Geeze this conclusion seems rather counter intuitive, a much smaller human population's pre-modern agriculture had enough of an impact on climate so much as to prevent an ice age?
Mainly because of deforestation (one of the causes mentioned in the article). French Gaul in Roman times had already been largely deforested in the previous centuries, some historians even suggest that the process had been underway since Neolithic times, that’s about 4,000-5,000 years ago. It’s relatively unconmplicated to get rid of large swaths of forrest without having to have access to modern industrial tools, you just need to put some good old physical work into it.
As a reference, I’m doing a mapping of Southern Romania’s forrests as they showed up in a 1860s Austrian map (it’s a hobby project that I work on during my spare time) and it’s crazy when comparing it to more modern times. As early as the 1920s (that is in less than 60 years) 60-to-80% of the forrests from the plane areas had disappeared, and that was not because of the introduction of modern technology, because most of the tree-cutting was being done with good old axes, but only because Romania had entered the international agricultural markets and that land was more economically valuable with grain or cows on it compared to trees (the same thing is now happening in Brazil).
Sure, but the study places the "pivot point" much back in time in the 7,000 to 5,000 years ago range (probably give or take a thousand years):
>"I noticed that methane concentrations started decreasing about 10,000 years ago and then reversed direction 5,000 years ago and I also noted that carbon dioxide also started decreasing around 10,000 years ago and then reversed direction about 7,000 years ago," says Ruddiman. "It alerted me that there was something strange about this interglaciation ... the only explanation I could come up with is early agriculture, which put greenhouse gases into the atmosphere and that was the start of it all."
Yeah, I had noticed that, too. To give a totally unsubstantiated opinion (i.e. mine) I'd say that that only helps make the case for the massive deforestation happening as soon as the Neolithic Revolution got under way in earnest, starting about 5,000 BCE, that would explain the rise in CO2 concentration (also see the Cucuteni–Trypillia culture - https://en.wikipedia.org/wiki/Cucuteni%E2%80%93Trypillia_cul...). The rise in methane concentration which happened 2,000 years after that might be explained by people raising a lot more cows around 3,000 BCE (the start of "civilization" on the Nile and Euphrates, for example).
> It’s relatively unconmplicated to get rid of large swaths of forrest without having to have access to modern industrial tools, you just need to put some good old physical work into it.
Merely killing off predators like wolves will contribute to deforestation, because runaway deer populations will stop new tree growth.
They only argue that the impact was enough to offset a decline from 270 ppm CO2 to 250 ppm CO2 to instead wind up at 280 ppm CO2 at the preindustrial stage. Over about 8,000 years. Call it 40 ppm CO2 over 7000 years to make the number a little larger and its 0.005 ppm CO2/yr.
We're currently increasing CO2 in the atmosphere by 2 ppm CO2/yr or 350 times faster.
So he's not claiming that pre-modern agriculture had a huge impact on CO2 levels, but that those CO2 levels do have rather large climate impact over time. He does this by comparisons with prior ice age due to similarities in the precession of the Earth. The paper is about MIS19 which is 18 interglacials ago, which provides a good analog in the orbital properties, and which there is CO2 data for in ice cores.
That can then be correlated with the paleoclimate record showing glacial inception happening ~10,000 years after the maximum in MIS19. Using the actual paleoclimate data from MIS19 to control a GCM for MIS1 they run the GCM forwards from the glacial maximum and find that the mean global temperature falls by 1.2K and that the high Arctic cools by 5-6K with 3K falls in temperature over most of the Arctic and Siberia.
That paints a consistent picture where by right before the pre-industrial revolution the Earth should have been cooler and continental shield glaciers should have started forming across Northern Canada and Siberia. And this matches the paleoclimate record of MIS19.
If this seems counter intuitive at all, then your calibration of how much human impact is necessary to produce global climate changes (particularly over thousands of years) is likely off.
That also means that the argument that climate change will be beneficial is correct in a way, but pre-industrial humans already made that impact. We're well beyond the climate impact that would produce a consistent, beneficial boost to global temperature and are blasting the furnace.
Remember that modern agriculture is very efficient from a land POV. Historically slash and burn or other techniques just chewed up farmland and led to mass deforestation.
Consider Vermont, which was basically clear-cut in the mid 19th century.
