Uncertain current and future ocean deoxygenation due to internal climate variability and observational gaps

Abstract. Observed declines in oceanic oxygen (O₂) over recent decades are subject to substantial uncertainty due to internal climate variability (ICV) and limited observational coverage. Here, we quantify how observational uncertainty affects the assessment of both historical and future ocean deoxygenation by combining multiple observational datasets with a large ensemble simulation of the Max Planck Institute Earth System Model (MPI-ESM). We find that observational biases in ICV can amplify global and regional O₂ variability by 150 %–500 % in annual time series over the past 50 years. The combined effect of ICV and sampling bias can also introduce deviations of 5 %–25 % in estimated multi-decadal O₂ trends. Moreover, time-dependent changes in observational coverage complicate the interpretation of historical O₂ trends. Our results underscore the crucial need for a sustained, globally uniform ocean observing system to monitor long-term deoxygenation, assess its impact on marine ecosystems, and detect the anthropogenic signal in O₂ trends. We further show that near-future trend detection will remain sensitive to ICV, and observational gaps may distort the detection of scenario-based projections of O₂ trends, especially in the context of climate mitigation efforts.