North American Tree Rings, Climatic Extremes, and Social Disasters - Dendroclimatology - page 321

Evidence for the geographical impact of the eighth-century drought is limited,
but tree-ring and lake sediment data indicate that multidecadal drought centered
near AD 750 extended from the northern Great Plains, across the southwestern
United States, and into central Mexico (e.g., Fig. 10.16 ) . Haug et al. ( 2003 ) and
Peterson and Haug ( 2005 ) documented multidecadal pulses of drought over northern
Venezuela and the Caribbean Sea in the sediment record of the Cariaco Basin, begin-
ning in the eighth century and extending into the mid-tenth century. They argued that
the anomalies in the Intertropical Convergence Zone (ITCZ) implied by this record
would have impacted rainfall over the Mayan lowlands. Intense drought during the
Terminal Classic Period has been reconstructed by Hodell et al. ( 1995 , 2005 ) in
lake sediment records from the Yucatan, and it has been implicated in the Mayan
collapse (Hodell et al. 1995 ; Gill 2000 ) . Hunt and Elliott ( 2005 ) have simulated
severe multidecadal drought over the Mesoamerican sector in a 10,000-year run of
the CSIRO Mark 2 global coupled climatic model based only on naturally occurring
global climatic variability, demonstrating the plausibility of the prolonged drought
identified in the proxy records from the Yucatan Peninsula and elsewhere.
These are interesting potential associations between the Classic Period decline
and drought. The only certainty is that the eighth-century megadrought—and sub-
sequent droughts in the ninth and tenth centuries evident in the North American,
Yucatan, and Cariaco records— may have interacted with anthropogenic environ-
mental degradation, epidemic disease, and social upheaval to contribute to the
collapse of the Classic Period in Mesoamerica. More paleoclimatic and archaeo-
logical information will be required to constrain these hypotheses, including the
development of long, climate-sensitive tree-ring chronologies for Mesoamerica and
realistic agent-based modeling of Classic Period societies.
10.5 Summary
Tree-ring-reconstructed climatic extremes contemporaneous with severe socioeco-
nomic impacts can be identified for the modern, colonial, and precolonial eras.
These events include the drought of the 1950s, the 1930s Dust Bowl, mid- and late
nineteenth-century Great Plains droughts, El Año del Hambre, and the seventeenth-
and sixteenth-century droughts in the English and Spanish colonies. The new tree-
ring reconstructions confirm the severe, sustained Great Drought over the Colorado
Plateau in the late thirteenth century identified by Douglass ( 1935 ) , and they doc-
ument its spatial impact. The available tree-ring data indicate a succession of
severe droughts over the western United States during the Mesoamerican Terminal
Classic Period, but these are located far from the cultural heartland of Mesoamerica.
Recently, Montezuma bald cypress ( Taxodium mucronatum ) more than 1000 years
old have been discovered in central Mexico (Villanueva et al. 2004 ) , and if they can
be exactly dated may provide tree-ring reconstructions of precipitation useful for
testing the role of drought in cultural decline during the Classic Period.
The only clear connections between climate extremes and impacts on humans
are found during the period of written history—including the prehispanic Aztec