While most discussions of climate change focus on modern history, discussing the change from pre-industrial CO2 levels of roughly 275 parts per million to today’s 410 (or so), humanity has been impacting the climate for millennia. Essentially since the emergence of agriculture, humans have been putting their thumb on the scales and influencing climate. While it is the industrial era and the ever-mounting burning of fossil fuels that have driven the greatest change, agriculture (such as methane emissions due to rice cultivation and forestry (think the forests that became the Royal Navy’s ships)) has had enough emissions to tip the scale toward warming.
Generally, we think of humanity’s impacts as warming the planet. There are, however, other ways in which human impact has actually contributed to cooling the planet. Well known is the mid-20th century cooling influence from all the particulates and pollution in the atmosphere that reflected solar radiation and acted to counter the warming contributes of increased greenhouse gases. (This, plus the studied impact from volcanoes, is behind much of the thinking behind geo-engineering concepts like ejecting sulphur in the atmosphere to counteract warming.)
Until reading a paper earlier today, this seemed to be the major example of humanity driving ‘cooling’ (rather than warming) globally.
In “Earth system impacts of the European arrival and Great Dying in the Americas after 1492,” scientists put figures behind the impacts of the genocidal 16th century. When Columbus ‘sailed the oceans blue’, there were approximately 60 million people living in the Americas. A century later, the population was a tenth that. With 90% fewer people, less agriculture was required (and sustainable), fewer other elements of human civilization (cities, roads, …) were required and maintained.
The scientists’ analysis suggests that some 56 million hectares (or roughly 140 million acres or over 200,000 square miles (nearly the size of France (250,000 square miles))) of agricultural land was overrun by fast growing plants and forests. These soaked up massive amounts of carbon dioxide. The scientists assess that, over a century, this amounted to 7 to 10 ppm.
In this period, there was a climate event called the Little Ice Age that, in at least part, seems to have been driven by human action.
“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 CO?” explained co-author Prof Mark Maslin.
Let us be clear, as Prof Maslin’s comment does state, that the “marked cooling” was not solely driven by human action: the genocide (and resultant natural process of trees reclaiming cleared land) might have been half the reason for the Little Ice Age cooling.
Prof. Ed Hawkins, 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 CO? 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.”
While a genocide is not the path for climate action, this study has interesting lessons for today:
- Human action can cool the planet just as it is warming it today.
- Reforestation — on sufficient scale — can have a climate-level impact.
- This climate impact can occur quickly — with the same sort of rapidity
- This massive reforestation (essentially a France of trees) reduced carbon emissions roughly equivalent to two years of today’s emissions.
Tree-planting (such as massively accelerating green walls) can (should be) part of humanity’s decision to act on climate, to mitigate future climate change. The ‘natural’ reforestration due to the 16th genocide in the Americas makes clear that it can have a meaningful impact. And, it provides a simple rule of thumb: we need to push humanity toward a carbon-neutral emissions economy as rapidly as possible while reforesting roughly six Frances to bring the carbon system below 350 ppm.
Notes:
- The study is well-documented. The references are rich and are worth mining is this topic is of interest.
- Professor Bill Ruddiman, Emeritus-UVA, is perhaps appropriately called the doyen of studying humanity’s impact on the climate in the pre-industrial period. Here is his Tyndall prize lecture laying out his hypothesis and 2013 state of the research. In 2010, he published Plows, Plagues, and Petroleum: How Humans Took Control of Climate.
- Other human ‘genocidal’ actions might have had climate impacts. For example, Genghis Khan is estimated to account for killing about 5% of the then world’s population with a resultant impact (through reforestation) of perhaps 700 million tons of carbon dioxide removed from the atmosphere (about 1/10th the impact in the Americas).
4 responses so far ↓
1 John Egan // Feb 3, 2019 at 8:33 pm
I question the broad assumptions made in this study. When I read the so-called findings, I was stunned at their ahistoricity. My doctoral field was in environmental history with a minor field in Native American history. So, I have a horse in the race. (And you have a historical background, as well.)
First, I must ask whether there was a Medieval Warm Period followed by a Little Ice Age or not. One cannot accept the records as they exist – even if rudimentary and incomplete – for one and not the other. The null hypothesis should be that climate cycles are random and naturally occurring unless significant evidence suggests otherwise. The level of significance of this study is low, indeed.
First, the issue of Native American populations has been fraught with controversy since Denevan published his 50+ million estimate in the 1970s – replacing Kroeber’s ridiculously low 10 million. Of course, estimates also reflect the prevailing historical-cultural contexts. Thus, in an era when Native American culture was devalued, their numbers were devalued, as well. And as Native American culture became valued, their numbers were revalued. Still, even at 50 million, the vast majority of this number would have been found in Mesoamerica and the Andean Highlands. There is little doubt that roughly 90% of native peoples died from introduced European diseases, enslavement, and the destruction of the native social fabric.
