Widespread population decline in South America correlates with mid-Holocene climate change

Abstract

Quantifying the impacts of climate change on prehistoric demography is crucial for understanding the adaptive pathways taken by human populations. Archaeologists across South America have pointed to patterns of regional abandonment during the Middle Holocene (8200 to 4200 cal BP) as evidence of sensitivity to shifts in hydroclimate over this period. We develop a unified approach to investigate demography and climate in South America and aim to clarify the extent to which evidence of local anthropic responses can be generalised to large-scale trends. We achieve this by integrating archaeological radiocarbon data and palaeoclimatic time series to show that population decline occurred coeval with the transition to the initial mid-Holocene across South America. Through the analysis of radiocarbon dates with Monte Carlo methods, we find multiple, sustained phases of downturn associated to periods of high climatic variability. A likely driver of the duration and severity of demographic turnover is the frequency of exceptional climatic events, rather than the absolute magnitude of change. Unpredictable levels of tropical precipitation had sustained negative impacts on pre-Columbian populations lasting until at least 6000 cal BP, after which recovery is evident. Our results support the inference that a demographic regime shift in the second half of the Middle Holocene were coeval with cultural practices surrounding Neotropical plant management and early cultivation, possibly acting as buffers when the wild resource base was in flux.

Figure 1

Archaeological sites and radiocarbon data: (a) Kernel-smoothed intensity of sites (white dots) for 12–2 k ¹⁴C years before present, measured in points/km², (b) Histogram of median calibrated radiocarbon ages placed in 200-year bins, (c) Summed probability distribution of calibrated radiocarbon dates for entire South American dataset with a 100-year rolling mean (black solid line), shown with the highly correlated exploratory model fitted to data (exponential red dotted line), R² = 0.971, Pearson.

Figure 2

Test of summed probability distribution of calibrated archaeological ¹⁴C dates against a null model (grey shading) and climatic variability index. Left: Starting at 8.6 k cal BP South America experiences three phases of significant population deflation (blue shading). By the end of the mid-Holocene, the continental summed probability curve exceeds the null model (red shading). Right: (I) Exceptionally high climatic variability characterises the beginning of the mid-Holocene, with three time steps within 200 years of 8.2 k cal BP having an incidence of anomalies more than two standard deviations (light blue dashed line) above the mean (dark blue dashed line). (II) A second phase of cyclical high variability persists in the early mid-Holocene until 6.5 k cal BP.

Figure 3

Permutation test of regional summed probability distributions, highlighting mid-Holocene asynchrony in the period 9 k – 3 k cal BP. Top: Tropical Highlands, the Northern Andes and Pacific Coast, Middle: Tropical Lowlands, Amazonia and circum-Amazonia, Bottom: Southern Cone, Southern Andes, Patagonia, and Pampas. Regional SPDs are compared against a 95% confidence envelope generated by randomly permuting the regional affiliation of each radiometric date (1000 runs). Significant deviations above (magenta) and below (cyan) the continental trend (95% confidence, grey) are asynchronous and frequently in antiphase between tropical South America and the Southern Cone.

Figure 4

Correlating variance in Austral Summer (December-January-February) and Winter (June-July-August) precipitation during period of highest Mid-Holocene instability (8.4–7.9 k cal BP) and tropical versus extra-tropical demographic patterns. Top: Maps are based on 11 simulated grids of the TRaCE-21ka experiment in 50-year time steps in 50-year intervals. Grid cell resolution of the circulation model is 2.5°, projected to Albers Equal Area Conic for South America. The Southern Cone displays the overall lowest variance in precipitation over the mid-Holocene in both summer and winter. Bottom: Inverse demographic trends in the tropical highlands and lowlands during the Middle Holocene.