Thread by @DagomarDegroot

So, I wanted to comment on a major #envhist article that’s been making the rounds lately. It argues that the Spanish arrival in the Americas in 1492 set in motion a chain of events that cooled Earth’s climate. Here’s a BBC summary. 1/15 #twitterstorians bbc.com/news/science-e…

While journalists report as though the article makes an entirely new argument, its core concept actually dates back to 2003, when climatologist William Ruddiman first proposed it. The new article is really a comprehensive attempt to test an old idea. 2/15 link.springer.com/article/10.102…

Ruddiman suggested that when Old World epidemics killed millions in the New World, agricultural practices ceased across much of the Americas. Plant biomass increased, drawing CO2 out of the atmosphere and cooling the Earth during an already-cold period: the “Little Ice Age.” 3/15

This is an elegant, disturbing idea (with potential irony, because cooling also undermined Spain's attempt to retain the Low Countries and conquer England). The new article introduces exciting new evidence and wisely qualifies its conclusions. But I do have problems with it. 4/15

For this idea to work, first, the pre-contact pop of the Americas must have been big enough to manage ecosystems on a huge scale. The article argues for a pop of ~60-64 million. To reach that number, it combines all previous population estimates and finds the average. 5/15

Yet as John McNeill reminded me, not all estimates are of equal value. The authors draw from a great deal of flawed, older scholarship, and choose not to cite important, newer work. They also use population estimates inconsistently. 6/15

While they conclude that most estimates have the population of the Caribbean at between 300-500K before European arrival, they later chart a decline in the population of Hispaniola alone – from a high of 4 million! 7/15

Second, one way to trace past atmospheric CO2 is to sample bubbles in ice cores. Multiple ice cores show an abrupt decline in atmospheric CO2 at around 1590 – corresponding to an especially cold part of the Little Ice Age (which some call the “Grindelwald Fluctuation”). 8/15

Yet if virgin soil epidemics quickly spread from Europeans and the animals they brought with them to indigenous communities, why would plant biomass only surge in the 1590s? Shouldn’t the decline start earlier and have been more gradual? 9/15

Third, events in the Americas did not, of course, unfold in isolation. In order to prove that surging plant biomass in the New World cooled Earth’s climate, we need to show that plant biomass remained steady – or at least did not sharply decline – elsewhere. 10/15

Yet in the 16th and 17th centuries, forests shrank across Asia and perhaps Europe, as governments and merchants used wood for commerce, industry, and conflict on an unprecedented scale. Did that deforestation balance the climatic effects of afforestation in the Americas? 11/15

Other aspects of the climatic picture seem equally murky. A 2016 paper, for example, argues that cooling temperatures in the Little Ice Age globally diminished gross primary production and ecosystem respiration. 12/15

In other words: there's evidence to suggest that cooling caused the drop in atmospheric CO2, partly by reducing plant activity . . . not the other way around. 13/15 nature.com/articles/ngeo2…

I will leave more qualified scholars – such as @monicaMedHist, whom you should really follow – to comment on other parts of the paper that I know less about, including the way it models the spread of virgin soil epidemics. 14/15

To conclude, I'll ask my students to read this paper, and I think it’s important, but it definitely leaves me with as many questions as answers . . . just like Ruddiman’s original work. And here's an article I wrote about all this, way back in 2016. 15/15 historicalclimatology.com/blog/-did-huma…