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A Solar-Influenced Little Ice Age and Medieval Warm Period in Tropical Venezuela
Reference
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.

What was done
The authors examined the titanium and iron concentrations of an ocean sediment core taken from a depth of 893 meters in the Cariaco Basin on the Northern Shelf of Venezuela (1042.73'N, 6510.18'W) to infer variations in the hydrologic cycle over northern South America over the past 14,000 years.

What was learned
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 (10.5 to 5.4 thousand years ago), however, concentrations of these metals remained at or near their highest values, suggesting wet conditions and an enhanced hydrologic cycle for over five thousand years. Closer to the present, the largest century-scale variations in precipitation are inferred in the record between approximately 3.8 and 2.8 thousand years ago, as the amounts of these metals in the sediment record varied widely over short time intervals. Higher precipitation was 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).

What factor(s) might best explain the regional changes in precipitation inferred from the Cariaco metals' records of the past 14,000 years? According to the authors, "these regional changes in precipitation are best explained by shifts in the mean latitude of the Atlantic Intertropical Convergence Zone," which, in turn, "can be explained by the Holocene history of insolation, both directly and through its effect on tropical Pacific sea surface conditions."

What it means
The results of this study add further credence to the climate realist claim that both the Little Ice Age and the Medieval Warm Period were global climatic anomalies, not limited to the mid- and upper-latitudes of the Northern Hemisphere. Indeed, both events were strong enough to exert a measurable influence in the Northern Hemisphere tropics. Furthermore, this study demonstrates the important influence of solar variations on climate.