Can High-altitude Water-ice Clouds Sustain Dry–wet Cycles in Early Mars Climate?

Geological evidence indicates that Mars experienced multiple lake-forming climates lasting longer than 100 years around 3–4 billion years ago. These early warm climates cannot be explained solely by the greenhouse effect of carbon dioxide and water vapor. Recently, a warming mechanism driven by high-altitude water-ice clouds has been proposed for early Mars under surface water-limited climatic conditions. Here, we develop a general circulation model for terrestrial planetary atmospheres capable of simulating both early and modern climates of Earth and Mars. Simulation results show that the radiative effect of clouds can lead to two distinct climate states: when low-latitude surface regions are relatively arid, cloud radiative effects are dominated by warming, which can sustain dry–wet cycles in early Mars climate; however, when surface meltwater in low-latitude regions exceeds a critical threshold, cloud radiative effects shift to cooling, maintaining the climate in a cold, stable state. This work provides a new perspective for studying the climate evolution of early Mars.