MAMMOTH BURPS: DID HUMAN IMPACTS ON THE ENVIRONMENT PREDATE THE HOLOCENE?

The human-mediated extinction of large herbivorous megafauna in the late Pleistocene had profound effects on terrestrial community structure and function, but may also have influenced atmospheric gas exchange. Herbivorous mammals are major producers of methane, a potent greenhouse gas with a high global warming potential; domestic livestock currently contribute ~20% of the annual input. Here, using allometric relationships between body mass and density, methane production and estimates of geographic range, we calculate the annual decrease in methane production resulting from the extinction of 114 species of large-bodied herbivore from the Americas ~13,400-11,500 ybp. Our results suggest a loss of at least ~9.6 Tg (upper limit, 25.5 Tg) CH4 annually, sufficient to explain a significant portion (12.5-100%) of the ~200 ppbv drop detected in isotopic analyses of ice-core records. We suggest humans measurably influenced global biogeochemical processes long before the development of agriculture, complex civilizations and the obvious impacts of the industrial age.