Does an improved ocean carbon budget lead to greater constraints on terrestrial carbon fluxes and a better understanding of global actions to reduce atmospheric CO2?

Today, the primary estimate of the net terrestrial carbon flux (the sum of fluxes due to land use change and terrestrial carbon sinks) is the difference between anthropogenic carbon emission (fossil fuels and cement) and the atmospheric CO2 growth and ocean CO2 sink [Le Quere et al., 2015]. In this scheme, improvements in our understanding of the ocean carbon sink will lead to an improved understanding of the net terrestrial carbon flux [NRC, 2010]. Such improved estimates are a key step towards understanding the success of global carbon agreements such as the Paris Agreement reached at the UN COP21 Conference.

The authors of the Global Carbon Project report [Le Quere et al., 2015] note that an important recent development in constraining ocean carbon fluxes, and subsequently the net terrestrial carbon flux, is the yearly estimate of the ocean carbon sink now produced by the SOCAT project. While the primary estimate of the temporal trend in the ocean sink is model-based, the yearly, observational estimates provide an assessment of the confidence in the trend. Today, the uncertainties in the observational estimates are relatively large, due to the scarcity of pCO2 measurements in any one year. The observations of pH on profiling floats can be combined with estimates of total alkalinity [Carter et al., 2016] to provide assessments of surface ocean pCO2 over complete annual cycles. These float-based observations provide complete annual cycles of pCO2, and can do so throughout the ocean. A system of profiling floats throughout the ocean would substantially improve our estimates of air-sea CO2 exchange derived from the SOCAT program by providing accurate estimates of the amplitude of the annual pCO2 cycle. This, in turn, could lead to greater confidence in the terrestrial carbon flux, which cannot be measured directly.

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