In Argentina, Cioccale (1999) assembled what was known at the time about the climatic history of the central region of that country over the past 1400 years, highlighting a climatic "improvement" that began some 400 years before the start of the last millennium, which ultimately came to be characterized by "a marked increase of environmental suitability, under a relatively homogeneous climate."  As a result of this climatic amelioration that marked the transition of the region from the Dark Ages Cold Period to the Medieval Warm Period, Cioccale says "the population located in the lower valleys ascended to higher areas in the Andes," where they remained until around AD 1320, when the transition to the stressful and extreme climate of the Little Ice Age began.

Down at the southern tip of the country in Tierra del Fuego, Mauquoy et al. (2004) inferred similar changes in temperature and/or precipitation from plant macrofossils, pollen, fungal spores, testate amebae and humification associated with peat monoliths collected from the Valle de Andorra.  These new chronologies were compared with other chronologies of pertinent data from both the Southern and Northern Hemispheres in an analysis that indicated there was evidence for a period of warming-induced drier conditions from AD 960-1020, which, in their words, "seems to correspond to the Medieval Warm Period (MWP, as derived in the Northern Hemisphere)."  They note that "this interval compares well to the date range of AD 950-1045 based on Northern Hemisphere extratropical tree-ring data (Esper et al., 2002)," and they conclude that this correspondence "shows that the MWP was possibly synchronous in both hemispheres, as suggested by Villalba (1994)."

In Chile, Jenny et al. (2002) studied geochemical, sedimentological and diatom-assemblage data derived from sediment cores extracted from one of the largest natural lakes (Laguna Aculeo) in the central part of the country.  From 200 BC, when the record began, until AD 200, conditions there were primarily dry, during the latter stages of the Roman Warm Period.  Subsequently, 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.  This more benign period was then followed by another period of frequent flooding from 1300-1700 that was coincident with the Little Ice Age, after which flooding picked up again after 1850.

In Peru, Chepstow-Lusty et al. (1998) derived a 4000-year climate history from a study of pollen in sediment cores obtained from a recently in-filled lake in the Patacancha Valley near Marcacocha.  Their data indicated a several-century decline in pollen content after AD 100, as the Roman Warm Period gave way to the long and dreary Dark Ages Cold Period.  However, a "more optimum climate," as they describe it, with warmer temperatures and drier conditions, came into being and prevailed for several centuries after about AD 900, which was, of course, the Medieval Warm Period, which was followed by the Little Ice Age, all of which climatic periods are in nearly perfect temporal agreement with the climatic history derived by McDermott et al. (2001) from a study of a stalagmite recovered from a cave nearly half the world away in Ireland.

Subsequent work in this area was conducted by Chepstow-Lusty and Winfield (2000) and Chepstow-Lusty et al. (2003).  Centered on approximately 1000 years ago, the former researchers identified what they describe as "the warm global climatic interval frequently referred to as the Medieval Warm Epoch."  This extremely arid interval in this part of South America, in their opinion, may have played a significant role in the collapse of the Tiwanaku civilization further south, where a contemporaneous prolonged drought occurred in and around the area of Lake Titicaca (Binford et al., 1997; Abbott et al., 1997).

Near the start of this extended dry period, which had gradually established itself between about AD 700 and 1000, Chepstow-Lusty and Winfield report that "temperatures were beginning to increase after a sustained cold period that had precluded agricultural activity at these altitudes."  This earlier colder and wetter interval was coeval with the Dark Ages Cold Period of the North Atlantic region, which in the Peruvian Andes had held sway for a good portion of the millennium preceding AD 1000, as revealed by a series of climatic records developed from sediment cores extracted from yet other lakes in the Central Peruvian Andes (Hansen et al., 1994) and by proxy evidence of concomitant Peruvian glacial expansion (Wright, 1984; Seltzer and Hastorf, 1990).

Preceding the Dark Ages Cold Period in both parts of the world was what in the North Atlantic region is called the Roman Warm Period.  This well-defined climatic epoch is also 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 relative warmth and significant aridity on both sides of it.

Returning to the Medieval Warm Period and preceding towards the present, the data of Chepstow-Lusty et al. (2003) reveal the occurrence of the Little Ice Age, which in the Central Peruvian Andes was characterized by relative coolness and wetness.  These characteristics of that climatic interval are also evident in ice cores retrieved from the Quelccaya ice cap in southern Peru, the summit of which extends 5670 meters above mean sea level (Thompson et al., 1986, 1988).  Finally, both the Quelccaya ice core data and the Marcacocha pollen data reveal the transition to the drier Modern Warm Period that occurred over the past 100-plus years.

