The dynamics of climate in the central and northern lowlands are driven by shifts in the boundary between the southeastern tropics and the northwestern subtropics (e. g., Folan et al. 1983; Gunn et al. 1994, 1995). The regional atmospheric mechanism that causes this change is a double sea-breeze effect that normally brings rainy season precipitation through the convergence of eastern and northern air streams (see Folan et al. 1983). The former, supported by the eastern trade winds, carries moist air and tropical storms from the Caribbean. Dry air comes from the north and has its origins in the Bermuda-Azores high. During the rainy season, the collision of these two sea breezes over the course of the day creates a line of northeast to southwest precipitation. Global warming leads to greater amounts of moisture, an earlier rainy season, and a northwest shift in the tropical-subtropical boundary. Conversely, cooling leads to a later rainy season, less precipitation, and southeast movement of the tropical-subtropical boundary.
One approach to assessing the regional effects of global warming on the climate of the Maya lowlands is to analyze the discharge of rivers. The Calakmul Basin forms part of the Rfo Candelaria watershed. We have found that a comparison of the annual discharge of the Rio Candelaria with global temperature fluctuations during the period 1958-1990 strongly supports the double sea-breeze model (Gunn et al. 1994, 1995). This model, abstracted as a regression equation predicting river discharge from mean global temperature, was projected over the last three thousand years to estimate the impact of global temperature regimes on local climate. The model indicates extended droughts at the end of the Preclassic, Classic, and Postclassic periods, and overly moist conditions during the Early Postclassic period (Figure 9.8; Gunn et al. 1994, 1995). The longest period of extended drought during the Classic period began about a. d. 750 and lasted for approximately 200 years.
Subsequent analyses of lake sediment cores provide an empirical assessment of the double sea-breeze model. Hodell et al. (1995) analyzed a core from the northern Maya lowlands that records a very strong drought during the Terminal Classic period. Another core studied by Curtis et al. (1996) also shows a drought at the end of the Classic period. Nonetheless, a sample taken from Laguna Tamarindito, located in the Petexbatun region of southwestern Peten, Guatemala, does not provide unambiguous evidence for a Terminal Classic drought. Increased charcoal levels and a shift from gilled to gilled-and-lunged snails were observed in the core. These indicate a local drying trend in the Terminal Classic, but there is no evidence that climatic change—rather than human action—was responsible (Dunning et al. 1998: 147).
Subregional variation may reconcile our double sea-breeze model and data from the northern lowlands with the Laguna Tamarindito core. The Petexbatun receives some of its water from an uplift of the Maya Mountains and the northern foothills of the Sierra Chama. These hills may have continued to generate precipitation despite generally drier conditions to the north. Alternatively, global cooling may not have been sufficient to push the tropical-subtropical boundary far enough to the south to affect southern Peten. Laporte (Chapter 10, this volume; Laporte and Quezada 1998) has documented a heavy Terminal Classic occupation of south-
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9.8 Fluctuations in the annual discharge of the Rw Candelaria during the late Holocene (after Gunn et al. 1995: Figure 7).
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World History
