Climate Emergency Institute
Climate Science Library
​​Ocean Oxygen Decline
​Karen Villarante-Tonido, Philippines
​Original Post: March 18, 2012

I. ​​Introduction

​​The rising levels of atmospheric carbon dioxide (CO2) due to unabated carbon emissions have been given much attention in recent years because of its major impact on global temperatures, climate, ocean chemistry, etc. Recently, it has also been reported to affect the level of oxygen (O2) in the atmosphere. Dr. Ralph Keeling estimated that about three O2 molecules are lost every time a single CO2 molecule is produced by fossil fuel combustion (Johnston, 2007). A 0.0317% decline in atmospheric oxygen has been recorded thus far (for the period 1990 to 2008) (Klusinske, 2010).

​​Fortunately, the world’s oceans (which cover 70% of the Earth’s surface) function as an efficient carbon sink. They absorb about half of the anthropogenic CO2 in the atmosphere (Sabine et al., 2004), thereby buffering the effects of excess atmospheric CO2. Unfortunately however, this is not without consequence to ocean chemistry. Elevated atmospheric CO2 have been reported to cause ocean warming, acidification and recently, the decline in ocean oxygen levels.

II. Declining Ocean Oxygen Levels

​​Scientists have recorded a steady decline in ocean oxygen in open waters of the North Pacific Ocean of about 0.3% per year since the 1950s. This “steady decline in ocean oxygen levels was discovered among a roughly 20-year cycle of fluctuating oxygen that is driven by the effects of lunar procession on the tides,” according to Conners (2011) (See Figure 1 below).


Fig. 1. Declines and oscillations in oxygen concentrations in the North Pacific Ocean

​​Marine Scientists from the Leibniz Institute of Marine Sciences in the University of Kiel, Germany have also observed the decline in ocean oxygen levels in some regions of the world oceans. According to the study, oxygen levels in tropical oceans at 300 to 700 meters depth have declined during the past 50 years (Stramma et al., 2008).

​​Since the oceans are not homogeneous in terms of oxygen concentration, this intermediate-depth low oxygen layers (200 to 700-dbar depth), termed the oxygen-minimum zones (OMZ) are most sensitive to further declines in oxygen levels. The team of Lothar Stramma from Kiel University in Germany estimated the differences in dissolved oxygen levels within the OMZ across the global tropical and subtropical oceans at two time periods: between 1960-1974 (early period with reliable data) and 1990-2008 (recent period capturing ocean response to planetary warming). They reported the following observations:

"In most regions of the tropical Pacific, Atlantic, and Indian Oceans the oxygen content in the 200–700-dbar layer has declined. Furthermore, at 200 dbar, the area with O2 <70 μmol kg−1, where some large mobile macro-organisms are unable to abide, has increased by 4.5 million km2. The tropical low oxygen zones have expanded horizontally and vertically. Subsurface oxygen has decreased adjacent to most continental shelves. However, oxygen has increased in some regions in the subtropical gyres at the depths analyzed” (Stramma et al., 2010).

The rise in oxygen levels in some regions (i.e. part of the Atlantic Ocean extending from Florida to North Africa) reported by the Stramma team are consistent with predicted changes in ocean circulation, according to Chameides (2010).

​​Nevertheless, oxygen concentrations have also been noted to drop at the subarctic Pacific Ocean from depths of 100 to 400 meters between 1956 and 2006 while constant fluctuations in oxygen levels have been observed in the upper 100 meters of the ocean waters (Ho, 2009). It is unclear whether this observed ocean oxygen decline is only due to natural cyclical processes or it is a long-term trend brought about by climate change. Nevertheless, after reviewing several geologic records, scientists agreed that climate is in fact a critical factor that influences ocean oxygen depletion (Conners, 2011).
​​​III. Causes

​​Global warming and consequently, ocean warming causes a decline in dissolved oxygen for two reasons. First is that the solubility of oxygen decreases as the ocean waters get warmer. In fact, zones of low oxygen in the ocean were found to be contracted in cold periods and expanded in warm periods based on geological records (Conners, 2011). Second, warm ocean waters are more stable, thereby slowing down the “ocean’s thermohaline ‘conveyor belt’ circulation system that…overturns surface layers of the water into the deep and vice versa…” The result is less oxygen carried from the surface layers of the water (which is in intimate contact with air) into the deeper layers (NASA, 2009). This leads to further oxygen depletion of the region between the surface and the deep ocean, the oxygen-minimum zone or OMZ (Chameides, 2010).

