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How long have human activities been affecting the climate system?

Final Report Summary - EARLYHUMANIMPACT (How long have human activities been affecting the climate system?)

The overarching objective of the Early Human Impact project was to quantify the temporal and spatial changes in Holocene fire activity. The project aimed to assess the impact of human activities and associated biomass burning on the global climate system, even before the Industrial Revolution. The use of ice and lake cores as climate archives allows to reconstruct fire history that can be coupled with the early agriculture development. This approach was based on the analysis of specific molecular markers for fire activity and human presence. The main scientific outcomes of this project are following summarized:
• As evidenced in many continental and ice core records analyzed in this project, at millennial time scales, global biomass burning showed an increase during the Late Glacial-Holocene transition, consistent with an increase of temperatures, CO2 concentrations and biomass availability. However, during the Holocene (before Industrial Revolution), the correlation between the major climate parameters respect with changes in biomass burning was not as much as evident. A more detailed understanding of the relation between fire and climatic parameters came out from regional reconstructions. Here, we clearly showed that fire is mainly sensitive to drier/wet climate oscillations, vegetation distribution and/or anthropogenic agricultural practices and land use. In turn, fire mainly acted in changing vegetation regime, thus influencing regional moisture balance and impacting on the landscape, the ecosystem and people.
• At global scale, during the Holocene (before Industrial revolution), global syntheses obtained from levoglucosan records, showed a progressive increase in fire activity, peaking at ~3000 yrs BP. The intensity of these signals suggested that past human impacts on fire activity could be greater than or equal to modern human contribution on fire.
• Biomass burning in continental records resulted much more fragmented along time and space, during the Holocene. Indeed, the timing of fire occurrence differs from continent to continent, since both natural and anthropogenic regional forcing were not synchronous.
• We argued the Ruddiman hypothesis which stated that the reversal increase in methane and carbon dioxide concentration, observed in ice core records at ~7000 and ~5000 yr BP, respectively, was due to early agriculture activities. Although the Ruddiman hypothesis raised up many debates in the recent years, biomass burning was unlikely the main cause of the CO2 and CH4 increase observed at 7000 and 5000 yr BP. In contrast, we propose that the first global anthropogenic signature on the environment was likely the anomalous fire activity increase observed at ~3000 yrs BP. No real detectable influence on climate parameters (e.g. temperature and/or precipitation) can be evinced from our results.
• In the framework of this project we also developed many analytical methods for determining a number of paleoproxy in ice and lacustrine sediment cores. These can successfully used for reconstructing fire and human presence, but also, moving to future applications in paleoclimatology, they can potentially provide information on sea-ice and biosphere dynamics.