I tend to be rather cautious about grand theories connecting everything to everything. So I wanted to look very closely at a new study arguing that “America colonisation ‘cooled Earth’s climate’” .
This winter researchers from the UK report evidence that the “Great Dying” in the Americas following European contact caused a period of global cooling .
“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. “ (, p. 13)
Following the contact and invasions of the early 1500’s, vast numbers of Americans died from European diseases, particularly in the Caribbean, Southeast US, Central America and Northern South America.. This depopulation had crippling effects on the social, political, and economic lives, and left the people open to relatively easy conquest.
The new paper seeks to estimate the ecological effects of this massive tragedy. Specifically, the sudden disappearance of the human inhabitants led to large areas of land reverting from agriculture and other human uses to regrown forests. In short, the paper interprets this as an accidental episode of mass reforestation.
Coinciding with this era, analysis of ice cores indicates a drop in atmospheric CO2, and historical records document a cool period, the Little Ice Age. Are these events connected?
This hypothesized connect involves a chain of connections. First, the pre-Colombian population would have to be large enough, and use enough land for the dieback to have an impact. Second, the dieback and putative reforestation would have to be large enough to account for the uptake of that much CO2. The study musters evidence for these factors, and models the probably effects on climate.
There are limited historical records from the pre-Colombian period, and no population censuses. However, many methods have been used to estimate the populations of the Americas from archeological remains. Combining these estimates, this study finds the population to be something like 60 million people (which might have been about 10% of the world’s human population).
“all 1492 CE population estimates require data to be combined with assumptions to arrive at estimates. “ (, p. 15)
Other studies have estimated the amount of land used to support these populations. These estimates are difficult to make precisely, because they rely on assumptions about not only populations, but also agricultural and cultural practices. This study proposes that the total land under human use was about 50 million hectares (about the extent of France or Texas).
The Great Dying was a massive disaster for the American peoples, but estimates of the magnitude of the disaster vary. This study considers a number of studies and takes the median of the estimates: which is 90% death rate! This would make the post contact (circa 1600) population about 6 million people, which is at the low end of previous estimates.
Finally, when there was a depopulation, the land use reverted to forest. Obviously, this process is specific to different locations. In some locations, there is evidence of the reduction of fires, which may be due to the end of intentional burning. Archaeological pollen indicates that some areas had significant reductions in cultivated plants at the time of the die off. There is also archaeological evidence of abandoned settlements and apparent agricultural and other infrastructure.
Taking the estimated population decline and the estimated cultivation per capita, this study suggests that over 50 million hectares of agricultural land was abandoned due to depopulation. (This is about 1% of the total land of the Americas.)
These estimates rely on a variety of assumptions and imperfect data. The total abandoned land is higher than some estimates, but roughly in the midst of other estimates.
So, if 50 million hectares reverted to forest in a relatively short period, what might the effect have been? The researchers indicate that carbon uptake increases rapidly in the first 20 years of such a transition, and slower after that. With plausible estimates for different areas, this study yields an estimate of about 7.4 petagrams of Carbon sequestered in the abandoned lands. This is higher than some estimates, partly because it depends heavily on estimates of the population loss and corresponding area of abandoned land.
At the end of this long chain of inference, the estimated Carbon uptake corresponds to a decrease in atmospheric CO2 of 3 ppm or more.
In fact, ice core data shows a sudden decrease of atmospheric C02 during that period. How much of this can be attributed to the effects of abandoned land, rather than other sources? The researchers argue that the isotope data from ice cores indicates that the decrease in atmospheric CO2 during the was due to uptake in soil and vegetation
“This unusual event of a rapid and large increase in terrestrial carbon stocks is consistent with a role for secondary succession following epidemics in the Americas after the arrival of Europeans, although there are other potential interpretations in the literature which we discuss in the next section. “(, p. 27)
Data from many studies shows that there was atmospheric cooling coinciding with the decreased atmospheric CO2, indeed “1577-1694 CE is the only period of significant global cooling within the past two millennia”, and is recognized as “the Little Ice Age”. This cooling is in alignment with theoretical models of the effects of decreased CO2. The researchers find that is no evidence for similar changes in total solar irradiation or volcanic aerosols during this period.
“Through multiple routes we arrive at the conclusion that LUC [land use change[ in the Americas played an important role in driving lower atmospheric CO2 in the late 1500s and early 1600s.” (, p. 29)
Altogether, the study concludes that the Little Ice Age was probably caused by the depopulation of the Americas and corresponding changes in land use.
This result, if sustained, is an interesting perspective on the complex world-wide effects history of the European invasion of the Americas. It is already known that the transfer of species (e.g., food plants) changed diets and health. It is also known that the massive transfer of precious metals, particularly silver, generated substantial inflation throughout Europe, and increased trade between China and the world.
If the American genocide triggered or exacerbated the Little Ice Age, then its indirect effects included bad weather and famine in Europe and elsewhere, with concomitant social stress and unrest. Karma, anyone?
This is a fascinating study, connecting two obvious dots with a line through a variety of evidence. (I’m a big fan of using multiple lines of evidence.) They make a plausible case.
We’ll have to see how well this holds up.
There are competing hypotheses, starting with disagreements about the supposed dating of the “Little Ice Age”. And while 1492 is pretty firmly dated, the course of contact and depopulation (and subsequent population increase from European immigration) are far harder to date because they occurred over the whole western hemisphere for several centuries.
In addition, estimates for the pre-contact populations and land us are uncertain, as are the estimates for the magnitude and pace of the die off. In short, refined estimates for these factors could change the conclusions.
And, of course, the climate models used to connect the dots from depopulation to cooling are uncertain. However plausible the hypothesis, there is a huge amount of uncertainty in the argument.
Nevertheless, this study certainly lays out an interesting case. The Little Ice Age does not seem to have a clear cause. And the Great Dying was a once-in-history “cause”, potentially of just the right magnitude. If there isn’t a connection, it’s a hell of a coincidence.
The authors also point out that the scenario in this study gives perspective on current preoccupation with reducing atmospheric CO2 . This was a massive unplanned reforestation (of an area equivalent to all of France) which had a strong, almost immediate effect on the atmosphere and climate. That’s good, but only a drop in the bucket compared to current emissions of Carbon. It’s going to take a heck of a lot of reforestation to put a dent in our current situation.
- Jonathan Amos, America colonisation ‘cooled Earth’s climate’, in BBC News – Science & Environment. 2019. https://www.bbc.com/news/science-environment-47063973
- Alexander Koch, Chris Brierley, Mark M. Maslin, and Simon L. Lewis, Earth system impacts of the European arrival and Great Dying in the Americas after 1492. Quaternary Science Reviews, 207:13-36, 2019/03/01/ 2019. http://www.sciencedirect.com/science/article/pii/S0277379118307261