In harmony with these several findings are the related observations of Rein et al. (2004) -- described more fully in our Editorial of 15 Dec 2004 -- who derived a high-resolution flood record of the entire Holocene from an analysis of the sediments in a 20-meter core retrieved from a sheltered basin situated on the edge of the Peruvian shelf about 80 km west of Lima.  These investigators found a major Holocene anomaly in the flux of lithic components from the continent onto the Peruvian shelf during the Medieval period.  Specifically, they report that "lithic concentrations were very low for about 450 years during the Medieval climatic anomaly from A.D. 800 to 1250."  In fact, they state that "all known terrestrial deposits of El Niño mega-floods (Magillian and Goldstein, 2001; Wells, 1990) precede or follow the medieval anomaly in our marine records and none of the El Niño mega-floods known from the continent date within the marine anomaly."  In addition, they report that "this precipitation anomaly also occurred in other high-resolution records throughout the ENSO domain," citing eleven other references in support of this statement.

Consequently, because heavy winter rainfalls along and off coastal Peru only occur during times of maximum El Niño strength, and because El Niños are typically more prevalent and stronger during cooler as opposed to warmer periods (see El Niño (Relationship to Global Warming) in our Subject Index), the lack of strong El Niños from A.D. 800 to 1250 suggests that this period was truly a Medieval Warm Period; and the significance of this observation was not lost on Rein et al.  In the introduction to their paper, for example, they note that "discrepancies exist between the Mann curve and alternative time series for the Medieval period."  Most notably, to use their words, "the global Mann curve has no temperature optimum, whereas the Esper et al. (2002) reconstruction shows northern hemisphere temperatures almost as high as those of the 20th century" during the Medieval period.  As a result, in the final sentence of their paper they suggest that "the occurrence of a Medieval climatic anomaly (A.D. 800-1250) with persistently weak El Niños may therefore assist the interpretation of some of the regional discrepancies in thermal reconstructions of Medieval times," which is a polite way of suggesting that the Mann et al. (1998, 1999) hockeystick temperature history is deficient in not depicting the presence of a true Medieval Warm Period.

In Venezuala, Haug et al. (2001) found a temperature/precipitation relationship that was different from that of the rest of the continent.  In examining the titanium and iron concentrations of an ocean sediment core taken from the Cariaco Basin on the country's northern shelf, they determined that the concentrations of these elements were lower during the Younger Dryas cold period between 12.6 and 11.5 thousand years ago, corresponding to a weakened hydrologic cycle with less precipitation and runoff, while during the warmth of the Holocene Optimum of 10.5 to 5.4 thousand years ago, titanium and iron concentrations remained at or near their highest values, suggesting wet conditions and an enhanced hydrologic cycle.  Closer to the present, higher precipitation was also noted during the Medieval Warm Period from 1.05 to 0.7 thousand years ago, followed by drier conditions associated with the Little Ice Age between 550 and 200 years ago.

In an update of this study, Haug et al. (2003) developed a hydrologic history of pertinent portions of the record that yielded "roughly bi-monthly resolution and clear resolution of the annual signal."  This record revealed that "before about 150 A.D.," which according to the climate history of McDermott et al. corresponds to the latter portion of the Roman Warm Period (RWP), Mayan civilization had flourished.  However, during the transition to the Dark Ages Cold Period (DACP), 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 RWP-to-DACP transition, which was centered on about the year 250 A.D.  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 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."

In assessing the significance of these several observations near the end of their paper, Haug et al. conclude that the latter droughts "were the most severe to affect this region in the first millennium A.D."  Although some of these spectacular droughts were "brief," lasting "only" between three and nine years, Haug et al. report "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 that these droughts within dry periods were likely the proverbial straws that broke the camel's back.

Although the Mayan civilization thus faded away, Haug et al.'s data soon thereafter depict the development of the Medieval Warm Period, when the Vikings established their historic settlement on Greenland.  Then comes the Little Ice Age, which just as quickly led to the Vikings demise in that part of the world.  This distinctive cold interval of the planet's millennial-scale climatic oscillation must have also led to hard times for the people of Mesoamerica and northern tropical South America; for according to the data of Haug et al., the Little Ice Age produced by far the lowest precipitation regime (of several hundred years duration) of the last two millennia in that part of the world.

In conclusion, it is difficult to believe that the strong synchronicity of the century-long Northern Hemispheric and South American warm and cold periods described above was coincidental.  It is much more realistic to believe it was the result of a millennial-scale oscillation of climate that is global in scope and driven by some regularly-varying forcing factor.  Although one can argue about the identity of that forcing factor and the means by which it exerts its influence, one thing should be clear: it is not the atmosphere's CO2 concentration, which has only exhibited a significant in-phase variation with global temperature change over the Little Ice Age to Modern Warm Period transition.  This being the case, it should be clear that the climatic amelioration of the past century or more has had nothing to do with the concomitant rise in the air's CO2 content but everything to do with the influential forcing factor that has governed the millennial-scale oscillation of earth's climate as far back in time as we have been able to detect it.

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