The Guardian ran an article today claiming that the mass death of Native Americans and thus re-wilding of land they'd farmed, caused a CO2 sink that reduced global temperatures. Is that a reasonable theory?
Yeah, it is certainly reasonable. The dip in atmospheric CO2 was dubbed the Orbis Spike by Lewis and Maslin (2015). The Orbis Spike marked a decline in atmospheric CO2 of 7-10 p.p.m. between 1570 and 1620, which is documented in two high-resolution Antarctic ice core records. Lewis and Maslin argue that this dip in atmospheric CO2 marks the most prominent feature, in terms of both rate of change and magnitude of change, in pre-industrial atmospheric CO2 records over the past 2,000 years. That is because with a loss of ~54 million people in Americas over a period of about a century, over 50 million hectares of forest, woody savanna, and grassland regenerated and locked in atmospheric carbon.
But it wasn't just the ice cores that marked this global cooling effect. Lewis and Maslin also note that "a range of deposits showing distinct changes at that time, including tephra and other signatures from the 1600 Huaynaputina eruption detected at both poles and in the tropics; charcoal reductions in deposits in the Americas and globally; decreases in atmospheric methane, enrichment of methane d13C, and decreases in carbon monoxide in Antarctic ice cores; pollen in lacustrine sediments showing vegetation regeneration; proxies indicating anomalous Arctic sea-ice extent; changing d18O derived from speleothems from caves in China and Peru and other studies noting changes coincident with 1600 and the coolest part of the Little Ice Age (1594–1677, a relatively synchronous global event noted in geologic deposits worldwide" (Lewis and Maslin 2015: 175).
The meeting of the Old and New World resulted in an unprecedented interaction, not only in the exchange of culture and knowledge between peoples, plants and animals between continents, and deadly diseases, but also in a fundamental shift in our environment that was counteracted and made worse by later industrialization. This is not a radical or fringe proposition, but one that we can see occur via multiple lines of solid, supporting evidence.
Lewis, Simon L., and Mark A. Maslin. "Defining the anthropocene." Nature 519.7542 (2015): 171.