We usually do not think about how we breathe unless our breath becomes labored, like when we have a cold or have been exercising. Of course, we all know that breathing is essential for human life and many life forms on Earth, because it brings oxygen (O2) into our bodies. Why do we need oxygen? Our food can only be converted to energy when oxygen is available. When the food we eat is processed in our bodies, it gets converted to carbon dioxide (CO2), amongst other things, and the CO2 is exhaled. The Ocean also breathes: it inhales oxygen at certain places, but it also loses it in others. This happens naturally, through a combination of physical and biological processes. Nowadays, we humans produce increasing amounts of CO2; because we burn a lot of fuels in our industries, we drive cars, and we do many other things that increase the amount of CO2 in our atmosphere. This disturbs the natural functioning of the Earth, because CO2 contributes to global warming. Droughts, floods, and the melting of the polar ice caps are some of the consequences of global warming, but global warming may also disturb the way the Ocean breathes.
How Does the Ocean Breathe In?
In this article, we are going to talk about the different ways that the Ocean can breathe. First there are tiny organisms; including cyanobacteria and phytoplankton, which take up CO2 from the atmosphere and produce oxygen in a process called photosynthesis (see Figure 1). Since photosynthesis needs light, this process can only occur in the water layers closest to the surface, about the top 10–100 meters of the Ocean. Even though this area seems very thin, photosynthesis in the Ocean is very important for the Earth, as it is estimated to produce up to 70% of the oxygen on our planet  and to remove 30% of human-made CO2 from the atmosphere . It is important that the Ocean keeps doing this, to keep our planet in a stable condition where the life we know today can continue to exist.
Another way that oxygen gets into the Ocean is when the waves break, and small air bubbles are pushed down into the water. This mechanism mostly takes place in surface waters and at the coasts, and the air bubbles in the water are the main reason waves look white when they break on the beach.
However, there is a place in the Ocean where oxygen is inhaled so deeply that it is taken down into the deep waters where it can stay for thousands of years. This happens in the very north of the Atlantic Ocean, in the Greenland Sea and the Labrador Sea , but also in the Southern Ocean in the Weddell and Ross Seas off the Antarctic coast (see Figure 2). Here, oxygen dissolves in the cold water, and the colder the water is, the more oxygen can dissolve (The opposite happens when you boil water, for example when making pasta. There you can see that gases, including oxygen, start to bubble out of the water, and the warmer the water is, the more bubbles come out). Most of the oxygen is therefore taken up in the winter, and because cold water is also denser than warm water, the cold water with oxygen in it sinks down into the deep Ocean, where oxygen would otherwise be rather scarce. This process is called deep convection.
The way the water circulates through the Oceans is called “the global conveyor belt” and is shown in Figure 3. This movement of water through the ocean starts in the areas of deep water (Figure 2), such as the Labrador Sea, where warm water from the Gulf Stream (see Figure 3) heats the atmosphere in those cold regions. When the water cools down, it becomes denser. This cold, dense water then sinks down to ocean floor. More and more warm surface water is transported to the Labrador Sea by wind currents, where the cold water is continuously sinking down to the deep sea, thus making room for the incoming warm water. When the deep water reaches the bottom of the ocean, it flows south and passes through the Atlantic Ocean, then through the Indian Ocean, and then through the Pacific Ocean. In the end, the cold water returns to the surface mostly through mixing and a process called upwelling. When the cold water reaches the surface, the water warms up and is transported back to the Labrador Sea by the winds. Thus, the global conveyor belt is a big circulation system connecting all the Oceans.
How Does the Ocean Lose Its Breath?
Deep convection in the Ocean depends on the water temperature, but also on the salinity (the “saltiness”) of the water. The colder and the saltier the water is, the more oxygen it can take up. Now that the Earth is warming up, snow, glaciers, and the polar ice caps may melt. This is particularly bad in the Polar regions, because the fresh water from this melting ice flows into the sea and forms a layer of water that is far less salty than the seawater. This may lead to less oxygen being taken up by the ocean, which means there will be less oxygen for life in the oceans.
Another reason that the Ocean is losing its breath is that, if the surface layer of water becomes warmer, it does not mix that well with deeper water layers. When the layers stop mixing, the oxygen that is produced by photosynthesis and by exchange with the air cannot get into deeper waters anymore. A lot of life forms in the deeper Ocean breathe oxygen, including fish, starfish, shrimps, jellyfish, and microbes. Particularly in areas where there is a lot of life around, this leads to the formation of oxygen minimum zones, which are areas where the water has little or no oxygen left (Figure 4). Those oxygen minimum zones are mostly found in the tropical Oceans, where most of the fish are living. If those waters lose more and more oxygen, and if less oxygen reaches deeper waters, there is not much space left for fish and other animals to live. This also means that there will be fewer and fewer fish left that humans can eat. The decrease in fish is a big problem, since fish make up 16% of the animal protein consumed by humans around the world . Fish also provide healthy fatty acids that are not found in any other foods. When the oxygen supply of the world’s fish is threatened, so is one of our primary food sources .
What Will Happen If the Ocean Loses Its Breath?
