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Will open ocean oxygen stress intensify under climate change?

Key Findings

  • Our model results suggest that global warming increases the volume of hypoxic waters only if the condition for hypoxia is set relatively high, as the volume of the most hypoxic waters (suboxic) does not increase under global warming and actually become more oxygenated.
  • We show that the rise in oxygen is associated with a physical drop in ventilation time due to increased lateral diffusion.
  • This modeled enhanced lateral diffusive flux is the result of an increase of ventilation along the Chilean coast, as a drying of the region under global warming opens up a region of wintertime convection.

Gnanadesikan, A., Dunne, J. P., John, J. G.. Journal: Biogeosciences.

Summary

Ten percent of today’s ocean volume is characterized by low level of dissolved oxygen similar to those found in the well-known "dead zones" in the Gulf of Mexico with 35% of global surface waters overlying at least some of this "hypoxia" (O2 < 88 µM;2ml l-1). Under global warming, higher temperatures would be expected to directly lower oxygen concentrations and enhanced stratification to reduce the flow of well-ventilated surface waters to the interior. Under such circumstances, it has been hypothesized that the open-ocean dead zones could greatly expand and indeed changes in low-oxygen waters have been invoked as evidence of climate change.

The primary objective of this work was to assess the role of global warming on oxygen solubility and vertical exchange in the ocean with particular attention of impact on the volume of hypoxic and suboxic waters. A simulation made with a full earth system model (a prototype of ESM2.1) with dynamical atmosphere, ocean, sea ice and biogeochemical cycling shows that this holds true if the condition for hypoxia is set relatively high. However, the volume of the most hypoxic waters does not increase under global warming, as these waters actually become more oxygenated.

We show that the rise in oxygen is associated with a drop in ventilation time. A term-by-term analysis within the least oxygenated waters shows an increased supply of oxygen due to lateral diffusion compensating an increase in remineralization within these highly hypoxic waters. This lateral diffusive flux is the result of an increase of ventilation along the Chilean coast, as a drying of the region under global warming opens up a region of wintertime convection in our model.

Because the presence of oxygen is critical for habitat of living marine resources, this effort supports NOAA’s research into the future of the earth as a system under the influence of anthropogenic forcing to better quantify how anthropogenic warming may impact marine ecosystems. This work serves to broaden our understanding of the interplay between varieties of climatically and ecologically relevant processes at work in the present day marine environment with a glimpse into its possible future. In our efforts to quantify the impacts of anthropogenic warming on ocean hypoxia, this work helps us to reduce uncertainty of the future of living marine resources.

Methodology

We based the development of this new earth system model on GFDL’s highly successful CM2.1 climate model. We incorporated GFDL’s state of the art ocean biogeochemical model and assess the biogeochemical changes under Special Report on Emissions Scenarios (SRES) forcing.

Known uncertainties

While state of the art in its design, this model suffers from many of the weaknesses typical of this class of model including the double ITCZ, overly strong El Nino, and equator-ward constriction of the subtropical gyres. Most importantly, the model does not ventilate the eastern Equatorial Pacific nearly enough and thus severely overestimates the size of the hypoxic region. For these reason, the projected change in ventilation of the coast of Chile - the underlying mechanism of oxygen change – is highly suspect, as this ventilation mechanism is known to be sporadically active in the present day oceans yet is only exhibited in the model under intense climate warming and expansion of the subtropical gyres.


Figure 2a and 2b from Gnanadesikan et al, 2012: Oxygen changes under the A2 scenario relative to the control. (a) Relative changes in O2 inventory (black line), volume of hypoxic water (red line) and volume of suboxic water (blue line). (b) Oxygen change (mmolm−3) at 300m depth at year 2300 between A2 run and 1860 control.