In addition, the slowing down of the ocean’s circulation system also brings fewer nutrients from the deep layers into the ocean surface. With fewer nutrients available in the surface layers, oxygen-producing phytoplankton that drift in the ocean surface may be grossly affected. In fact, the declining numbers of phytoplankton species (which dropped by 40 percent from 1950 to 2010) noted in a study published July 29 in Nature was attributed to this nutrient deprivation (Morello, 2010). Phytoplankton organisms produce half of the world’s oxygen output (the other half is produced by plants on land). Hence, with decreasing numbers of these oxygen producers, the level of oxygen in the ocean (and the atmosphere as well) is bound to decline further.

Another reason for the reduced oxygen levels in the deep ocean mentioned by Stramma et al. (2008) is the reduced production of oxygen-rich deep water in polar regions. Furthermore, pollution has also been cited as one reason for the decline in ocean oxygen. Pollutants such as discharged sewage and industrial waste, farm fertilizer run-off, etc. trigger oxygen-depleting algal blooms. However, this only explains oxygen reduction in some coastal waters. In contrast, global warming justifies (at least partly) ocean oxygen decline across the globe.

IV. Implications

​​Oxygen is the most important constraint or limiting factor on the growth of many marine organisms according to Daniel Pauly, a Fisheries Biologist at the University of British Columbia (Zimmer, 2010). When ocean oxygen levels run low, it is harder for aerobic marine animals to respire (extract oxygen from seawater for use in respiration). This in turn, makes it more difficult for these animals “to find food, avoid predators, and reproduce” (NASA, 2009).

​​Hence, decreased ocean oxygen levels can have devastating effects on marine life. Many marine organisms are stressed or cannot survive under hypoxic conditions (between 60 to 120 m mol/kg depending on the species) (Ho, 2009). For instance, during the Permian-Triassic extinction event about 252 million years ago, extremely low oxygen conditions led to the loss of approximately 90% of marine animal taxa (Conners, 2011).

To avoid the risk of local extinction, many organisms move to non-hypoxic areas. Therefore as oxygen-poor regions expand, there are less and less suitable areas for aerobic marine organisms to thrive and enter into in search of food. The result is habitat compression for these hypoxia-intolerant species, decreases in biodiversity, shifts in animal distributions and changes in ecosystem structure (Stramma et al., 2010).

​​​​​​​​​Besides this, some possible scenarios have been suggested by Pauly and his colleagues. One such scenario is that jellyfish will flourish in these oxygen-deficient waters. Unlike fish and many other marine organisms that are stressed under low oxygen conditions, jellyfish can tolerate hypoxia because their jelly can store reserves of oxygen. With less competition and threat from predators (that have high oxygen demands), it is not unlikely that jellyfish becomes abundant in such conditions. (Zimmer, 2010) Another possible scenario is that the habitat for Blue marlin, other billfish and tropical tuna will be restricted to surface waters of the ocean. These organisms have high oxygen demands. As dead zones (oxygen-free zones) in the ocean expand, the habitat for these organisms shrinks. In effect, these organisms are forced to remain in surface waters where there is enough oxygen for them to survive. Unfortunately, this makes them more vulnerable to fishing. (Goldstone, 2011) It was also suggested that anaerobic (organisms that do not need oxygen to survive) bacteria may thrive in lieu of aerobic bacteria in oxygen-free zones of the ocean. Some of these anaerobic bacteria produce nitrogen compounds (i.e. nitrous oxide), which are powerful greenhouse gases. This could further worsen the problem of global warming. (Zimmer, 2010)

