The loss of oxygen from the world’s ocean is increasingly leaving sea life, fisheries and marine species such as tuna, marlin and sharks gasping for breath, a new report by the International Union of Conservation of Nature (IUCN) has warned.
In its report "Ocean deoxygenation: Everyone's problem", released at the UN Climate Change conference in Madrid, the global conservation body said that about 700 sites had been identified globally with low oxygen levels - up from only 45 in the 1960s.
In the same period, the IUCN warned in the largest peer-reviewed study to date that the volume of anoxic waters - areas totally devoid of oxygen - have quadrupled.
"With this report, the scale of damage climate change is wreaking upon the ocean comes into stark focus. As the warming ocean loses oxygen, the delicate balance of marine life is thrown into disarray," said Dr Grethel Aguilar, IUCN Acting Director General.
"The potentially dire effects on fisheries and vulnerable coastal communities mean that the decisions made at the ongoing UN Climate Change Conference are even more crucial. To curb ocean oxygen loss alongside the other disastrous impacts of climate change, world leaders must commit to immediate and substantial emission cuts."
"We are now seeing increasingly low levels of dissolved oxygen across large areas of the open ocean. This is perhaps the ultimate wake-up call from the uncontrolled experiment humanity is unleashing on the world's ocean as carbon emissions continue to increase," said Dan Laffoley, Senior Advisor Marine Science and Conservation in IUCN's Global Marine and Polar Programme and a co-editor of the report.
"Ocean oxygen depletion is menacing marine ecosystems already under stress from ocean warming and acidification. To stop the worrying expansion of oxygen-poor areas, we need to decisively curb greenhouse gas emissions as well as nutrient pollution from agriculture and other sources."
Deoxygenation is starting to alter the balance of marine life , favouring low-oxygen tolerant species (e.g. microbes, jellyfish and some squid) at the expense of low-oxygen sensitive ones (many marine species, including most fish). Some of the ocean's most productive biomes - which support one fifth of the world's wild marine fish harvest - are formed by ocean currents carrying nutrient-rich but oxygen-poor water to coasts that line the eastern edges of the world's ocean basins.
As naturally oxygen-poor systems, these areas are particularly vulnerable to even small changes in ocean oxygen. Impacts here will ultimately ripple out and affect hundreds of millions of people.
"What we are seeing is a decline of 2 percent in the global oxygen level [in the oceans]. It doesn't sound like a lot but this small change will have enormous ramifications," Minna Epps, the IUCN's global marine and polar programme director.
"Deoxygenation will have an impact on biodiversity, on biomass of commercially important species and on vulnerable rare species. It will also have an impact on habitats. We are seeing species migrating because of this," she added.
The ocean is expected to lose 3-4% of its oxygen inventory globally by the year 2100 under a business-as-usual scenario, but the global average masks local changes that are predicted to be, for example, more severe in mid to high latitudes. Most of the losses are predicted to be concentrated in the upper 1000m of the water column, which is richest in marine biodiversity.
