581:
I know that the above is from H's post but it caught my eye because 'Indian fire' has two different meanings: a planned forest burn or a skin disease (impetigo). Kinda confusing since both contagious diseases as well as ecological disasters had been mentioned in both your posts.
Also Keithmasterson at 576.
First, I hadn't heard impetigo called Indian Fire. Thanks! I'll be more careful.
As far as the Little Ice Age ("LIA") goes, yes there was a drop of atmospheric CO2 from 284 ppm to 272 ppm around 1600 ( https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2006GL026152 ).
What caused it? Well, there are a couple of theories, but the thing to realize is that, absent industry (which at that point, was essentially absent modulo Ming China), atmospheric CO2 is primarily about the balance of plant uptake (e.g. carbon turns into wood, roots, or soil carbon) and respiration, which is primarily PLANT respiration (there's a lot more plant biomass than animal biomass).
Plants do exhale CO2 just as we do, and there's a neat, recent NOVA episode that points out that trees are actually net carbon emitters for the first few decades of their lives, before settling in to become net carbon stores. Yeah, we've been screwing up big time with the forest industry by harvesting young trees and replanting. Anyway, another point here is that there are big trees globally, but the reason there tend to be lots of really big trees in temperate rainforests, more than in tropical rainforests, is that the tropics are hotter. Trees are ectothermic, so the hotter they are, the more they respire. In the cool temperate zones, they respire less, and that means they lay on more wood.
So, what happened in the Little Ice Age? One possibility is the abandoned farmland in the New World hypothesis: virgin ground plagues caused mass casualties (historic numbers of around 1/3 or more are recorded), farms were abandoned, trees grew, and a few decades later CO2 levels dropped.
The other possibility is that something else drove the initial temperature spike, plant respiration decreased, existing trees laid on more wood, and that drove the temperature drop.
One study (paywalled in Nature Geoscience, rough draft here: https://core.ac.uk/download/pdf/77027602.pdf ) did something cool. The glacial air samples used to determine atmospheric CO2 concentrations back ca. 2000 years had other molecules, notably carbonyl sulfide. This is taken up by plants too, but unlike CO2, it is not re-emitted except by biomass burning. So if there's a drop in carbonyl sulfide, what that means is that there's more biomass and it's being taken out of the air. If there's a spike in carbonyl sulfide, it means that there's no growth in biomass, or even a biomass loss.
Turns out, that when CO2 dropped during the Little Ice Age, carbonyl sulfide spiked. If their interpretation is correct, it means that something else caused the air to cool, this decreased the respiration of existing plants, and that in turn drove down atmospheric CO2 levels. This directly contradicts the notion that a lot of trees grew before the LIA.
What caused the cooling in the LIA? There aren't any perfect candidates, although things like Milankovich cycles, sun spot activities, and of course colonization of the New World have all been fingered.
One wacko possibility is that, had industrialization not happened, the Holocene interglacial would have ended and we'd be heading into the next ice age now. Conceivably the LIA was the aborted start of the next ice age, and the fact that civilizations especially in Eurasia adapted to the cold, rebuilt after losing 1/3 of their people, and Europe started industrializing, kept the glaciers from growing out of control. There's little proof for that. Given that our genus evolved during the ice ages, and given what our civilization is doing right now, I'm not entirely sure that this was a good thing. But it's far too late to choose the ice. If indeed that's what actually happened.
Anyway, it's worth reconsidering the whole New World pandemic and afforestation scenario as a driver. It's a great story, but the evidence doesn't really support it. It also conflicts with a lot of other evidence.
For one thing, we've got data that Eurasian populations decreased by about 1/3 during the LIA, but no one's going on about how reforestation in Europe and Asia drove the LIA. Why not? Abandoned fields in much of Europe and Asia grow trees just as readily as they do in North America. We also have the example of the Black Death and all the other virgin ground plagues in Eurasia, that, again, killed huge numbers of people without, so far as we know, causing little ice ages. We've also got examples of afforestation in North America up to the 20th century, without CO2 drawdowns. And, despite what was written in 1491, there is actually good evidence of huge numbers of bison, huge numbers of passenger pigeons, and huge numbers of vegetarian sea turtles in the New World prior to initial European colonization, so the idea that their numbers boomed when New World humans were decimated, and that this fiddled with atmospheric CO2 levels, doesn't wash.
Also, passenger pigeons and bison were decimated during the Gilded Age, not during the first colonization wave, and the American Great Plains were thinly populated with humans, but hugely populated with grazers, before the Spanish introduced guns and horses. So that doesn't work. And we can get into the details of how a bunch of North American cities were already being depopulated in response their rulers' increasingly authoritarian politics, well before the LIA started.
And, just this week, Science published a note that new radiocarbon dating techniques used on old villages is radically revising the dates of occupation. Apparently before, site dating depended on the presence or absence of European artifacts, because radiocarbon dating on villages of that time is tricky ( https://www.science.org/content/article/redated-artifacts-rewrite-how-indigenous-people-responded-european-contact? ).
Among other details, they reported that one of the sites De Soto is thought to have visited turns out to have been occupied a full century after his visit. It wasn't rapidly abandoned as previous archeologists had thought. As one researcher noted, just because European artifacts were available doesn't mean that the Indians wanted them, and previous archeological dating efforts had assumed otherwise.
So it may well be that Charles Mann in 1491 probably got some major details wrong, just as Jared Diamond turns out to have gotten a lot of things wrong in Collapse. It's sad, because they're great books. Fortunately, they inspired a lot of good science. Unfortunately, that science marched over them, instead of supporting them. Such is science.
