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

Abstract

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 pCO2 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.