The mechanisms which amplify small orbitally driven changes in insolation and drive the glacial-interglacial cycles of the past 2.7 million years are poorly understood. Previous studies indicate that cloud feedbacks oppose ice sheet initiation at times when orbital configuration supports ice sheet growth. A recent study in which cloud phase was observationally constrained by satellite measurements provides evidence for a weaker opposing cloud feedback than previously found in response to carbon dioxide doubling (Tan et al., 2016). We observationally constrain cloud phase in the Community Earth System Model. We find a weaker cloud phase feedback, which unmasks water vapor and cloud feedbacks that extend cooling to lower latitudes. Snowfall accumulation and ablation metrics also support ice sheet expansion as seen in proxy records. Our results indicate that well understood cloud and water vapor feedbacks are the amplifying mechanism driving orbital climates.