Importance of Longwave Radiative Forcing by Icy Clouds in Maintaining Miocene High-Latitude Warmth

During the early-to-middle Miocene, global mean surface temperature (GMST) was approximately 8°C warmer than preindustrial, with a greater temperature increase in polar regions than the tropics. However, existing Miocene simulations underestimate this warmth, particularly in northern high latitudes. To address this discrepancy, we investigate the potential role of cloud phase. Using the Community Earth System Model, we conduct a paleoclimate sensitivity study focused on modifying ice nucleation and cloud phase partitioning schemes. These modifications increase the GMST, with a strong temperature rise in high latitudes and a muted increase in the tropics. These increases are driven by enhanced longwave cloud forcing, resulting from increased ice cloud amounts and cloud water content, and are amplified by water vapor and lapse rate feedbacks in the Arctic. Our study highlights that the improved parameterizations of cloud phase processes enhance models' capability to simulate Miocene high-latitude warmth and potentially other warm climates.