Anoxic Oceans: Is CO2 Even More Harmful To Our Seas Than Our Skies?

Our oceans are being starved of oxygen, which is killing wildlife and raising sea temperatures. Action must be taken to clear the atmosphere of carbon dioxide (anoxic) before it’s too late, writes Cora Moran

As well as providing us with abundant food, we have much to thank the oceans for.

Approximately half of the carbon dioxide emitted since the beginning of the industrial era has been absorbed by the earth’s oceans, along with much of the heat trapped by increasing atmospheric carbon dioxide levels.

The rising carbon dioxide levels in the water are making the oceans less alkaline, which makes it difficult for creatures such as molluscs to form shells. What’s more, the expansion of water is leading to sea levels rising, compounding the effects of glacial melt.

The global ocean is the oft-overlooked big sister of the atmosphere in the Earth’s climate system – it’s 1000 times the volume! While the effects of rising carbon dioxide emissions in the air are well documented and widely reported, it is only recently that the impacts on the world’s oceans have become widely mentioned in the media. While people have often heard about rising sea level and temperature, plastic pollution and the increasing acidity of the oceans, few have heard of ocean anoxia, which will arguably have the worst impact of all.

The problem with warm water

What’s more, warm water holds less oxygen. This is demonstrated by the greater levels of biological productivity in temperate and arctic waters than in the tropics. Levels of oxygen in seawater have been declining over the past half-century, which is only compounded by increased nutrient runoff into the oceans from agriculture, causing algal blooms.

As the algal blooms die, they encourage the growth of microbes that reduce oxygen levels creating ‘hypoxic’ or low oxygen levels, which can develop into ‘anoxic’ areas that have extremely low oxygen levels. A number of areas have already developed these conditions around the globe, such as the Gulf of Mexico, which has become known as a ‘dead zone’.

As the oceans warm up, they are also becoming more stratified; which means the various layers of the ocean with different properties are mixing less which leads to less heat, nutrient and oxygen exchange into the deep ocean. The resulting lower oxygen levels in the deep ocean encourage the growth and expansion of anaerobic bacteria, which produce hydrogen sulphide gas when respiring.

For now, anaerobic bacteria only reside very deep in the ocean. However, there is a feedback mechanism; the more bacteria there are, the higher the hydrogen sulphide levels in the water become and – at a certain threshold – the water becomes highly sulphidic, or ‘euxinic’. These conditions kill most life of other kinds, which encourages the growth of more yet more bacteria and anoxic conditions over larger areas. If this condition spreads sufficiently it could reach the ocean’s surface and the hydrogen sulphide gas could leach into the air, becoming toxic to terrestrial life and even the earth’s ozone layer.

Anoxic oceans have occurred a number of times before in the earth’s geological record and have typically been caused by rapidly increasing levels of carbon dioxide in the atmosphere. These carbon dioxide spikes are also directly correlated with the timing of mass extinctions, the worst of which was the ‘Great Dying’ 250 million years ago where 97 percent of marine life and 70 percent of terrestrial life became extinct.

If we continue to burn fossil fuels at a colossal rate, we, too, are headed for dwindling biodiversity, more erratic weather patterns and very serious long-term ecological consequences.

For the balance of our oceans to be restored, key actions must be taken, including: reducing river runoff of agricultural fertilisers; reducing overfishing; creating large scale marine reserves; and reducing carbon emissions.

Currently there are global efforts and initiatives in place to revive our oceans, but these must be sustained and elevated if they are to make a long term, positive contribution and reverse the damage.

Cora Moran is an experienced researcher who has worked in the Built Environment & Renewable Energy sectors for a number of years and writes for the European Energy Centre about a range of environmental issues.

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3 comments

  • Andrew B

    May 28, 2018 at 6:06 pm

    I think that the above commenter is missing the point of what the author is saying in her piece. She is describing a trend based on evidence and has precedence in the geological record; ocean anoxia can be caused by rapidly increasing carbon emissions. This has happened naturally in the past naturally due to very large scale volcanic eruptions burning through coal deposits and is currently happening today due to human GHG emissions (e.g. https://www.theguardian.com/environment/climate-consensus-97-per-cent/2018/mar/12/burning-coal-may-have-caused-earths-worst-mass-extinction). That has nothing to do with a mindset, nor is it discredited by an unrelated reference to natural variability in systems such as declining oxygen levels. What is required is for us all to look at where the evidence is pointing and what seem the most likely, rather than indulging in ad hominem arguments.

    Reply

  • Andrew B

    May 29, 2018 at 11:36 am

    I think that the above comment is missing the point of what the author is saying in her piece. She is describing a trend based on evidence and has precedence in the geological record; ocean
    anoxia can be caused by rapidly increasing carbon emissions. This has happened naturally in the past naturally due to very large scale volcanic eruptions burning through coal deposits and is
    currently happening today due to human GHG emissions (e.g.
    https://www.theguardian.com/environment/climate-consensus-97-per-cent/2018/mar/12/burning-coal-may-have-caused-earths-worst-mass-extinction ). That has nothing to do with a mindset, nor is it discredited by an unrelated reference to natural variability in systems such as declining oxygen levels. What is required is for us all to look at where the evidence is pointing and what seem the most likely, rather than indulging in ad hominem arguments.

    Reply

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