Contrasting response of hydrological cycle over land and ocean to a changing CO₂ pathway

The hydrological cycle has a significant impact on human activities and ecosystems, so understanding its mechanisms with respect to a changing climate is essential. In particular, a more detailed understanding of hydrological cycle response to transient climate change is required for successful adaptation and mitigation policies. In this study, we exploit large ensemble model experiments using the Community Earth System Model version 1.2.2 (CESM1) in which CO₂ concentrations increase steadily and then decrease along the same path. Our results show that precipitation changes in the CO₂ increasing and decreasing phases are nearly symmetrical over land but asymmetric over oceans. After CO₂ concentrations peak, the ocean continues to uptake heat from the atmosphere, which is a key process leading the hydrological cycle’s contrasting response over land and ocean. The symmetrical hydrological cycle response over land involves a complex interplay between rapid responses to CO₂ and slower responses to ensuing warming. Therefore, the surface energy constraints lead to the contrasting hydrological response over land and ocean to CO₂ forcing that needs to be verified and considered in climate change mitigation and adaption actions.