Several methods have been developed to quantify the oceanic accumulation of anthropogenic carbon dioxide (CO2) in response to rising atmospheric CO2. Yet, we still lack a corresponding estimate of the changes in the total oceanic dissolved inorganic carbon (DIC). In addition to the increase in anthropogenic CO2, changes in DIC also include alterations of natural CO2. Once integrated globally, changes in DIC reflect the net oceanic sink for atmospheric CO2, complementary to estimates of the air-sea CO2 exchange based on surface measurements. Here, we extend the MOBO-DIC machine learning approach by Keppler et al. (2020a, https://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.nodc%3A0221526) to estimate global monthly fields of DIC at 1° resolution over the top 1,500 m from 2004 through 2019. We find that over these 16 years and extrapolated to cover the whole global ocean down to 4,000 m, the oceanic DIC pool increased close to linearly at an average rate of 3.2 ± 0.7 Pg C yr−1. This trend is statistically indistinguishable from current estimates of the oceanic uptake of anthropogenic CO2 over the same period. Thus, our study implies no detectable net loss or gain of natural CO2 by the ocean, albeit the large uncertainties could be masking it. Our reconstructions suggest substantial internal redistributions of natural oceanic CO2, with a shift from the midlatitudes to the tropics and from the surface to below ∼200 m. Such redistributions correspond with the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. The interannual variability of DIC is strongest in the tropical Western Pacific, consistent with the El Nio Southern Oscillation.

Key Points

From 2004 through 2019, the global oceanic dissolved inorganic carbon (DIC) pool increased at an average rate of 3.2 ± 0.7 Pg C yr−1

Most of this increase is associated with the uptake of anthropogenic CO2, while natural CO2 is mostly redistributed within the ocean

The interannual variability of DIC is largest in the tropical Pacific Ocean

Plain Language Summary

Human activities, such as the burning of fossil fuels, increase the amount of the greenhouse gas carbon dioxide (CO2) in the atmosphere. A large portion of this additional CO2 is being taken up by the ocean. Several methods have been developed to quantify how much of this anthropogenic CO2 is taken up by the ocean. However, in addition to the anthropogenic CO2, there can also be changes in the naturally occurring CO2 in the ocean. Both the anthropogenic and the natural changes would be reflected in changes of the total dissolved inorganic carbon (DIC) stored in the ocean. Here, we extend a machine learning approach to estimate global monthly fields of DIC from 2004 through 2019. We find that over these 16 years, the oceanic DIC pool in the upper 4,000 m increased at an average rate of 3.2 ± 0.7 Pg C/yr. This trend is statistically indistinguishable from current estimates of the oceanic uptake of anthropogenic CO2 over the same period. Thus, our study implies no detectable net loss or gain of natural CO2 by the ocean, albeit the large uncertainties could be masking it. Our reconstructions suggest substantial internal redistributions of natural oceanic CO2.