Even before industrialization, humans were having an effect on atmospheric carbon dioxide levels by transforming natural land for agricultural uses, report researchers from the Max Planck Institute for Meteorology in Hamburg, Germany.
Julia Pongratz and her co-workers combined a reconstruction of historical land cover with a coupled biosphere–atmosphere–ocean general-circulation model going back over more than a millennium. The work suggests that land-use changes resulted in the release of 53 gigatonnes of carbon between AD 800 and 1850. Only 21% of this remained airborne, with the rest being reabsorbed by the oceans and biosphere, but that was enough to bump CO2 emissions up above background levels by the late medieval period.
The team also looked at the impact of wars and epidemics, and found that the thirteenth-century Mongol invasion of China resulted in carbon sequestration, because the massive death toll led to vegetation regrowth on abandoned farmlands.
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Biogeochemistry: Preindustrial carbon. Nature 461, 850 (2009). https://doi.org/10.1038/461850e
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DOI: https://doi.org/10.1038/461850e
