A growing oceanic carbon uptake: Results from an inversion study of surface pCO₂ data

Concerted community efforts have been devoted to producing an authoritative climatology of air-sea CO₂ fluxes, but identifying decadal trends in CO₂ fluxes has proven to be more challenging. The available surface pCO₂ estimates are too sparse to separate long-term trends from decadal and seasonal variability using simple linear models. We introduce Markov Chain Monte Carlo sampling as a novel technique for estimating the historical pCO₂ at the ocean surface. The result is a plausible history of surface pCO₂ based on available measurements and variability inferred from model simulations. Applying the method to a modern database of pCO ₂ data, we find that two thirds of the ocean surface is trending toward increasing uptake of CO₂, with a mean (year 2000) uptake of 2.3 ± 0.5 PgC yr^-1 of anthropogenic carbon and an increase in the global annual uptake over the 30 year time period of 0.4 ± 0.1 PgC yr^-1 decade^-1. The results are particularly interesting in the Southern Ocean, where we find increasing uptake of carbon over this time period, in contrast to previous studies. We find evidence for increased ventilation of deep ocean carbon, in response to increased winds, which is more than offset by an associated surface cooling. Key Points Surface pCO2 measurements are analyzed for global CO2 air-sea flux trends Model and data-based information is combined using Bayesian probability Interannual variability is included to prevent bias from sampling errors