Linking carbon cycling to climate feedbacks in a simple climate model for decarbonization

Complex models of the Earth system are increasingly able to represent processes that make up the carbon-climate system, but a variety of simple climate models (SCMs) use parameterized representations of the Earth system, which make them easily deployed tools for climate mitigation assessment and accessible tools for conceptual understanding. However, SCMs vary in their approach to simplifying the Earth system, especially in their representation of the carbon cycle. We examine how two distinct carbon cycle structures within one SCM, FaIR, produce differing constrained projections of future climate under idealized decarbonization. We find that differences in carbon cycle structure lead to differences in the timescales of carbon uptake, which do not directly lead to or explain differences in warming under the same decarbonization emissions scenario. Differences in the metrics of warming are instead primarily explained by assumptions about climate feedbacks and non-carbon cycle forcing, which are parameterized separately from carbon cycling. When we introduce a physically-motivated link reflecting the connection between ocean circulation and energy balance, we see a change in the set of climate feedbacks necessary to explain our observed carbon-climate system. The result is a shift in TCRE, ZEC, and consequent necessary mitigation.