AbstractBiocultural systems adapt to global climate change through its regional manifestations. Subsistence customs are the cultural interface between regional climate and culture at large. Swidden horticulturalists in Campeche, Mexico report that dry April followed by early onset of the wet season enhances the productivity of upland tropical gardens, or milpas. To relate regional seasonality of moisture to global climate, growing season discharge for Mexico's Candelaria River from 1958 to 1990 was analyzed relative to global average temperature. Analysis of covariance revealed a statistically significant relationship (p<0.001). Further analysis showed that hot global climate eliminates the dry season, which lowers milpa productivity by preventing burning of the slash. Cold global climate delays the wet season and planting, also at cost of productivity. Intermediate global temperature fosters optimal wet—dry season combinations. Productivity of milpas is therefore directly related to global climate through the intervening mechanism of seasonality of moisture. A regression model reflecting these findings is used to retrodict paleohydrology for the last 3000 years. The pattern of ascendancy and decline of ancient southern Maya lowland urban centers is reviewed in the perspective of changing hydrological conditions. The model indicates that fluorescence occurred with optimal balance of wet and dry season duration and catastrophes unfolded during extended wet or dry periods. We suggest that the southern Maya lowlands have had a precipitous record of urban development and collapse in part because of complex interactions of global climate and upland horticulture. The most productive conditions for milpa issue from an inherently unstable overlay of global climate on a relatively narrow band of partially developed karst (semikarst) geological formation