Wild claim: America colonisation ‘cooled Earth’s climate’

O……K…..

Earth system impacts of the European arrival and Great Dying in the Americas after 1492

Here’s a link to the full paper.

Highlights

• Combines multiple methods estimating pre-Columbian population numbers.
• Estimates European arrival in 1492 lead to 56 million deaths by 1600.
• Large population reduction led to reforestation of 55.8 Mha and 7.4 Pg C uptake.
• 1610 atmospheric CO2 drop partly caused by indigenous depopulation of the Americas.
• Humans contributed to Earth System changes before the Industrial Revolution.

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.

1. Introduction

The arrival of Europeans in the Americas in 1492 CE marks the onset of disease epidemics resulting in the loss of the majority of indigenous people living in the Americas over the subsequent century (Berlinguer, 1993; Cook, 1998; Crosby, 1972, 1976; Nunn and Qian, 2010). Indigenous land use was widespread before European arrival, particularly in Mexico, Central America, Bolivia and the Andes where terraced fields and irrigated agriculture was practised (e.g. Abrams and Nowacki, 2008; Chepstow-Lusty and Jonsson, 2000; Heckenberger et al., 2003; Hunter and Sluyter, 2015; Whitmore and Turner, 1992), and across Amazonia where diverse pre-Columbian land uses left its traces in the composition of contemporary Amazon forests (Clement et al., 2015; Levis et al., 2017; Maezumi et al., 2018a; Watling et al., 2018). Thus the Great Dying of the Indigenous Peoples of the Americas (Wolf, 1982) after 1492 CE likely led to a reduction in land use. Fields and fallow areas then underwent secondary succession and in many cases increased carbon stocks as they reverted towards similar prior states – with local, regional and potentially global consequences for the Earth System.

The uptake of carbon on the abandoned anthropogenic lands after European contact may have been large enough to impact the atmospheric CO₂ record (Dull et al., 2010; Faust et al., 2006; Lewis and Maslin, 2015; Nevle and Bird, 2008; Nevle et al., 2011; Ruddiman, 2005). Furthermore, at the same time high-resolution Antarctic ice-core records of atmospheric CO₂ concentration show an anomalously large decline of ∼7–10 ppm (Ahn et al., 2012; MacFarling Meure et al., 2006) beginning in the 1500s with a minimum in the early 1600s (Fig. 1). Isotope analysis shows that the anomaly was driven by an increase in the terrestrial carbon sink (Fig. 1B, Bauska et al., 2015; Francey et al., 1999; Trudinger et al., 1999). Hence, the carbon uptake that is thought to have occurred following the arrival of epidemics in the Americas may have reduced atmospheric CO₂ levels and led to a decline in radiative forcing that may then have contributed to the coldest part of the Little Ice Age (Faust et al., 2006; Neukom et al., 2014).

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Fig. 1. Global atmospheric CO₂ time series. (A) The CO₂ concentrations recorded in two Antarctic ice cores: Law Dome (grey, MacFarling Meure et al., 2006) and West Antarctic Ice Sheet (WAIS) Divide (blue, Ahn et al., 2012). (B) Carbon isotopic ratios recorded in CO₂ from the WAIS Divide ice core (black, Bauska et al., 2015) showing an increased terrestrial carbon uptake over the 16th century (B). Yellow box is the span of the major indigenous depopulation event (1520–1700 CE). Loess smoothed lines for visual aid. Error bars are the 1-σ standard deviation. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

8. Conclusion

We estimate that 55 million indigenous people died following the European conquest of the Americas beginning in 1492. This led to the abandonment and secondary succession of 56 million hectares of land. We calculate that this led to an additional 7.4 Pg C being removed from the atmosphere and stored on the land surface in the 1500s. This was a change from the 1400s of 9.9 Pg C (5 ppm CO₂).

Including feedback processes this contributed between 47% and 67% of the 15–22 Pg C (7–10 ppm CO₂) decline in atmospheric CO₂ between 1520 CE and 1610 CE seen in Antarctic ice core records. These changes show that the Great Dying of the Indigenous Peoples of the Americas is necessary for a parsimonious explanation of the anomalous decrease in atmospheric CO₂ at that time and the resulting decline in global surface air temperatures.

These changes show that human actions had global impacts on the Earth system in the centuries prior to the Industrial Revolution. Our results also show that this aspect of the Columbian Exchange – the globalisation of diseases – had global impacts on the Earth system, key evidence in the calls for the drop in atmospheric CO₂ at 1610 CE to mark the onset of the Anthropocene epoch (Lewis and Maslin, 2015, 2018).

We conclude that the Great Dying of the Indigenous Peoples of the Americas led to the abandonment of enough cleared land in the Americas that the resulting terrestrial carbon uptake had a detectable impact on both atmospheric CO₂ and global surface air temperatures in the two centuries prior to the Industrial Revolution.