A Prognostic Theory for the Land-Ocean Warming Contrast

Comprehensive simulations and observations consistently show that surface temperatures warm faster over land than over the ocean in response to increased CO2. While several diagnostic explanations related to climatological moisture availability have been proposed (such as the lapse rate mechanism and the Bowen ratio mechanism), they do not offer a prognostic prediction of the land/ocean warming contrast under a prescribed, arbitrary CO2 forcing. In this work, we use the local tropospheric heat budget to write a prognostic reduced-complexity model that calculates the response of land and ocean surface temperatures to any arbitrary forcing. The model is a simple linear dynamical system (3 coupled ODEs), is based on only a few parameters (radiative feedbacks, heat capacities and strength of the energetic linkage between ocean and land), and is flexible enough to accurately mimic (or “emulate”) different CMIP6 coupled models. We use our simplified prognostic description of the land/ocean contrast to address three key questions: (1) What causes the land/ocean warming contrast from an energy balance perspective, and how does it relate to previous diagnostic, moisture-related theories? (2) Why does the land/ocean contrast decrease over time in abrupt 4xCO2 simulations but remain approximately steady in historical and typical SSP scenarios? (3) What drives the intermodel spread in the CMIP6-predicted land/ocean contrast? We argue that these questions are difficult to answer only with CMIP6-grade models, but are readily answered once the full-complexity models are “summarized” into the few parameters that govern the evolution of land and ocean temperatures.