Although comprising less than 4% of all terrestrial plant species, C4 plants are an essential component of low-latitude ecosystems, and in recent modeling simulations have been shown to strongly influence the stable carbon isotope ratio of the atmosphere. We used a fully coupled Earth system model (HadCM3LC) to evaluate the contribution of C4 plants to the exchange of energy and water between the biosphere and atmosphere. Our simulations indicate that the presence or absence of C4 plants is important for understanding regional climate, specifically with respect to seasonal climate patterns. When C4 plants are absent from simulated tropical ecosystems, the percentage of bare soil increases regionally, Amazonia becomes warmer during the dry season, South Africa becomes drier during the dry season, the African Sahara-Sahel boundary becomes hotter during the dry season, and yet temperatures cool in central Australia during the Austral winter. Our modeling study provides the first insights into the potential climate feedbacks of late-Miocene C4-C3 ecosystems on surface energy and water exchange.