Second, any population/climate arguments must consider not only populations in the Americas, but populations worldwide. Median estimated world population are 1200 – 405M; 1300 – 395M; 1400 – 360M; 1500 – 480M; 1600 – 560M; 1700 – 640M – – with world population declining between 1200 and 1400 and surging afterwards. The primary demographic even was the Black Death – killing upwards of 100 million, with tens of millions more dying of plague and famine in China in the early 1300s. Furthermore, dramatic climate change – particularly cooling – was recorded in Europe and China prior to the plague events with population declines prior to the epidemics.
Third, if the loss 45 million Native Americans produced cooling, then would not the loss of 130 Eurasians have an even greater impact? And sooner? Eurasia had a more intensely developed field agriculture with deep plowing and with nearly all arable land utilized. There are written records in Europe and China of reforestation following the Black Death. The Mayan milpas were reclaimed by forests following the Mayan Collapse; yet, this took place centuries before European contact. And if the majority of population lost was in Mesoamerica and the Andes, how did the resulting encomienda and repartimiento systems import land use – with growing exports of foodstuffs to European consumers?
Fourth, the degree of population loss in the Americas must be considered as well. Even if 45 million Native Americans died, Europeans and African slaves rapidly filled the population vacuum – especially in the Caribbean and Brazil. Furthermore, the types of agriculture practiced by Europeans in the Americas – sugar, tobacco, and livestock grazing – was far more destructive of forest ecosystems than prior native agriculture.
Finally, there must be added the impacts of mining on forests in the Americas. Spain imported 180 tons of gold and 16,000 tons of silver from the Americas in the 16th and early 17th centuries. All of that gold and silver had to be smelted with charcoal. Mining in the 19th century American West denuded forests for miles in boom regions. Did the authors consider the impact of Spanish mining on forests? No.
Climate activists are always complaining about nonscientists failing to understand the complexity of climate science. That climate discussion should be left to the scientists. Well, history should, perhaps be better left to historians who might approach the issue without preconceived conclusions and who might also weigh the collective historical evidence instead of only one small piece.
2 A Siegel // Feb 4, 2019 at 11:47 am
This paper presents a hypothesis that is backed up with literature.
Why not, if you wish, consider doing a letter / such response challenging what you see as gaps in the analysis?
Some quick comments:
1. Most powerful, from your comment, seems to be the question/issue of wood for charcoal. Did they consider charcoal burning?
2. There are analyses/hypotheses about other major population changes and climate implications. For example, Ghenghis Khan is estimated to have killed (directly/indirectly) roughly 5% of the then global population.
That 700m tons is about 1/10th the CO2 impact of what the America’s study presents/discusses.
Other that some exposure to work like Ruddiman’s (both peer-reviewed and general discussions), the pre-industrial is far from a focus of my interests and study … I am not in a position to delve into great detail on this study.
3 Greg Laden // Feb 4, 2019 at 12:00 pm
The null hypothesis (when examining human effects such as this paper does) is not that climate “cycles” are random (cycles are not random), but rather, the best model based on known forcings other than the one examined in the paper.
It is not true that the numbers of Native Americans or the number killed are a direct function of how politically correct society is. Partly, perhaps, but if you look carefully at that literature, it is clear that the estimates changed over time primarily as the scholarship got better.
I agree with the comments about world wide population being very important, as well as worldwide changes in agriculture regardless of population.
I’ve not examined the paper closely enough to have a strong opinion about it, but if it does in fact hold the world constant while attributing change in North America, rather than estimating the global changes while examining a major regional change, then that is a problem.
For instance, my own research shows a very large change that I think would be going in the opposite direction, over an overlapping time period, in Africa.
I don’t think it is legitimate to bring in whinging about “climate activists” in discussing this academic research. But yes, this is a complicated problem and this paper may be a single stroke of color on a very unfinished canvas.
4 John Egan // Feb 6, 2019 at 4:02 am
Greg –
The null hypothesis is ALWAYS that there is no significant difference between populations – that any differences are due to random variation.
The serious scientific problem that I see in much climate research is that the “model” takes precedence – as you state above. Which is fundamentally unscientific.
The ability of the biases of the researcher to impact outcomes has been shown repeatedly. That is the primary rationale for the double-blind trial. Not only can prior knowledge of group membership affect subjects in a study, but is can impact researchers as well, as shown by Rosenthal’s expectancy effect.
It is disheartening that – 50 years after the development of critical structures in research design – these appear to be increasingly discarded.