The debate over the early Martian climate is among the most intriguing in planetary science. Although the geologic evidence generally supports a warmer and wetter climate, climate models have had difficulty simulating such a scenario, leading some to suggest that the observed fluvial geology (e.g., valley networks, modified landscapes) on the Martian surface could have formed in a cold climate instead. However, as we have originally predicted using a single-column radiative-convective climate model (Ramirez, Kopparapu, Zugger, et al., 2014, https://doi.org/10.1038/ngeo2000), warming from CO2-H-2 collision-induced absorption on a volcanically active early Mars could have raised mean surface temperatures above the freezing point, with later calculations showing that this is achievable with hydrogen concentrations as low as similar to 1%. Nevertheless, these predictions should be tested against more complex models. Here we use an advanced energy balance model that includes a northern lowlands ocean to show that mean surface temperatures near or slightly above the freezing point of water were necessary to carve the valley networks. Our scenario is consistent with a relatively large ocean as has been suggested. Valley network distributions would have been global prior to subsequent removal processes. At lower mean surface temperatures and smaller ocean sizes, precipitation and surface erosion efficiency diminish. The warm period may have been approximately <10(7) years, perhaps suggesting that episodic warming mechanisms were not needed. Atmospheric collapse and permanently glaciated conditions occur once surface ice coverage exceeds a threshold depending on collision-induced absorption assumptions. Our results support an early warm and semiarid climate consistent with many geologic observations. Plain Language Summary Mars today is a dry and cold planet that looks quite "Moon-like." However, ancient landscapes on Mars depict a world that may have been much more Earth-like billions of years ago. The surface is filled with ancient riverbeds, deltas, and even features that look a lot like ancient ocean shorelines. Some scientists think that such features suggest a warmer and wetter early planet whereas others believe that these features had formed when the planet was not too different from today. Here we test the idea that a warm early Mars could have had a large northern ocean sustained by a thick carbon dioxide and hydrogen atmosphere. We first confirm previous studies that showed that such an atmosphere could have made Mars warm. We then demonstrate that a relatively large ocean would have been required to form the water features we see on the ancient surface. It may have taken as little as 10 million years or less to form them. In contrast, cold and icy climates, even if they were sporadically warmed from time to time, may not have produced enough precipitation to form these features. We confirm that rain would have been the dominant precipitation on a warm early Mars.
