Did human land use affect atmospheric CO2 before industrialization?

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Researchers ran transient simulations over the entire last millennium using a general circulation model coupling the atmosphere, ocean and land surface with a closed carbon cycle. They used a high-detail reconstruction of anthropogenic land-cover change (ALCC) as the climate system’s only forcing, to isolate its effects on the carbon cycle and climate from other natural and human disturbances and to assess preindustrial human activities.

With ALCC as the only forcing, the terrestrial biosphere lost a net 96 Gt C over the millennium, raising atmospheric CO2 by 20 ppm. Biosphere–atmosphere coupling restored 37% of primary emissions during the industrial period (A.D. 1850–2000) and 48% during the preindustrial period (A.D. 800–1850). Because the coupling flux was stronger in the preindustrial period, only 21% of its 53 Gt C of emissions remained airborne.

Despite this low airborne fraction, atmospheric CO2 rose above natural variability by late medieval times, suggesting that human influence on CO2 began before industrialization. Global mean temperatures, however, were not significantly altered until the strong population growth of the industrial period.

The researchers also investigated how historic events such as epidemics and warfare affected the carbon budget. Only long-lasting events such as the Mongol invasion led to carbon sequestration. Indirect emissions from past ALCC offset carbon uptake by regrowing vegetation for several decades, limiting sequestration. The researchers therefore considered drops in ice-core CO2 unlikely to be attributable to human action.

The results indicate that climate–carbon cycle studies of present and future centuries, which usually start from an equilibrium state around 1850, start from a significantly disturbed state of the carbon cycle.