Earth system impacts of European colonization of the Americas

The potential impacts of human actions on the Earth system prior to the Industrial Revolution are not well constrained. We investigate whether the decline in global atmospheric CO2 concentration by ca. 7–10 ppm in the late 1500s and early 1600s and globally lowered surface air temperatures of 0.1–0.2°C were generated by natural forcing or were a result of the large-scale depopulation of the Americas after European arrival, subsequent abandonment of agriculture and regrowth of natural vegetation. We quantitatively review the evidence for (i) the pre-Columbian population size, (ii) their per capita land use, (iii) the depopulation rates, (iv) the resulting carbon uptake of the abandoned anthropogenic landscapes, and (v) assess the scale of this carbon uptake compared to other carbon cycle and radiative forcing changes occurring at that time. From 119 published regional population estimates we find a pre-1492 population of 60.5 million (interquartile range, IQR 44.8–78.2 million), utilizing 1.04 ha land per capita (IQR 0.98–1.11, n = 18). European epidemics removed 90% (IQR 87–92%) of the indigenous population over the next century. This resulted in the return of 55.8 Mha (IQR 39.0–78.4 Mha) farmland to its prior state, sequestering 7.4 Pg C (IQR 4.9–10.8 Pg C), equivalent to a decline in atmospheric CO2 of 3.5 ppm (IQR 2.3–5.1 ppm CO2). Subsequent temperature-carbon cycle feedback processes further increased land carbon uptake thereby driving a small and relatively short-lived global impact on the Earth System in the centuries prior to the Industrial Revolution.