Ocean (de)oxygenation from the Last Glacial Maximum to the twenty-first century: insights from Earth System models

Abstract

All Earth System models project a consistent decrease in the oxygen content of oceans for the coming decades because of ocean warming, reduced ventilation and increased stratification. But large uncertainties for these future projections of ocean deoxygenation remain for the subsurface tropical oceans where the major oxygen minimum zones are located. Here, we combine global warming projections, model-based estimates of natural short-term variability, as well as data and model estimates of the Last Glacial Maximum (LGM) ocean oxygenation to gain some insights into the major mechanisms of oxygenation changes across these different time scales. We show that the primary uncertainty on future ocean deoxygenation in the subsurface tropical oceans is in fact controlled by a robust compensation between decreasing oxygen saturation (O) due to warming and decreasing apparent oxygen utilization (AOU) due to increased ventilation of the corresponding water masses. Modelled short-term natural variability in subsurface oxygen levels also reveals a compensation between O and AOU, controlled by the latter. Finally, using a model simulation of the LGM, reproducing data-based reconstructions of past ocean (de)oxygenation, we show that the deoxygenation trend of the subsurface ocean during deglaciation was controlled by a combination of warming-induced decreasing O and increasing AOU driven by a reduced ventilation of tropical subsurface waters.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

“…The simulated glacial salinity between 3500 and 5500 m in the Pacific Ocean is 36.2, indistinguishable from the LGM salinity of 36.1 ± 0.1 reconstructed from porewater extractions at five sites spanning the same depth range (Insua et al 2014). We are not aware of any other comprehensive climate model that has previously reproduced the reconstructed salinities of Bopp et al 2017). The simulated salinity at Shona Rise is only 36.4, rather than the reconstructed 37.1 ± 0.2, but the robustness of the Shona Rise reconstruction has been questioned (Wunsch 2016) so we are unsure whether or not this discrepancy is significant.…”

“…A 200-year bias in ventilation age would suggest an underestimate of O 2 utilization in the deep sea of 60-80 µM, according to a regression of simulated age vs. O 2 utilization (Eggleston and Galbraith 2018), which would bring the simulated glacial O 2 back down to interglacial levels. Others have argued that ventilation of the LGM deep ocean was significantly slower than PI, based on model simulations forced with varying freshwater inputs (Bopp et al 2017;Menviel et al 2017), which would clearly help to achieve low glacial O 2 concentrations; however it is not obvious that a glacial steady-state deep ocean could have been substantially older than PI without violating the observed constraints on deep ocean radiocarbon and salinity.…”

Will ocean zones with low oxygen levels expand or shrink?

“…Modelled changes in oxygen solubility are relatively well constrained and are tied to surface warming 14,15 . The reduction in biological export identified by Fu et al is robustly produced by other models 11,14 , and most models also project a strengthening of the ventilation in tropical OMZs 16 .…”

“…Uncertainties arise from the differences in the magnitude and timing of these changes. Simulated reductions in biological export by 2100 vary between 1% and 40% 15 , and changes in ventilation can vary by a factor of two between models 16 . These differences, even when small, can tip the balance of oxygen levels in OMZs, shifting them from expansion to contraction.…”

Ocean ventilation and deoxygenation in a warming world: introduction and overview

“…Twelve papers were presented at the meeting, and the final versions of 10 of these are included in this volume [9][10][11][12][13][14][15][16][17][18]. Ralph Keeling (paper not included in this volume) reviewed what we know about recent oxygen trends based on results from oceanic and atmospheric O 2 measurements.…”