PNNL

Abstract

The oceans slow the rate of global warming by absorbing each year about 25% of the an-thropogenic CO2 emissions and 94% of the additional heat resulting from the radiative forc-ing of greenhouse gases1,2. The interplay between the uptake of heat and carbon by the oceans, also known as the “Ocean Heat-Carbon Nexus”, links together the responses of the Earth climate and the global carbon cycle to cumulative CO2 emissions1-4. This nexus results from a suite of processes involving the exchange of heat and carbon across the sea-air interface and their storage below the mixed-layer depth. Because this suite of processes is well understood, the representation of the Ocean Heat and Carbon Nexus is assumed to be correctly captured by the current generation of climate mod-els. However, we show here that the representation of the ocean heat-carbon nexus within state-of-the-art Simple Climate Models (SCMs) differs substantially from that of the current gener-ation Earth system models (ESMs) as used in the latest IPCC assessments4,6. A meta-analysis conducted on a large array of SCMs suggests that a physical inconsistency arises from the structure of SCMs, which crudely accounts for the coupling between the ocean thermal and carbon cycle modules. We show that this physical inconsistency may have important consequences in future warming as modelled by the SCMs both in terms of magnitude and timing. We point out that a more realistic heat-to-carbon uptake ratio lead to exacerbate the projected warming by 0.1°C in the low overshoot scenarios and up to 0.2°C in high overshoot scenarios. Although small, this difference in projected warming is enough as to impact the scenario classification as well as the characterization of key geophysical properties of overshoots scenarios.