Abstract
The Southern Ocean between 30°S and 55°S is a major sink of excess heat and anthropogenic carbon, but model projections of these sinks remain highly uncertain. Reducing such uncertainties is required to effectively guide the development of climate mitigation policies for meeting the ambitious climate targets of the Paris Agreement. Here, we show that the large spread in the projections of future excess heat uptake efficiency and cumulative anthropogenic carbon uptake in this region are strongly linked to the models’ contemporary stratification. This relationship is robust across two generations of Earth system models and is used to reduce the uncertainty of future estimates of the cumulative anthropogenic carbon uptake by up to 53% and the excess heat uptake efficiency by 28%. Our results highlight that, for this region, an improved representation of stratification in Earth system models is key to constrain future carbon budgets and climate change projections.
Highlights
The Southern Ocean between 30°S and 55°S is a major sink of excess heat and anthropogenic carbon, but model projections of these sinks remain highly uncertain
We find that stratification biases in CMIP5 and CMIP6 Earth system models (ESMs) in the region between 30°S and 55°S are strongly related to the amount of their future uptake of excess heat per degree of transient global warming (Hexcess uptake efficiency) and anthropogenic carbon (Fig. 2)
This depth range has been chosen as it encompasses the Mode Water (MW) and Intermediate Water (IW) formation and subduction pathways in Coupled Model Intercomparison Project (CMIP) ESMs7,28, and modern observational coverage is good, since it is covered by standard ARGO floats
Summary
The Southern Ocean between 30°S and 55°S is a major sink of excess heat and anthropogenic carbon, but model projections of these sinks remain highly uncertain Reducing such uncertainties is required to effectively guide the development of climate mitigation policies for meeting the ambitious climate targets of the Paris Agreement. Despite improvements in model performance in successive phases of the Coupled Model Intercomparison Project (CMIP), this progress might be too slow to warrant significantly reduced uncertainty of ESM projections within the decade[16] Since this is the time horizon for framing climate mitigation policies that allow for meeting stringent climate targets[17,18], more efforts have to be put into model analysis, i.e., understanding the roots of this uncertainty and reducing uncertainty in key climate metrics such as the projected carbon and heat uptake. We note that our definition of Cant and Hexcess includes changes induced by climate change such as changes in ocean circulation, wind conditions and primary production (Methods)
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