Exploring Sensitivity of Carbon Dioxide and Aerosol Forcing When Modelling the Mid-Miocene Climatic Optimum

Modelling the mid-to-high latitude warmth of the Mid-Miocene Climatic Optimum (MMCO-17-14 Ma) in comparison to available proxy records has proven to be challenging. Previous MMCO modeling has been unsuccessful in trying to replicate the mid-latitude warmth at reconstructed CO₂ levels through changing ice sheets, orbital variability, vegetation, sea-ways, and other boundary conditions. Several hypotheses have been invoked to explain the discrepancy. These include, our inability to identify missing paleo-climate forcing, models lacking sufficient positive feedbacks, or misinterpretation of the signals in proxy records.

Here we explore two mechanisms which could be important in our understanding of past warmth of the MMCO. First, we employ the recently released Community Atmosphere Model (CAM5) from the National Center for Atmospheric Research (NCAR) in fully coupled mode, which has higher sensitivity to CO₂ forcing than its predecessor CAM4. Second, we explore the significance of changing aerosols within the CAM5 model framework, which includes cloud aerosols indirect effects in its cloud microphysical schemes.

Preliminary results in comparison to compiled MMCO proxy records show that CAM5 is a better match at specific locations and in the mean annual when compared to CAM4. Specifically, a reduction of aerosol concentrations within CAM5 produces additional warming in the high latitudes. While the higher sensitivity to CO₂ forcing, in CAM5, induces a general increase in the mean annual temperature in the simulation. The combination of these two forcings is able to keep the CAM5 MMCO simulations relatively sea-ice free year round.