Long-term and orbital-scale climate and carbon cycle change across the Miocene Climatic Optimum

Abstract

Climate reconstructions show that during the relatively warm Miocene Climate Optimum (MCO), ~17 million years ago, atmospheric CO2 concentrations were in the same range as predicted by models for future climate change. Therefore, the MCO is also referred to as a historical climate analogue as it can offer crucial information to better predict future climate change. To better understand global climate variability during the MCO (~18 – 15 million years ago), I present new records of sea surface temperature and atmospheric CO2 concentrations. These new records were generated using methods involving organic geochemistry and reconstructions of microfossil assemblages, which were conducted on sediments cored from the ocean floor in, a.o., the tropical Atlantic Ocean and the Southern Ocean. The results describe past climate variability with relatively high temporal resolution, allowing us to interpret variations on both short (~10,000 years) and long (>1,000,000 years) timescales. We show that sea surface temperatures in the eastern equatorial Atlantic Ocean abruptly increased by ~2 °C at the start of the MCO. This warming was accompanied by an intensification of the West African monsoon, resulting in notable peaks in productivity and upwelling of cold oceanwater. Furthermore, we demonstrate for the first time that atmospheric CO2 levels increased concurrent with this sea surface temperature increase. However, in contrast to these results, a compilation of global sea surface temperature records shows that global warming during the MCO was limited. Therefore, we suggest that, in addition to atmospheric CO2, ocean temperatures were also forced by changes in ocean circulation, which we confirm with model simulations.