Collaborative Research: P2C2--Late Holocene Climate: Natural or Anthropogenic?

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This collaborative project generally aims to address a fundamental and yet unresolved question about Holocene climate which is: When did humans assume the "driver's seat" of the global climate system? Did it coincide with the advent of the Industrial Era, as traditionally believed, or did it instead occur many millennia ago with the emergence of agriculture? The scientific community remains divided on whether carbon emissions released by early farming activities were large enough to explain the unprecedented rise in atmospheric greenhouse gas (GHG) concentrations during the late Holocene compared with declines in these climatic drivers during recent previous inter-glacial periods. This research will test the "Early Anthropogenic Hypothesis," which posits that anthropogenic global climate change was initiated by agriculture in the middle Holocene, by identifying the "fingerprint" of human influence on global climate during the past 6,000 years.The project will capitalize on state-of-the-art earth system models to distinguish between the actual climatic evolution of the late Holocene and the hypothesized "natural" trajectory that would have been followed in the absence of human land clearance and greenhouse-gas overprints. To achieve this goal, a pair of 6,000-year long transient simulations will be run with the Community Earth System Model (CESM). In one experiment CESM will be driven by historical variations in orbital parameters, GHG concentrations, and regional land clearance, whereas the latter two forcings will differ in the counterpart experiment according to the hypothesized declines in carbon dioxide and methane and the associated global vegetation patterns compatible with "natural" climate evolution.The Broader Impacts involve the potential for a unique and potentially transformative shift in the scientific understanding of human influence on global climate.