There is nothing more fundamental to humans than the availability of oxygen. We think little about the oxygen we need, we just breathe, but where does it come from?
To shed light on this, statements like “the ocean provides 50 percent of the oxygen we breathe” or the equivalent “every second breath we breathe comes from the ocean” have become popular mantras to highlight human dependence on the ocean and the risk of decreased oxygen supply due to climate change and environmental degradation.
These mantras are repeated by high-ranking politicians, including US climate officials John kerry and French President Emmanuel Macron, international organizations like UNESCO and the European Commission, and even prominent reports from the IPCC and other serious ones scientific institutions.
While they may be good fodder for speech, these claims misrepresent where the oxygen we breathe actually comes from and thereby mislead the public as to why we should step up our role as ocean caretakers.
Where do we get our oxygen from?
The earth’s atmosphere has wasn’t always as oxygenated as it is today. The atmosphere now consists of 21 percent oxygen, but it was only 0.001 percent of today’s level for the first 2 billion years of Earth’s history.
It is the emergence of microscopic marine bacteria and plants (phytoplankton) and later larger plants on land that have caused the amazing increase in oxygen in our atmosphere. This oxygen comes from photosynthesis – the process by which plants convert carbon dioxide and water into organics and oxygen.
Oxygen has been relatively stable at a high level for 500 million years. Today, About half of photosynthesis takes place in the sea and on land.
So yes, the ocean is responsible for about 50 percent of the oxygen produced on the planet. But it’s not responsible for 50 percent of the air we humans breathe. Most of the oxygen produced by the ocean is consumed directly by the microbes and animals living there, or it falls to the sea floor as plant and animal products. In fact, the net production of oxygen in the ocean is close to zero.
A tiny fraction of primary production, around 0.1 percent, escapes degradation and is stored as organic carbon in marine sediments – a process known as biological carbon pump. This organic carbon can eventually become fossil fuels like coal, oil, and gas. The tiny amount of oxygen that was created in the manufacture of this carbon reservoir can later be released into the atmosphere. A similar process also takes place on land, with some of the carbon stored in the soil.
Therefore, the oxygen we breathe now comes from the slow accumulation of O₂ in the atmosphere, which is supported by the storage of organic substances over very long periods of time – hundreds of millions of years – and not from today’s production by land or Ocean biosphere.
Fossil fuels and the air we breathe
What about future trends in atmospheric oxygen? As early as 1970, the prominent geochemist Wally S Broecker recognized that if we burned all known fossil fuel reserves, we would use less than 3 percent of our oxygen reservoir.
If we were to cut down or burn down all forests and oxidize all of the organic carbon stored in vegetation and topsoil around the world, this would result in little depletion of atmospheric oxygen. If photosynthesis in the ocean and on land stopped producing oxygen, we could breathe for millennia, although we would certainly have other problems.
Even in the worst-case scenarios with massive burning of fossil fuels and deforestation, the predicted decrease in atmospheric oxygen will be very small in relation to the very large atmospheric reservoir. Models show that oxygen levels in the atmosphere will change slightly over the next 100,000 years in response to fossil fuel use. So as long as it is there many things to worry about in our climate future, the availability of oxygen to air-breathing organisms (including humans) is not one of them.
Oxygen decline in the ocean
There are significant ones Cause for concern however, with regard to the oxygen content in the ocean. The ocean’s O₂ reservoir is vulnerable because it contains less than 1 percent of the oxygen stored in the atmosphere. In particular, ocean regions with very little or no oxygen, so-called Oxygen minimum zones, expand as the planet warms, making new regions habitable for breathing organisms such as fish.
Lost the open sea 0.5 to 3.3 percent of its oxygen supply in the upper 1000 meters from 1970-2010, and the volume of the oxygen minimum zones increased by 3-8 percent.
This loss of oxygen is primarily due to increasing Ocean layering. The mixing of the warmer and lighter surface ocean with the deeper and denser sea layers is less efficient, which restricts the penetration of oxygen. The activity of enzymes, including those involved in breathing, also generally increases with temperature. The oxygen consumption of the sea creatures increases with the warming of the ocean.
One recently to learn found that the open ocean oxygen minimum zones have expanded millions of square kilometers and hundreds of coastal areas now have oxygen concentrations low enough to limit animal populations and alter the cycle of important nutrients. The volume of the deoxygenated areas is projected onto grow by about 7 percent by 2100 under a scenario of high CO₂ emissions.
Such deoxygenation affects biodiversity and food webs; and has a negative impact on the food security and livelihood of the people who depend on it.
So where is our mantra?
While it is wrong to say that the ocean provides 50 percent of the oxygen we breathe, it is correct to say that, on geological time scales, the ocean provides much of the oxygen we take in today. It is also completely correct to say that the ocean is responsible for 50 percent of the primary production on earth and maintains our food system.
And while we shouldn’t worry about the future oxygen supply for humans to breathe, we should worry that fish are increasingly being displaced from the expanding deoxygenated marine areas.
This article was written by several authors: Jean-Pierre Gattuso, Research Professor, CNRS, Iddri, Sorbonne University; Carlos M. Duarte, Honored Professor of Marine Science, King Abdullah University of Science and Technology; Fortunat Joos, Professor, University of Bern, and Laurent Bopp, Research Professor, CNRS, cole normal supérieure (ENS) – PSL