A warm Miocene climate at low atmospheric CO2 levels

Proxy records from the Miocene epoch (∼23‐5 Ma) indicate a warmer climate than today in spite of lower atmospheric carbon dioxide (CO2) concentrations in the range of preindustrial levels. As yet the simulation of a warm Miocene climate with these low CO2 values has proven to be a challenge. In this study we present climate simulations of the Late Miocene (11‐7 Ma) with a preindustrial CO2 level, using a coupled atmosphere‐ocean general circulation model (AOGCM). The simulated global mean surface temperature of ∼17.8 °C represents a significantly warmer climate than today. We have analyzed the relative importance of tectonic and vegetation changes as forcing factors. We find that the strongest temperature increase is due to the Late Miocene vegetation distribution, which is more than three times stronger than the impact induced by tectonic alterations. Furthermore, a combination of both forcing factors results in a global temperature increase which is lower than the sum of the individual forcing effects. Energy balance estimates suggest that a reduction in the planetary albedo and a positive water vapor feedback in a warmer atmosphere are the dominating mechanisms to explain the temperature increase. Each of these factors contributes about one half to the global temperature rise of ∼3 K. Our results suggest that a much warmer climate during the Late Miocene can be reconciled with CO2 concentrations similar to preindustrial values. Key Points We simulate a warm climate of the late Miocene at preindustrial CO2 levels The temperature rise is largely due to the late Miocene vegetation distribution Our results support CO2 reconstructions based on marine geochemical proxies