We do not know what will happen. Scientists have confirmed that the Ocean is indeed losing its breath, which is to say that oxygen concentrations are going down, at least in certain areas of the Ocean. This means that the oxygen minimum zones are expanding . It is hard to predict what else will happen, mainly because we only have data from the last 50–60 years. This is not a whole lot of time, if you compare it with how long it takes for water to move all the way through the global conveyor belt (thousands of years) or if you compare it with the age of our planet, which is around 4.6 billion years.
One way that scientists try to understand the situation is to compare Earth’s present to its past: the Earth and the Oceans have been through similar or worse times, in terms of global warming and CO2 levels. For example, massive changes occurred in the Cretaceous, a time in Earth’s history that started 145 million years ago and ended 65 million years ago. In this period, there was more than three times as much CO2 in the Earth’s atmosphere than there is now. This CO2 came from volcanic eruptions . The ocean surface waters had temperatures of up to 40°C, which is more than twice as high as today’s ocean temperatures. In the Cretaceous, large amounts of oxygen were lost from the Ocean, until it had no oxygen left for two long episodes called ocean anoxic events. During these events, many life forms became extinct. However, afterwards, new life evolved and the Earth and the Oceans recovered again.
Considering all of this, most people understand why scientists want to find ways around the problem of global warming and stop the Ocean from losing its breath. We want to do something about climate change because we do not want to become extinct. So, we as humans need to become active and stop global warming. This is not so easy to do, because we must drastically reduce the amount of CO2 we produce; each one of us currently produces tons of CO2 per year. Everything in our life that needs electricity, like our fridges and computers, as well as wearing clothes that are industrially produced, using cars, eating meat, and many more things in our daily life all produce CO2.
What Can We Do to Help the Ocean Continue to Breathe?
The most important way to help keep oxygen in the oceans is to slow down global warming. Around the world, there are several groups working on this. One initiative to track climate change and to study the impact of global warming on the Earth and its oceans is called the Intergovernmental Panel on Climate Change (IPCC). This group collects data, predicts what exactly could happen if the climate keeps getting warmer, and gives recommendations to politicians . By doing these things, the IPCC hopes to directly reduce the CO2 output on our planet. Many countries also signed an agreement called the Kyoto protocol . This protocol contains guidelines on how much every country has to reduce its CO2 emissions in order to reduce global warming. By doing these things, scientists and politicians hope to reduce global warming, so that, among other things, the polar ice will stop melting and the Ocean can continue to breathe so that oxygen minimum zones will stop expanding and ocean life will continue to exist.
If you would like to check how much CO2 you are producing in your daily life, to see if you can do your own part to reduce global warming and help the oceans continue to breathe, you can use these links: https://www.earthday.org/take-action/footprint-calculator, https://www.carbonfootprint.com/calculator.aspx.
Cyanobacteria: ↑ Bacteria that can perform photosynthesis and produce oxygen. They have different shapes; some are small round cells, others are chain-like colonies (one of those chain-like colonies is shown in Figure 1A).
Phytoplankton: ↑ Free-floating tiny organisms, living in the Ocean and other waters, which can perform photosynthesis. Phytoplankton contains a pigment called chlorophyll, which gives them their green color. They have many different shapes and can consist of one or many cells. They are different from cyanobacteria because phytoplankton cells have a nucleus and a cell wall, and their cells also contain other special structures.
Photosynthesis: ↑ The process by which organisms, such as plants and algae, and microorganisms like cyanobacteria and phytoplankton, take CO2 from the atmosphere and turn it into oxygen and sugar. Photosynthesizing organisms are usually found only in the top 10–100 meters of the ocean, because these organisms need sunlight as an energy source for photosynthesis.
Deep Convection: ↑ When surface waters in the polar seas cool down, they absorb oxygen and sink to the bottom of the ocean. This process provides oxygen rich water for the global conveyor belt to transport around.
Global Conveyor Belt: ↑ When waters circle through first the Atlantic Ocean, then through the Indian Ocean, and then through the Pacific Ocean, we think of the water as traveling along the global conveyor belt. This conveyor belt provides oxygen for a variety of life forms in the ocean.
Oxygen Minimum Zones: ↑ A zone of the ocean that is very low in oxygen.
Global Warming: ↑ When gases, such as CO2 increase in quantity in the Earth’s atmosphere, the Earth heats up. This effect is known as global warming. CO2 can come from volcanos, but these days it often comes from burning fossil fuels, which include the oil that was buried deep under the ocean. Fossil fuels are used to generate electricity, fuel our cars, and produce/run a lot of things in our daily lives, such as clothing, fridges, computers, etc.
Cretaceous: ↑ A time in earth’s history that started 145 million years ago and ended 65 million years ago. During this period the Earth was very warm, CO2 levels are high, and the Ocean turned anoxic several times.
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
 ↑ Walker, J. C. G. 1980. “The oxygen cycle,” in The Oxygen Cycle in the Natural Environment and the Biogeochemical Cycles, ed O. Hutzinger (Berlin: Springer-Verlag), 87–104.
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 ↑ Schmidtko, S., Stramma, L., and Visbeck, M. 2017. Decline in global oceanic oxygen content during the past five decades. Nature 542:335–9. doi: 10.1038/nature21399
 ↑ Turgeon, S. C., and Creaser, R. A. 2008. Cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode. Nature 454:323–6. doi: 10.1038/nature07076
 ↑ IPCC. 2013. “Climate change 2013: the physical science basis,” in Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, eds T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, et al. (Cambridge; New York, NY: IPCC), 1535.
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