​​V. Summary and Conclusions

​​Ocean oxygen levels are declining and this is partly due to climate change. Compounded by the effects of ocean warming and acidification, ocean oxygen decline is bound to worsen the negative impact on the ocean’s biogeochemical cycles and ecosystems. This could lead to more ocean dead zones (NASA, 2009) and consequently, decreases in the ocean’s biodiversity and productivity. Pauly and his colleagues predicted that the synergy between low ocean oxygen levels and pH will decrease the world’s fish catch by 20 to 30 percent by Year 2050 (Zimmer, 2010).

​​Despite these predictions however, it is still quite unclear how exactly this trend of declining ocean oxygen levels will affect marine life and ocean health in the future. Scientists agree that there is a need for further research in order to collect better data on ocean oxygen levels and consequently, make better predictions about their future impact on marine life. An international collaboration that began in 1995 known as the Climate Variability and Predictability Repeat Hydrography Program (CLIVAR) has been gathering such data although Ralph Keeling of Scripps Institution of Oceanography warns that this could take 20 to 30 years to establish long-term trends in oxygen levels. He suggested that researchers should instead employ a global network of floating sensors called Argo, where oxygen sensors will be placed in some of these units. With this strategy, Keeling estimates that it will take only about five years for sufficient data to be collected, enough to establish a clear pattern. (Zimmer, 2010) So far, computer models developed by climate scientists predict a drop in ocean oxygen levels by 1 to 7 percent in the next century and this could persist for a thousand years or more (Conners, 2011). And since the primary driver of these changes (not only ocean oxygen decline but also ocean warming and acidification) on the ocean physical, chemical and biological environment is the rising levels of anthropogenic CO2 in the atmosphere, there is no better solution than to reduce carbon emissions to zero the soonest time possible.


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Conners, Deanna. “Declining oxygen concentrations might threaten ocean health.” 30 Nov. 2011. EarthSky. 15 Feb. 2012. <>

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​​Gruber, Nicolas. “Warming up, turning sour, losing breath: ocean biochemistry under global change.” Phil Trans R Soc A 369 (2011): 1980-1996.

​​Ho, Mae-Wan. “Warming Oceans Starved of Oxygen.” 08 Oct. 2009. Institute of Science in Society. 15 Feb. 2012. <>

​​Johnston, Mike. “Atmospheric Oyxgen Levels Fall As Carbon Dioxide Rises. 14 Dec. 2007. Blogcritics. 11 Feb. 2012. <

​​Klusinske, Elizabeth. “Atmospheric Oxygen Depletion: Should We Be Worried?” 2010. World Green. 25 Jan. 2012. <

​​Morello, Lauren. “Phytoplankton Population Drops 40 Percent Since 1950.” Scientific American (2010). 21 Oct. 2011. <>

​​Sabine, C.L., Feely, R.A., Gruber, N. et al. “The oceanic sink for anthropogenic CO2”. Science 305 (2004):367-371.

​​Stramma, Lothar, Gregory Johnson, Janet Sprintall and Volker Mohrholz. “Expanding oxygen-minimum zones in the Tropical Oceans. Science 320 (2008): 655-658.

​​Stramma, Lothar, Sunke Schmidtko, Lisa Levin and Gregory Johnson. “Ocean oxygen minima expansions and their biological impacts.” Deep Sea Research Part 1: Oceanographic Research Papers 57.4 (2010): 587-595.

​​Zimmer, Carl. “A Looming Oxygen Crisis and Its Impact on World’s Oceans. 05 Aug. 2010. Yale Environment 360. 15 Feb. 2012.

​ “Increasing Carbon Dioxide and Decreasing Oxygen in the Oceans will Make it Harder for Deep-Sea Animals to Breathe.” 17 Apr. 2009. NASA Earth Observatory. 15 Feb. 2012. <>