Evaluating Climate Emulation: Unit Testing of Simple Climate Models

Abstract

Abstract. Simple climate models (SCMs) are numerical representations of the Earth’s gas cycles and climate system. SCMs are easy to use and computationally inexpensive, making them an ideal tool in both scientific and decision-making contexts (e.g., complex climate model emulation; parameter estimation experiments; climate metric calculations; and probabilistic analyses). Despite their prolific use, the fundamental responses of SCMs are often not directly characterized. In this study, we use unit tests of three chemical species (CO2, CH4, and BC) to understand the fundamental gas cycle and climate system responses of several SCMs (Hector v2.0, MAGICC 5.3, MAGICC 6.0, FAIR v1.0, and AR5-IR). We find that while idealized SCMs are widely used, they fail to capture important global mean climate response features, which can produce biased temperature results. Comprehensive SCMs, which have non-linear forcing and physically-based carbon cycle representations, show improved responses compared to idealized SCMs. Even some comprehensive SCMs fail to capture response timescales of more complex models under BC or CO2 forcing perturbations. These results suggest where improvements should be made to SCMs. Further, we provide a set of fundamental tests that we recommend as a standard validation suite for any SCM. Unit tests allow users to understand differences in model responses and the impact of model selection on results.

Impacts of Observational Constraints Related to Sea Level on Estimates of Climate Sensitivity

“…Recently, by combining simple contributor model components, reduced complexity models have been developed and applied to uncertainty quantification surrounding sea level projections (Bakker et al, ; Nauels et al, ; Wong, Bakker, Ruckert, et al, ). Hector (v2.0), the reduced complexity model used in this study, is an open source, object‐oriented, simple global climate carbon cycle model (Hartin et al, ; Schwarber et al, ). It has been applied to ocean acidification projections and sensitivities (Hartin et al, ) and is the default carbon cycle climate module of the Global Change Assessment Model (Calvin et al, ).…”