Climate change will deplete ocean oxygen by up to 4% by 2100

The global ocean oxygen inventory has declined by an additional 2 percent between 1960 and 2010 and is projected to shrink by an additional 3 to 4 percent by 2100 because of global warming, with dramatic effects on habitats and the coastal economies that depend on them.

This is clear from the report of the International Union for the Conservation of Nature (IUCN, for its acronym in English) “The deoxygenation of the oceans: a problem for all”, presented this Saturday at the XXV United Nations Conference on Climate Change (COP25).

According to the study, the loss of ocean oxygen is closely related to the warming and acidification of the oceans caused by the increase in carbon dioxide (CO2), in turn a consequence 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 through rivers promotes algal blooms and the consequent increase in oxygen demand, explained Dan Laffoley, of the IUCN Marine Science and Conservation programme.

According to the report, the global average of deoxygenation hides local changes that could be more severe in mid- or high latitudes and thus, some simulations of ocean models project by the year 2100 a decrease in the oxygen stock of the oceans of up to 7 percent in an unchanged scenario.

The study has identified more than 900 coastal areas and semi-enclosed seas worldwide subject to the effects of eutrophication (excessive enrichment of waters with nutrients or organic matter).

Of these, more than 700 have problems with hypoxia (lack of oxygen) – in the 60s there were 45 – and the volume of water completely depleted of oxygen has quadrupled, according to the report.

The researchers note that the combination of eutrophication-induced hypoxia can be reversed if the necessary measures are taken, but hypoxia caused by global warming is more difficult to combat.

Therefore, they urge to undertake a “drastic effort” to reduce GHG emissions to mitigate the rate of decrease in oxygen in the world’s oceans or deoxygenation, a new problem of global importance still unknown, said Professor Lisa Levin, of the Scripps Institution of Oceanography.

Because, beyond the damage caused by human actions such as overfishing, pollution, habitat destruction or plastics, there is no environmental variable of such ecological importance for marine ecosystems that has changed so drastically in such a short time as a result of human activities as dissolved oxygen, said John Baxter, expert in Protected Areas of the IUCN.

According to Baxter, although the causes are known, no attention is paid to the long-term consequences that this phenomenon will have for human health, the economy and society, such as the loss of biodiversity, changes in the distribution of species, displacement or reduction of fishery resources and changes in biogeochemical cycles.

The report points to the ocean as one of the sources of oxygen for the atmosphere, even though it accounts for only 0.6 percent of the atmosphere, and highlights the Baltic Sea and Black Sea as the largest semi-enclosed marine ecosystems with low oxygen content.

But the repercussions of deoxygenation are not limited only to enclosed or semi-enclosed seas, but those areas whose oxygen content is limited have expanded drastically in most of the Atlantic in the last 50 to 100 years, even in connected seas, such as the Mediterranean.

And, in addition to being found in many coastal waters, the lack of oxygen is also seen at medium depths, usually between 300 and 1,000 meters -the richest in biodiversity-, as in the Atlantic basin, with a significant reduction in the last 60 years.

For Lisa Levin, “there are solutions”, but these necessarily go through a “greater ambition” in the reduction of GHG emissions, an objective that is in the hands of governments “through multiple avenues”.

He also pointed to the need to reduce discharges from agriculture, industry or wastewater and avoid “other sources of stress for the oceans”, such as pollution and overfishing, and has advocated promoting the creation of marine protected areas and “including the oceans in the negotiations that are carried out in this Blue COP”.

Source: The Vanguard