Decadal Trends in the Oceanic Storage of Anthropogenic Carbon From 1994 to 2014

Abstract

The oceanic uptake and resulting storage of the anthropogenic CO2 (Cant) that humans have emitted into the atmosphere moderates climate change. Yet our knowledge about how this uptake and storage has progressed in time remained limited. Here, we determine decadal trends in the storage of Cant by applying the eMLR(C*) regression method to ocean interior observations collected repeatedly since the 1990s. We find that the global ocean storage of Cant grew from 1994 to 2004 by 29 ± 3 Pg C dec−1 and from 2004 to 2014 by 27 ± 3 Pg C dec−1 (±1σ). The storage change in the second decade is about 15 ± 11% lower than one would expect from the first decade and assuming proportional increase with atmospheric CO2. We attribute this reduction in sensitivity to a decrease of the ocean buffer capacity and changes in ocean circulation. In the Atlantic Ocean, the maximum storage rate shifted from the Northern to the Southern Hemisphere, plausibly caused by a weaker formation rate of North Atlantic Deep Waters and an intensified ventilation of mode and intermediate waters in the Southern Hemisphere. Our estimates of the Cant accumulation differ from cumulative net air‐sea flux estimates by several Pg C dec−1, suggesting a substantial and variable, but uncertain net loss of natural carbon from the ocean. Our findings indicate a considerable vulnerability of the ocean carbon sink to climate variability and change.

How Does the Pinatubo Eruption Influence Our Understanding of Long‐Term Changes in Ocean Biogeochemistry?

“…One of the key findings from this study is that, while the Pinatubo climate perturbation influences the distribution of preindustrial carbon, it has no discernible impact on the externally forced changes in the anthropogenic carbon distribution. This finding agrees with previous studies that find an important role for Pinatubo in preindustrial carbon variability (Eddebbar et al., 2019; Fay et al., 2023; McKinley et al., 2020), and gives us additional confidence that observation‐based estimates of changing anthropogenic carbon distribution (e.g., Gruber et al., 2019; Müller et al., 2023); (also Müller , Jens Daniel , Gruber , Nicolas , Carter , Brendan R ., Feely et al ., Decadal Trends in the Oceanic Storage of Anthropogenic Carbon from 1994 to 2014 , in preparation for Authorea ) are relatively unaffected by the Pinatubo climate perturbation. This confidence, however, is only valid to the extent that the methods employed can accurately separate anthropogenic carbon from the much larger preindustrial component.…”

Magnitude, Trends, and Variability of the Global Ocean Carbon Sink From 1985 to 2018

“…Combined with structural biases in the OCIM (such as neglect of seasonality and small-scale circulation features), means that the uncertainty of the values in Figure 7f is likely larger than their mean. Nevertheless, a recent update of the eMLR-C* estimates by Müller et al (2023) also suggests substantial climate-driven variability in the oceanic storage of anthropogenic carbon similar to that shown in Figure 7f.…”

“…This product provides near-global coverage, but is missing data in some marginal seas, and the analysis is cut off below 3,000 m where the anthropogenic DIC signal-to-uncertainty ratio is low. A recent update of the eMLR(C*) results by Müller et al (2023) resolves decadal trends in the anthropogenic carbon accumulation from 1994 to 2014, but was published after the completion of this study and could thus not be considered here.…”

Seasonal Variability of the Surface Ocean Carbon Cycle: A Synthesis

“…How is the ocean carbon cycle changing as a consequence of sustained increases in emissions of carbon to the atmosphere? Important steps toward answering this question over the last several decades have been provided via estimates of ocean carbon uptake from both interior hydrographic measurements (Gruber et al, 2019;Müller et al, 2023;Sabine et al, 2004), surface fluxes inferred from measurements of the sea surface partial pressure of CO 2 (pCO 2 ) (Chau et al, 2022(Chau et al, , 2023Gregor et al, 2019;Landschützer et al, 2014;Rödenbeck et al, 2015), global ocean biogeochemistry model simulations (Friedlingstein et al, 2022a(Friedlingstein et al, , 2022bHauck et al, 2020;Orr et al, 2001) and ocean inverse models (Gloor et al, 2003;Gruber et al, 2009). A first global synthesis was performed roughly a decade ago through the REgional Carbon Cycle Assessment and Processes (RECCAP) project (https://www.globalcarbonproject.org/reccap), highlighting inevitable forced carbon cycle changes, while also identifying sources of uncertainty.…”