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

Abstract

Changes of ocean ventilation rates and deoxygenation are two of the less obvious but important indirect impacts expected as a result of climate change on the oceans. They are expected to occur because of (i) the effects of increased stratification on ocean circulation and hence its ventilation, due to reduced upwelling, deep-water formation and turbulent mixing, (ii) reduced oxygenation through decreased oxygen solubility at higher surface temperature, and (iii) the effects of warming on biological production, respiration and remineralization. The potential socio-economic consequences of reduced oxygen levels on fisheries and ecosystems may be far-reaching and significant. At a Royal Society Discussion Meeting convened to discuss these matters, 12 oral presentations and 23 posters were presented, covering a wide range of the physical, chemical and biological aspects of the issue. Overall, it appears that there are still considerable discrepancies between the observations and model simulations of the relevant processes. Our current understanding of both the causes and consequences of reduced oxygen in the ocean, and our ability to represent them in models are therefore inadequate, and the reasons for this remain unclear. It is too early to say whether or not the socio-economic consequences are likely to be serious. However, the consequences are ecologically, biogeochemically and climatically potentially very significant, and further research on these indirect impacts of climate change via reduced ventilation and oxygenation of the oceans should be accorded a high priority. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.

Variations in Ocean Deoxygenation Across Earth System Models: Isolating the Role of Parameterized Lateral Mixing

“…Interestingly, the fact that biological drawdown is increasing implies that the preformed nutrient content of the ocean as a whole must decrease. While the general findings of this study are consistent with current understanding (Shepherd et al, ), our quantification of the impact of lateral mixing represents a new result.…”

“…Global warming is expected to strongly influence the biogeochemical and physical processes responsible for global oxygen distribution (Cabré et al, 2015;Frölicher et al, 2009;Gnanadesikan et al, 2012;Keeling et al, 2010;Oschlies et al, 2018;Shepherd et al, 2017;Stramma et al, 2010;Vaquer-Sunyer & Duarte, 2008). Three processes are expected to drive changes in oxygen: (i) warming of ocean waters reducing oxygen solubility; (ii) more stratification reducing vertical exchange and the supply of oxygen to the deep, and (iii) weaker vertical exchange reducing the supply of nutrients to the surface, thus driving down both productivity near the surface, and the consumption of oxygen in the ocean interior (Praetorius et al, 2015).…”

The Molecular Basis for Understanding the Impacts of Ocean Warming

“…But typically, a Q 10 of 2 is assumed for model purposes, and the actual values are not derived. Segschneider and Bendtsen () noted (astonishingly) that the temperature dependence of the decay of organic matter in the ocean “is, in general, not included in marine biogeochemistry models currently used for Coupled Model Intercomparison project Phase 5 (CMIP5) climate projections.” This topic was reviewed by Shepherd et al () who noted that some hybrid models combining both temperature and depth dependent functions have now been used.…”

“…The end result is that there is today a considerable disagreement between model predictions and observations of the trends in ocean oxygen concentrations (Shepherd et al, ; Stramma et al, ), and observations of microbial rates in marine sediments. The changes in ocean oxygen, beyond simple temperature‐dependent solubility, reflect changes in the flux of organic matter that provide the food supply for deep sea life, and possible widespread hypoxia, and that is a matter of concern.…”

“…Changes in ocean pH are having impacts on aragonite shell forming animals and coral reef systems (Gattuso & Hanson, ), and the physical chemistry basis for this is now well understood. But the impacts of ocean warming are far larger and are driving ocean oxygen losses (Shepherd et al, ). Ocean warming increases chemical rates of all kinds—diffusive, metabolic, and so forth—and the laws governing this are well known.…”