which was centered on about the year AD 250. Although the drought was devastating to the Maya, Haug et al. report that when it was over, "populations recovered, cities were reoccupied, and Maya culture blossomed in the following centuries during the so-called Classic period." Ultimately, however, there came a time of total reckoning, between about 750 and 950 A.D., during what Haug et al. determined was the driest interval of the entire Dark Ages Cold Period, when they report that "the Maya experienced a demographic disaster as profound as any other in human history," in response to a number of additional multi-year droughts. During this Terminal Classic Collapse, as it is called, Haug et al. say that "many of the densely populated urban centers were abandoned permanently, and Classic Maya civilization came to an end."
There are a number of conclusions that may be drawn from these several observations. One is that climatic history, like human history, tends to repeat itself. Another is that the millennial-scale oscillation of climate that manifests itself throughout glacial and interglacial periods alike [see Climate Oscillations (Millennial Variability: Oceans) in our Subject Index] does so totally independently of what the atmosphere's CO2 concentration does. Yet another logical conclusion is that the two nodes of this climate cycle, of which the Roman Warm Period and Dark Ages Cold Period are typical, are truly global climate states, manifesting themselves in some parts of the world primarily in terms of thermal extremes and in other parts of the world primarily in terms of moisture extremes. Most important of all, however, is that all of these conclusions clearly demonstrate there was nothing abnormal or unusual about 20th-century global warming. It was simply the natural transition from cool-node to warm-node global climate that was only to be expected with the "scheduled" demise of the Little Ice Age.
References
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Chepstow-Lusty, A., Frogley, M.R., Bauer, B.S., Bush, M.B. and Herrera, A.T. 2003. A late Holocene record of arid events from the Cuzco region, Peru. Journal of Quaternary Science 18: 491-502.
Chepstow-Lusty, A. and Winfield, M. 2000. Inca agroforestry: Lessons from the past. Ambio 29: 322-328.
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.
Haug, G.H., Gunther, D., Peterson, L.C., Sigman, D.M., Hughen, K.A. and Aeschlimann, B. 2003. Climate and the collapse of Maya civilization. Science 299: 1731-1735.
Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C. and Rohl, U. 2001. Southward migration of the intertropical convergence zone through the Holocene. Science 293: 1304-1308.
Jenny, B., Valero-Garces, B.L., Urrutia, R., Kelts, K., Veit, H., Appleby, P.G. and Geyh M. 2002. Moisture changes and fluctuations of the Westerlies in Mediterranean Central Chile during the last 2000 years: The Laguna Aculeo record (33°50'S). Quaternary International 87: 3-18.
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Moy, C.M., Seltzer, G.O., Rodbell, D.T. and Anderson D.M. 2002. Variability of El Niņo/Southern Oscillation activity at millennial timescales during the Holocene epoch. Nature 420: 162-165.
Webster, D. 2002. The Fall of the Ancient Maya. Thames and Hudson, London, UK.