A (RE-)EMERGING VIEW ON PAST LAND CLEARANCE

Most modelers who have estimated the history of land use have assumed that the amount of forest clearance was closely (almost linearly) tied to population. In this view, most deforestation occurred during the explosive increase from 600 million people in 1700 to the current 7 billion. These studies have concluded that only about one third of total forest clearance occurred before 1850, the start of the industrial-era CO2 rise. This view suggests that hydrologic changes caused by land disturbance in once-forested areas should also have mainly occurred during the industrial era. Yet scientists working in geoarcheology and related field-based disciplines know of evidence in many regions that contradicts this view. Erosion that began millennia ago caused increased silt fluxes to lakes, rivers and deltas. Silting of river mouths forced seaward relocation of many coastal harbors.

The reason for this mismatch in views lies in the invalid assumption that clearance is linearly tied to population. Ester Boserup long ago summarized evidence that early farmers practicing shifting cultivation used much more land per-capita than those who have recently cultivated the same plots of land every year. Historical evidence from China and Europe shows decreases in per-capita land use by a factor of four during the last 2000 years, and the total decrease from the middle to late Holocene may have been a factor of ten or more. New model simulations by Jed Kaplan and colleagues allowing for changing per-capita clearance indicate that as much as ¾ of total forest clearance (and related emissions of forest carbon) occurred prior to industrial times. These simulations suggest that pre-industrial soil disturbance and hydrologic disruption could have been very large millennia ago in intensively farmed areas. The evidence for early forest clearance also strengthens the claim that early farming contributed to the anomalous CO2 increase during the last 7000 years.