The amount of oxygen in the world's oceans decreased by 2 percent between 1960 and 2010 and is expected to go down by a further 3-4 percent by the year 2100 as a result of global warming, something that will have an effect on habitats and the coastal economies that depend on them.
A Starfish crawls along a Kelp trunk next to a Sea Urchin in the Indian Ocean, Cape Town, South Africa, 06 October 2019. (EPA-EFE/NIC BOTHMA / MANILA BULLETIN)
This situation was outlined in a report by the International Union for Conservation of Nature (IUCN) - "Ocean deoxygenation: Everyone's problem" - presented Saturday at the United Nations climate change summit in Madrid.
According to the study, this loss of oxygen is closely linked to the warming of the planet and the acidification of the oceans caused by an increase in carbon dioxide (CO2) as a result of greenhouse gas (GHG) emissions and the so-called fertilization of the oceans.
Most of the excess heat retained by the Earth is absorbed by the oceans, which inhibits the diffusion of oxygen from the surface to the depths, and the increase in nutrients that arrive via rivers leads to the proliferation of algae and an increase in the demand for oxygen, said Dan Laffoley of the IUCN Marine Science and Conservation program.
The study identified over 900 coastal areas and semi-enclosed seas around the world that are subject to the effects of eutrophication - the excessive enrichment of the water with nutrients or organic matter.
More than 700 of these areas have hypoxia problems (a lack of oxygen) compared to 45 of them back in the 1960s.
And the volume of water that has been completely depleted of oxygen has quadrupled, according to the report.
The researchers point out that eutrophication-induced hypoxia can be reversed if certain measures are taken, but hypoxia caused by global warming is more difficult to combat.
They are therefore calling for a "drastic effort" to reduce greenhouse gas emissions in order to reduce the rate of oxygen reduction in the oceans, a new global problem that remains relatively unknown, according to professor Lisa Levin from the Scripps Institution of Oceanography.
Beyond the damage caused by overfishing, pollution, habitat destruction and plastic, there is no environmental variable of such ecological importance to marine ecosystems that has changed so drastically in such a short period of time owing to human activities as that of dissolved oxygen, said John Baxter, an expert in Protected Areas of the IUCN.
According to Baxter, although the causes are known, no attention is being paid to the consequences that this will have in the long term for human health, the economy and society.
It will entail the loss of biodiversity, changes in the distribution of species, the displacement or reduction of fishery resources as well as changes in biogeochemical cycles.
The report highlights oceans as a source of oxygen for the atmosphere and the Baltic Sea and the Black Sea as the largest semi-closed marine ecosystems whose oxygen content is low.
But the repercussions of deoxygenation are not limited to closed or semi-enclosed seas - areas with limited oxygen content have also expanded in much of the Atlantic in the last 50-100 years, even in connected seas like the Mediterranean.
And besides affecting coastal areas, the lack of oxygen is also seen at medium depths between 300-1,000 meters - the richest in biodiversity -, as in the Atlantic basin, which has witnessed a significant reduction in the last 60 years.
The report pointed to the need to reduce waste from agriculture, industry, wastewater and to avoid other sources of stress for the world's oceans such as pollution and overfishing.
For Levin, there are solutions but there needs to be greater ambition when it comes to reducing greenhouse gas emissions - an objective that is in the hands of governments.