Changes in Oceanic Carbon Storage due to Anthropogenic Carbon Input over the Past Three Decades

While the ocean is known to be an important sink for anthropogenic CO2 emissions, assessing trends in ocean’s uptake and storage of atmospheric CO2 is complicated because changes in the ocean dissolved inorganic carbon (DIC) concentrations due to natural ocean circulation patterns and flux of anthropogenic CO2 need to be disentangled. In this study, we analyze the interannual and decadal changes in the ocean anthropogenic DIC storage from 1992 to 2022 using data from the physically and biogeochemically consistent ECCO-Darwin ocean state estimate model. We use the quasi-conservative tracer C∗ to represent the ocean anthropogenic DIC concentrations and offer several key extensions to previous studies: (1) a longer period of analysis (three decades), (2) analysis including the Arctic Ocean, (3) regular spatio-temporal coverage using annually-averaged data to more accurately estimate the rates of change of C∗. Over the 1992–2022 period, we estimate a total global ocean C∗ increase of 60 Pg C, corresponding to about 28% of total anthropogenic CO2 emissions during this time. The general temporal trend shows a nonlinear increase with accelerating rates of anthropogenic DIC accumulation especially in the last two decades (2002−2022), though a slowdown in the increasing rates is found in some parts of the ocean, in particular in high-nutrient low-chlorophyll regions. Empirical Orthogonal Function analysis of the vertically-integrated rates of change of C∗ reveals that the top three modes of interannual variability correspond to equatorial and tropical climate modes, such as El Niño Southern Oscillation, Atlantic Niño, and El Niño Modoki.