Tracer-based Rapid Anthropogenic Carbon Estimation (TRACE)

Abstract. The ocean is one of the largest sinks for anthropogenic carbon (C_anth) and its removal of CO₂ from the atmosphere has been valued at hundreds of billions of US dollars in climate mitigation annually. The ecosystem impacts caused by planet-wide shifts in ocean chemistry resulting from marine C_anth accumulation are an active area of research. For these reasons, we need accessible tools to quantify ocean C_anth inventories and distributions and to predict how they might evolve in response to future emissions and mitigation activities. Unfortunately, C_anth estimation methods are typically only accessible to trained scientists and modelers with access to significant computational resources. Here we make modifications to the transit-time-distribution approach for C_anth estimation that render the method more accessible. We also release software called “Tracer-based Rapid Anthropogenic Carbon Estimation version 1” (TRACEv1) that allows users—with one line of code—to obtain C_anth and water mass age estimates throughout the global open ocean from user-supplied values of coordinates, salinity, temperature, and the estimate year. We use this code to generate a data product of global gridded open-ocean C_anth distributions (TRACEv1_GGC_anth, Carter, 2024) that ranges from the preindustrial era through 2500 c.e. under a range of shared socioeconomic pathways (SSPs, or atmospheric CO₂ concentration pathways). We quantify the skill of these estimates by reconstructing C_anth in models with known distributions of C_anth and transient tracers and by conducting perturbation tests. In the model-based reconstruction test, TRACEv1 reproduces the global ocean C_anth inventory with reasonable skill (within ±12 % in 1980 and 2015). We discuss implications of the projected C_anth distributions and highlight ways that the estimation strategy might be improved. One finding is that the ocean will continue to increase its net C_anth inventory at least through 2500 due to deep ocean ventilation even with the SSP where intense mitigation successfully decreases atmospheric C_anth by ~60 % in 2500 relative to the 2024 concentration.