Demographics are tied to land use, are affected by and in turn affect climate

From Earth system impacts of the European arrival and Great Dying in the Americas after 1492 by Alexander Koch, Chris Brierley, Mark M.Maslin, and Simon L.Lewis.

Relates to my argument that much local climate change is driven by land use and that that does not adequately get factored into climate models. It is especially a challenge because land use impacts can be very large, and speed of change can be very fast. Measured in decades and centuries rather than millennia.

From the Abstract:

Human impacts prior to the Industrial Revolution are not well constrained. We investigate whether the decline in global atmospheric CO2 concentration by 7–10 ppm in the late 1500s and early 1600s which globally lowered surface air temperatures by 0.15∘C, were generated by natural forcing or were a result of the large-scale depopulation of the Americas after European arrival, subsequent land use change and secondary succession. We quantitatively review the evidence for (i) the pre-Columbian population size, (ii) their per capita land use, (iii) the post-1492 population loss, (iv) the resulting carbon uptake of the abandoned anthropogenic landscapes, and then compare these to potential natural drivers of global carbon declines of 7–10 ppm. From 119 published regional population estimates we calculate a pre-1492 CE 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). European epidemics removed 90% (IQR 87–92%) of the indigenous population over the next century. This resulted in secondary succession of 55.8 Mha (IQR 39.0–78.4 Mha) of abandoned land, 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). Accounting for carbon cycle feedbacks plus LUC outside the Americas gives a total 5 ppm CO2 additional uptake into the land surface in the 1500s compared to the 1400s, 47–67% of the atmospheric CO2 decline. Furthermore, we show that the global carbon budget of the 1500s cannot be balanced until large-scale vegetation regeneration in the Americas is included. The Great Dying of the Indigenous Peoples of the Americas resulted in a human-driven global impact on the Earth System in the two centuries prior to the Industrial Revolution.

A not completely new argument but one that has been percolating in the background for some time.

The Great Dying of the Indigenous Peoples of the Americas was primarily an unavoidable outcome of different biomes coming into contact with one another for the first time. Biome interaction was inevitable with global transport (circa 1500) and the Great Dying was not different in kind from comparable old world events such as the die-off in Europe as a consequence of the Black Death in 1346 or the Plague of Justinian in 542.

The ideological belief, not supported by serious historians, that the great dying occurred as a deliberate plan is passionately held, making the discussion of the event rare. Discuss it factually and you are held to be ignorant or a racist. Marginal Revolution mentions this paper and they have a dynamic commenting going between some postmodernists and the fact-based crowd.

Whether Koch et al have done their maths correctly or not, the underlying point is that demographic fluctuations in the Americas has fluctuated wildly over the centuries, causing dramatic changes in land use. In addition already established patterns in climate system (periodic drought, flooding, etc.) also have heavily influenced land use changes as well.