Jeffrey T. Kiehl (National Center for Atmospheric Research)
Earth has experienced warm climates throughout the geologic past. These deep past warm climates are associated with elevated atmospheric carbon dioxide. Proxy data from these warm time periods indicates extreme warmth at high latitudes. This excessive warm polar amplification has been very difficult to simulate with fully coupled climate system models and has been termed the Equable Climate Problem or Cold Pole Problem. Assuming we continue to burn fossil fuels at their current rate atmospheric CO2 concentrations will reach ~1000 ppmv by 2100, levels not seen for at least 30 million years in Earth’s history. Given this situation, it is imperative that we understand how Earth’s climate processes operate under enhanced greenhouse forcing. In this presentation, I review the observational evidence for Earth’s past warm climates and the various mechanisms proposed to explain the disparity between proxy data and model simulations. I then present results from new climate simulations of the Paleocene Eocene Thermal Maximum (PETM) using the Community Climate System Model (CCSM3) that include the effects of aerosols on cloud properties. These simulations are in better agreement with many of the proxy data for this time period. I discuss the relevance of these simulations to understanding future climate change and Earth’s climate sensitivity.