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. Graphic: Koch, et al., 2019 / Quaternary Science Reviews

By Jonathan Amos
31 January 2019

(BBC News) – Colonisation of the Americas at the end of the 15th Century killed so many people, it disturbed Earth's climate.

That's the conclusion of scientists from University College London, UK.

The team says the disruption that followed European settlement led to a huge swathe of abandoned agricultural land being reclaimed by fast-growing trees and other vegetation.

This pulled down enough carbon dioxide (CO₂) from the atmosphere to eventually chill the planet.

It's a cooling period often referred to in the history books as the "Little Ice Age" - a time when winters in Europe would see the Thames in London regularly freeze over.

"The Great Dying of the Indigenous Peoples of the Americas led to the abandonment of enough cleared land that the resulting terrestrial carbon uptake had a detectable impact on both atmospheric CO₂ and global surface air temperatures," Alexander Koch and colleagues write in their paper published in Quaternary Science Reviews.

The team reviewed all the population data it could find on how many people were living in the Americas prior to first contact with Europeans in 1492.

It then assessed how the numbers changed in following decades as the continents were ravaged by introduced disease (smallpox, measles, etc.), warfare, slavery, and societal collapse.

It's the UCL group's estimate that 60 million people were living across the Americas at the end of the 15th Century (about 10% of the world's total population), and that this was reduced to just five or six million within a hundred years.

The scientists calculated how much land previously cultivated by indigenous civilisations would have fallen into disuse, and what the impact would be if this ground was then repossessed by forest and savannah.

The area is in the order of 56 million hectares, close in size to a modern country like France.

This scale of regrowth is figured to have drawn down sufficient CO₂ that the concentration of the gas in the atmosphere eventually fell by 7-10ppm (that is 7-10 molecules of CO₂ in every one million molecules in the air).

"To put that in the modern context - we basically burn (fossil fuels) and produce about 3ppm per year. So, we're talking a large amount of carbon that's being sucked out of the atmosphere," explained co-author Prof Mark Maslin.

"There is a marked cooling around that time (1500s/1600s) which is called the Little Ice Age, and what's interesting is that we can see natural processes giving a little bit of cooling, but actually to get the full cooling - double the natural processes - you have to have this genocide-generated drop in CO₂." […]

Ed Hawkins, professor of climate science at Reading University, was not involved in the study. He commented: "Scientists understand that the so-called Little Ice Age was caused by several factors - a drop in atmospheric carbon dioxide levels, a series of large volcanic eruptions, changes in land use and a temporary decline in solar activity.

"This new study demonstrates that the drop in CO₂ is itself partly due the settlement of the Americas and resulting collapse of the indigenous population, allowing regrowth of natural vegetation. It demonstrates that human activities affected the climate well before the industrial revolution began." […]

"There is a lot of talk around 'negative emissions' approaches and using tree-planting to take CO₂ out of the atmosphere to mitigate climate change," he told BBC News.

"And what we see from this study is the scale of what's required, because the Great Dying resulted in an area the size of France being reforested and that gave us only a few ppm. This is useful; it shows us what reforestation can do. But at the same, that kind of reduction is worth perhaps just two years of fossil fuel emissions at the present rate." [more]

America colonisation ‘cooled Earth's climate’


Impact of changes in radiative forcing on atmospheric CO2 and temperature 1000–1800 CE. A) CO2 concentrations recorded in two Antarctic ice cores: Law Dome (grey) and West Antarctic Ice Sheet (WAIS) Divide (blue); B) Global mean temperature reconstruction from northern and southern hemisphere proxies, anomaly compared to the means from 1000 to 2000 CE; C) Total Solar Irradiation anomaly compared to the means from 1000 to 2000 CE; D) volcanic, radiative forcing (grey, individual eruptions; black, smoothed); E) Land-atmosphere flux based on double deconvolution of WAIS CO2 data; F) Land use change (LUC) in the Americas from the LUC reconstructions KK10 (purple), HYDE 3.1 (blue and grey. The 1600 CE value has been replaced with the LUC estimate from this review (solid lines) and the original LUC reconstruction is plotted for comparison (dashed lines); G) LUC in the two other regions with considerable agrarian societies at the time, Asia and Europe based on the three LUC reconstructions. Graphic: Koch, et al., 2019 / Quaternary Science Reviews

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 0.15C, 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.

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.

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

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