Haug et al. (2001) measured and analyzed the titanium and iron concentrations of an ocean sediment core extracted from the Cariaco Basin (10°42.73'N, 65°10.18'W) on the Northern Shelf of Venezuela for the purpose of inferring variations in the hydrologic cycle over northern South America over the past 14,000 years. This work revealed that titanium and iron concentrations 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. During the Holocene Optimum or thermal maximum of 10.5 to 5.4 thousand years ago, however, the concentrations of these metals were at or near their highest values, indicative of wet conditions and an enhanced hydrologic cycle. In addition, higher precipitation values were detected during the Medieval Warm Period from 1.05 to 0.7 thousand years ago, which was followed by drier conditions associated with the Little Ice Age between 550 and 200 years ago.

What factor or factors might best explain the regional changes in precipitation inferred from these observations? According to Haug et al., they are best explained by shifts in the mean latitude of the Atlantic Intertropical Convergence Zone," which in turn, in their words, "can be explained by the Holocene history of insolation, both directly and through its effects on tropical Pacific sea surface conditions."

Two years later, and based on a more detailed study of the titanium content of a smaller portion of the Cariaco Basin record, Haug et al. (2003) developed a hydrologic history that yielded, in their words, "roughly bi-monthly resolution and clear resolution of the annual signal." As for how this more detailed history was related to that of the Mayas, they tell us that the Pre-Classic period of that ancient civilization flourished "before about 150 A.D.," which according to the climate history that was developed by McDermott et al. (2001), corresponds to the latter portion of the Roman Warm Period (RWP). 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 first millennium A.D., 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 A.D." Although some of these spectacular droughts were "brief," lasting only between three and nine years, Haug et al. say "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 likely the proverbial "straws" that "broke the camel's back."

The Mayan civilization thus faded away sometime during the transition from the Dark Ages Cold Period to the Medieval Warm Period, when the Vikings established their historic settlement on Greenland. Then came the Little Ice Age, which just as quickly led to the Vikings' demise in that part of the world. But this last cold node 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.

There are a number of conclusions that may be drawn from these several observations. One is that both climatic and human history tend to repeat themselves. Another is that the millennial-scale climatic oscillation, which manifests itself throughout glacial and interglacial periods alike, does so totally independently of what the atmosphere's CO2 concentration is doing. Yet another is that the two nodes of this climate cycle, of which the Medieval Warm Period and Little Ice Age are typical, are truly global phenomena, 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 is that these several observations clearly demonstrate there was nothing unusual, unnatural of unprecedented about the warming of the 20th century, which was simply the most recent example of the periodically-recurrent transition from cool-node to warm-node global climate that was only to be expected with the demise of the Little Ice Age.

One year later, and based on the degree of unsaturation of certain long-chain alkenones synthesized by haptophyte algae contained in a sediment core retrieved from the eastern part of the Cariaco Basin (20°30'N, 64°40'W) on the continental shelf off the Venezuelan central coast, Goni et al. (2004) reconstructed a history of sea surface temperatures for that location that covered the past 6000 years. This work revealed that the highest temperatures of that extended period occurred during the Medieval Warm Period; and from the graph of their results that we have reconstructed below, it is further evident that the Little Ice Age was the coldest period of the last six 6000 years.

Insert graph here.

The results of this research are striking in the clarity of their depiction of both the Medieval Warm Period and Little Ice Age, as well as the warming that led to the development of the Current Warm Period; and they indicate that there was nothing unusual, unnatural or unprecedented about the latter phenomenon.

Moving inland, Polissar et al. (2006) derived continuous decadal-scale records of a number of climate-relevant parameters related to precipitation/evaporation balance -- and, therefore, to glacier activity -- from sediment cores extracted from Venezuela's Laguna Blanca (8°20'N, 71°47'W) and Laguna Mucubaji (8°47'N, 70°50'W), while data they obtained from the nearby Piedras Blancas peat bog yielded "pollen histories that chronicle vegetation change in response to climate." This work focused on the Little Ice Age, where they found that "four glacial advances occurred between AD 1250 and 1810, coincident with solar-activity minima," and they state that "temperature declines of 3.2 ± 1.4°C and precipitation increases of ~20% are required to produce the observed glacial responses," which indicates, in their words, "considerable sensitivity of tropical climate to small changes in radiative forcing from solar irradiance variability."

Finally, since Venezuela is the last country of South America (alphabetically speaking), and because it is considered an Andean country, we conclude this summary with a brief review of the findings of [not yet posted, so it needs a linkage] Rodbell et al. (2009), who updated "the chronology of Andean glaciation during the Lateglacial and the Holocene from the numerous articles and reviews published over the past three decades," and who noted that the Andes "offer an unparalleled opportunity to elucidate spatial and temporal patterns of glaciation along a continuous 68-degree meridional transect." Their work revealed, as they describe it, that "all presently glacierized mountain ranges contain multiple moraines deposited during the last 450 years," and they say that "these correlate with the Little Ice Age as defined in the Northern Hemisphere." In addition, they state that most Andean regions "reveal a nearly continuous temporal distribution of moraines during the Little Ice Age."

The temporal correspondence of the Little Ice Age in essentially all of the glacierized portions of the Northern Hemisphere and the great expanse of most of Andean South America, as well as the similar glacial activity of both parts of the planet during this time period, provide strong support for the proposition that much of the world commenced its return to its current more milder climatic state from the very bottom of what we could call the Holocene's "thermal basement." And this fact suggests that the 20th-century warming of the planet should have been rather dramatic, starting (as it did) from such a cold initial temperature.

Considered in this light, the significant warming that followed the Little Ice Age is readily recognized to not have been all that unusual, as it was only what should have been expected to occur, in light of the warming's unusual (extremely low) thermal point of origin. And recognizing that fact helps one to understand why there is no compelling reason to assume that the concurrent historical increase in the atmosphere's CO2 content had anything at all to do with the post-Little Ice Age warming. The time for the latter had simply come, based on the specific state of the prior millennial-scale warming-and-cooling cycle of the planet; and that state (record global cold) just happened to coincide with the start of the historical increase in the air's CO2 content that was caused by the historical increase in mankind's industrial activity, which has had essentially no impact on either the rate or magnitude of global warming over the last hundred or so years.

References
Goni, M.A., Woodworth, M.P., Aceves, H.L., Thunell, R.C., Tappa, E., Black, D., Muller-Karger, F., Astor, Y. and Varela, R. 2004. Generation, transport, and preservation of the alkenone-based U37K' sea surface temperature index in the water column and sediments of the Cariaco Basin (Venezuela). Global Biogeochemical Cycles 18: 10.1029/2003GB002132.

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.

McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem ð¹⁸O record from SW Ireland. Science 294: 1328-1331.

Polissar, P.J., Abbott, M.B., Wolfe, A.P., Bezada, M., Rull, V. and Bradley, R.S. 2006. Solar modulation of Little Ice Age climate in the tropical Andes. Proceedings of the National Academy of Sciences: 10.1073/pnas.0603118103.

Rodbell, D.T., Smith, J.A. and Mark, B.G. 2009. Glaciation in the Andes during the Lateglacial and Holocene. Quaternary Science Reviews 28: 2165-2212.

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