The Varying Earth’s Radiative Feedback Connected to the Ocean Energy Uptake: A Theoretical Perspective from Conceptual Frameworks

Abstract

Abstract When quadrupling the atmospheric CO2 concentration in relation to preindustrial levels, most global climate models show an initially strong net radiative feedback that significantly reduces the energy imbalance during the first two decades after the quadrupling. Afterward, the net radiative feedback weakens, needing more surface warming than before to reduce the remaining energy imbalance. Such weakening radiative feedback has its origin in the tropical oceanic stratiform cloud cover, linked to an evolving spatial warming pattern. In the classic linearized energy balance framework, such variation is represented by an additional term in the planetary budget equation. This additional term is usually interpreted as an ad hoc emulation of the cloud feedback change, leaving unexplained the relationship between this term and the spatial warming pattern. I use a simple nonlinearized energy balance framework to justify that there is a physical interpretation of this term: the evolution of the spatial pattern of warming is explained by changes in the ocean’s circulation and energy uptake. Therefore, the global effective thermal capacity of the system also changes, leading to the additional term. In reality, the clouds respond to what occurs in the ocean, changing their radiative effect. In the equation, the term is now a concrete representation of the ocean’s role. Additionally, I derive for the first time an explicit mathematical expression of the net radiative feedback and its temporal evolution in the linearized energy balance framework. This mathematical expression supports the new proposed interpretation. As a corollary, it justifies the 20-yr time scale used to study the variation of the net radiative feedback. Significance Statement Linearized energy balance models have helped the study of Earth’s radiative response. However, the present linear models are at the edge of usefulness to get more insights. In this work, I justify that part of the nonlinearity in the radiative response can be explained without peculiar atmospheric radiative feedback mechanisms or a nonlinearity in the radiative response. Instead, the concept of an evolving thermal capacity recovers the ocean’s role in redistributing the energy, changing the spatial warming pattern, and, finally, altering the atmospheric feedback mechanisms. This work also justifies the time scales used in the field for studying the variation of the net radiative feedback.