Climate change is going to wreak havoc on the world’s oceans, according to two new studies, depleting the warming waters of the oxygen that fish and other sea life depend on to survive.
By 2080, around 70 percent of the oceans on the planet will suffer from a lack of oxygen due to warmer temperatures, a study published in November by researchers with the American Geophysical Union’s journal Geophysical Research Letters concluded. The study finds that substantial deoxygenation of the middle ocean depths, where a large percentage of the fish that people eat are found, began occurring in 2021.
“This zone is actually very important to us because a lot of commercial fish live in this zone,” Yuntao Zhou, an oceanographer at Shanghai Jiao Tong University and lead author of the study, said in a statement.
The study’s models show deoxygenation will begin affecting all ocean depth zones by 2080, and that deoxygenation may be irreversible. Even if humans stopped emitting greenhouse gases and reversed global warming by sucking carbon dioxide from the air, the question of “whether dissolved oxygen would return to pre-industrial levels remains unknown,” Zhou said. This underscores the importance of halting climate change — and the resulting ocean deoxygenation — as quickly as possible, according to scientists.
The reason for the declining oxygen levels in oceans is that warmer waters hold less dissolved oxygen and ocean temperatures are rising at an alarming rate. According to another new study released this week and published in the scientific journal PLOS Climate, a majority of the world’s ocean surface has consistently exceeded the normal range since 2014.
High water temperatures threaten ecosystems such as coral reefs and kelp forests, which fish feed on. Researchers from the Monterey Bay Aquarium in Monterey, Calif., mapped 150 years of sea surface temperatures to establish the normal temperature ranges. They found that 2014 was the first year in which a majority of ocean surface areas went above what would normally be considered an extremely hot temperature by historical standards. That benchmark has been surpassed every year since then, making what was once extreme heat the new normal.
“Climate change is not a future event,” Dr. Kyle Van Houtan, who led the research team when he was chief scientist for the aquarium, said in a statement. “The reality is that it’s been affecting us for a while. Our research shows that for the last seven years more than half of the ocean has experienced extreme heat.”
While heat in and of itself threatens marine life, declining oxygen levels add an additional danger, and the middle ocean depths are most vulnerable because they do not have additional sources of oxygen. The ocean surface absorbs oxygen from the atmosphere, and the lowest sea depths gain oxygen from the decomposition of algae. But the ocean’s middle depths, which are from about 600 feet to 3,300 feet below the surface and are called mesopelagic zones, will be the first area to lose large amounts of oxygen because they do not get oxygen from either of those sources. They are also home to many of the world’s commercially fished species, including tuna, swordfish and anchovies.
Losing those fish species to deoxygenation could devastate fishing-based economies and cause shortages of seafood for communities that rely on it. “Deoxygenation affects other marine resources as well, but fisheries [are] maybe most related to our daily life,” Zhou said.
The researchers also found that oceans closer to the poles, where warming is occurring faster, are also losing oxygen more quickly.