Physical inconsistencies in the representation of the ocean heat-carbon nexus in simple climate models

The oceans slow the rate of global warming by absorbing each year about 25% of the anthropogenic CO₂ emissions and 90% of the additional heat resulting from the Earth energy imbalance induced by the accumulation of greenhouse gases in the atmosphere. The interplay between the ocean heat and carbon uptake, the “Ocean Heat-Carbon Nexus”, links together the responses of the Earth climate and the global carbon cycle to cumulative CO₂ emissions and to net zero CO₂ emissions. It results from a suite of processes involving the exchange of heat and carbon across the sea-air interface as well as their storage below the mixed-layer and redistribution by the ocean large-scale circulation. The Ocean Heat and Carbon Nexus is assumed to be consistently represented across two modelling platforms used in the latest IPCC assessments: the Earth System Models (ESMs) and the Simple Climate Models (SCMs). However, our research shows significant deficiencies in state-of-the-art SCMs in replicating the ocean heat-carbon nexus of ESMs due to a crude treatment of the ocean thermal and carbon cycle coupling. With one SCM, we show that a more realistic heat-to-carbon uptake ratio exacerbates the projected warming by 0.1°C in low overshoot scenarios and up to 0.2°C in high overshoot scenarios. It is therefore critical to explore how SCMs' physical inconsistencies, such as the representation of the ocean heat-carbon nexus, can affect future warming projections used in climate assessments, not just by SCMs in Working Group 3 but also by ESMs in Working Group 1 via SCM-driven emission-to-concentration translation.