(a) Evolution of simulated ensemble mean O2 inventory anomalies over the historical period and over the 21st century relative to the 1820–1829 period. The gray shaded band denotes one standard deviation among the ensemble members. (b) Changes in the vertical distribution of O2 over the 21st century (difference between 2090–2099 and 1990– 1999) for the Atlantic (left) and the Pacific Ocean (right).
Dissolved oxygen concentration is a key quantity for ocean biogeochemistry and ecology, because it is a particularly sensitive indicator of change in ocean transport and biology. We investigated the impact of natural variability on the dection of anthropogenic trends in oceanic oxygen. A six-member ensemble simulation with the NCAR CSM1.4-carbon model was performed over the period 1820 to 2100. The results show that simulated anthropogenically forced O2 decrease is partly compensated by volcanic eruptions, which cause considerable interannual to decadal variability. While well identified on global scales, the detection and attribution of local O2 changes to volcanic forcing is difficult because of unforced variability. Oxygen variability in the North Atlantic and North Pacific are associated with changes in the North Atlantic Oscillation and Pacific Decadal Oscillation indices. The future part of the global warming simulations shows that the oceanic O2 inventory is projected to significantly decrease over the next century, in agreement with earlier work. It is concluded that under the large interannual to decadal variations in oceanic oxygen and the limited data availability the detection of human-induced O2 changes is still challenging.
This paper was accompanied by a series of papers investigating changes in dissolved oxygen (e.g. Jaccard et al. 2014) and using large initial conditions Ensemble simulations (e.g. Rodgers et al. 2015).