Dark Ages Cold Period (South America) – Summary

http://www.co2science.org/scripts/Template/MainPage.jsp?Page=subject/d/summaries/dacpsamerica

The previous cycle of the millennial-scale oscillation of climate that brought the planet the Little Ice Age and Modern Warm Period introduced the world to the Dark Ages Cold Period and Medieval Warm Period.  We here report on this phenomenon as it occurred in South America, focusing on the Dark Ages Cold Period.

Chepstow-Lusty et al. (1998) analyzed pollen contained in sediment cores extracted from a lake that was in-filled 40 years previous to their study in the Patacancha Valley at Marcacocha, Peru, in a program designed to develop a history of the region's climate over the prior four millennia.  While engaged in this enterprise, they discovered a significant decline in pollen content over the period AD 100-1050, which they say is reflective of increasingly colder conditions relative to the period of time that preceded it.  In addition, they found a relative rise in sedges around AD 100 that they attributed to a concomitant shift to wetter conditions.  Hence, it would appear that the Dark Ages Cold Period in this part of Peru was both colder and wetter than the warmer and drier periods that preceded and followed it.

In a subsequent study of the same area, as related in our Editorial of 18 Feb 2004, Chepstow-Lusty and Winfield (2000) described the warming that produced the Medieval Warm Period by saying that between AD 700 and 1000 "temperatures were beginning to increase after a sustained cold period that had precluded agricultural activity at these altitudes."  This earlier interval of reduced temperatures, of course, was the Dark Ages Cold Period, which in this area had held sway for a good portion of the millennium preceding AD 1000, as revealed by a series of proxy climate records developed from sediment cores extracted from yet other lakes in the Central Peruvian Andes (Hansen et al., 1994), as well as by proxy evidence of concomitant Peruvian glacial expansion (Wright, 1984; Seltzer and Hastorf, 1990).  And preceding the Dark Ages Cold Period was the Roman Warm Period, which is strikingly evident in the pollen records of Chepstow-Lusty et al. (2003), straddling the BC/AD calendar break with one to two hundred years of significant warmth and aridity located on either side of it.

In a similar investigation, but conducted further north, Moy et al. (2002) retrieved two 8-m cores and two 0.5-m cores from the center of Laguna Pallcacocha in the southern Ecuadorian Andes; and from careful analyses of the cores' sediments, they derived a continuous history of El Niño/Southern Oscillation (ENSO) events over the past 12,000 years.  At approximately 2000 years BP, near the peak warmth of the Roman Warm Period, ENSO event frequency was very low, as it also was in AD 1000, in the midst of the Medieval Warm Period, when only 3 ENSO events per century were evident.  Between these two warm periods, however, during the Dark Ages Cold Period, the frequency of ENSO events was an order of magnitude greater, as they occurred at a rate of 33 events per century.  And so the cyclical relationship has continued right up to the present, with ENSO event occurrence rising to a frequency of 27 events per century in the midst of the Little Ice Age and dropping to a value of 4 to 5 events per century at the start of the Modern Warm Period.

Also working with lakes, but much further south, Jenny et al. (2002) studied geochemical, sedimentological and diatom-assemblage data derived from sediment cores extracted from one of the largest natural lakes in Central Chile (Laguna Aculeo), in order to obtain information about the hydrologic climate of that region over the past two millennia.  They found that from the start of the record to AD 200, conditions were primarily dry.  This period of time coincides with the latter part of the Roman Warm Period.  Then, from AD 200-700, with a slight respite in the central hundred years of that Dark Ages Cold Period, flood events were more frequent.  Subsequently, there was a several-hundred-year period of less flooding that was coeval with the Medieval Warm Period; and this more benign period was followed by another period of frequent flooding from 1300-1700 (which picked up again about 1850) that was of the same timeframe as the Little Ice Age.

Last of all, we consider some of the societal impacts of the Dark Ages Cold Period, concentrating on the Maya of Mesoamerica and northern tropical South America, as described by Haug et al. (2003), who used the study of Haug et al. (2001) as a springboard for their more recent work.  Based on a study of titanium and iron concentrations in an ocean sediment core extracted from the Cariaco Basin on the Northern Shelf of Venezuela, the earlier of the two studies led to the development of a hydrologic history of the entire Holocene for Mesoamerica and northern tropical South America.  Then, based on a more detailed study of the titanium content of a smaller portion of this record, the more recent of the two studies produced a hydrologic history of pertinent portions of the extended record that yielded, in the words of Haug et al. (2003), "roughly bi-monthly resolution and clear resolution of the annual signal."

How is this detailed hydrologic history related to the history of the Maya?  Haug et al. (2003) tell us that the Pre-Classic period of Maya civilization flourished "before about 150 AD," which corresponds to the latter portion of the Roman Warm Period.  However, during the transition to the Dark Ages Cold Period, which in contrast to what was found further south in Ecuador, Peru and Chile 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 250 AD.  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 AD, 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 other intense 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."

As they assess the significance of these several observations near the end of their paper, Haug et al. conclude that "given the perspective of our long time series, it would appear that the droughts we have highlighted were the most severe to affect this region in the first millennium AD." Although some of these spectacular droughts were "brief," lasting "only" between three and nine years, Haug et al. note that "they occurred during an extended period of reduced overall precipitation that may have already pushed the Maya system to the verge of collapse," which suggests to us that these droughts within dry periods were like the proverbial straw that broke the camel's back, causing the Maya civilization to fade away and never return.

In conclusion, the Dark Ages Cold Period was very real to the people of South America, just as it was to people the world over, as indicated in our reviews of its impacts on human societies of other continents.  And again, all of these societal impacts clearly indicate that the warm nodes of this naturally-recurring climate cycle (the Roman Warm Period, Medieval Warm Period and Modern Warm Period) are much to be preferred to their cooler counterparts (the Dark Ages Cold Period and Little Ice Age).

References
Chepstow-Lusty, A.J., Bennett, K.D., Fjeldsa, J., Kendall, A., Galiano, W. and Herrera, A.T.  1998.  Tracing 4,000 years of environmental history in the Cuzco Area, Peru, from the pollen record.  Mountain Research and Development 18: 159-172.

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.

Hansen, B.C.S., Seltzer, G.O. and Wright Jr., H.E.  1994.  Late Quaternary vegetational change in the central Peruvian Andes.  Palaeogeography, Palaeoclimatology, Palaeoecology 109: 263-285.

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.

Moy, C.M., Seltzer, G.O., Rodbell, D.T. and Anderson D.M.  2002.  Variability of El Niño/Southern Oscillation activity at millennial timescales during the Holocene epoch.  Nature 420: 162-165.

Seltzer, G. and Hastorf, C.  1990.  Climatic change and its effect on Prehispanic agriculture in the central Peruvian Andes.  Journal of Field Archaeology 17: 397-414.

Webster, D.  2002.  The Fall of the Ancient Maya.  Thames and Hudson, London, UK.

Wright Jr., H.E.  1984.  Late glacial and Late Holocene moraines in the Cerros Cuchpanga, central Peru.  Quaternary Research 21: 275-285.