The early impact of agriculture : how humans have been affecting climate for thousands of years

Abstract

Humans have started shaping the environment since their very first agricultural activities. Whether this influenced climate before the industrial era is object of a lively scientific discussion. Climate changes in the Late Pleistocene – Holocene transition allowed an unprecedented geographic expansion and population growth due to the possibility of producing food. Although centers of early origin and expansion of agriculture are well documented worldwide, researchers highlight the lack of instruments and data for assessing the local and regional effects of deforestation and farming activities on climate. In this work, specific molecular markers for tracing past fire events and evaluating human and animal presence are individuated. Namely, polycyclic aromatic hydrocarbons (PAHs), monosaccharide anhydrides and fecal sterols are used as source specific and stable indicators in lacustrine environments. In the first part, specific analytical methods for detecting and quantifying the selected tracers in lake sediment cores are developed. In the second part, the proposed methods are tested and applied on target locations. New Zealand is known to represent a particular site for the study of the ecological impacts of human settlement. It was occupied by Polynesian only 700-800 y BP and resulted in abrupt and huge landscape modifications. Here, the molecular marker methods were applied on samples from two lakes and results were compared with existing paleoecological records, proving their validity in order to confirm human presence and correlate it with intensity and frequency of fires. Results showed a dramatic increase in the fluxes of both fire and human tracers soon after the Māori arrival, consistent with intensive anthropogenic land clearance, as previously hypothesized from charcoal and pollen evidence. The European 19th century colonization is also evident in the flux of fecal sterols, that rapidly increased following the population growth. Molecular tracers were further analyzed on test sample batches from Lake Victoria (Uganda) and Flinders Island (Tasmania). In Africa, increased fire activity is observed in correspondence with the drier periods documented by paleolimnological proxies. the influence of the eastward migration of the Bantu speaking populations in the last 2000 years is visible in the sterol record, that shows good correspondence with changes in vegetation and fire regimes. In Tasmania, fecal sterols vary in accordance with the human presence, that was intermittent along the last 10,000 years, although higher resolution would be required in order to draw more precise conclusions. All results are presented, compared with literature data and interpreted according to paleoecological, anthropological and archaeological evidence, where possible. This work provided a reliable and objective instrument for future studies oriented to the creation of a spatial and temporal high resolution database of the early human impact on landscape and climate.