Late Holocene climate: Natural or anthropogenic?

Abstract:

In the mid-late Holocene, while Northern Hemisphere summer insolation has decreased, there was a reversal of the typical downward trends in CH₄ and CO₂ concentration beginning around 5,000 to 7,000 years ago. Because this anomalous behavior was presumably due to anthropogenic carbon emissions from early agriculture, the Holocene provides a unique opportunity to evaluate the relative strength of natural astronomical vs. early anthropogenic forcing. Here we present the most comprehensive transient and snapshot (time-slice) global climate simulations of the middle-late Holocene to investigate one of the fundamental questions of the Holocene: when did humans assume the "driver’s seat" of the global climate system?

Three 6,000-year long transient simulations have been performed in 2-degree resolution with the Community Earth System Model version 1.2 (CESM1.2). In one experiment CESM1.2 was driven by historical variations in orbital parameters, greenhouse gases (GHG), and land clearance from early agriculture, whereas the latter two forcings differed in the counterpart experiment by using the hypothesized declines in CO₂ and CH₄ and without land clearance. A third simulation was driven only by orbital parameters of the past 6000 years. A comparison between the proxy data reconstruction and the transient actual Holocene climate simulation addresses the uncertainties of actual Holocene climate evolution associated with both proxy data (e.g., effects of seasonality, spatial and temporal coverage) and climate models (e.g., climate sensitivity). A comparison between the transient simulations with twenty-four snapshot 1-degree CCSM4 simulations (1,000 years apart from the past 6,000 years with actual climate forcing as well as single forcing of actual GHG, orbital variations and land cover changes) shows that the transient simulation reproduces the results of snapshot simulations with ~0.3 °C cooling from orbital variations and ~0.15 °C warming from the combined forcing of GHGs, orbital and land clearance. The simulated global temperature differences between the actual and “natural” Holocene transient simulations amounts to a global warming of 0.8 °C during the preindustrial era, indicating that the early anthropogenic warming is nearly as much as the global warming during industrial times.