Ocean warming attributable to climate change is gradually reducing the amount of oxygen dissolved in the world’s oceans, and it’s likely to get worse with time, according to a study led by the National Center for Atmospheric Research.
"Loss of oxygen in the ocean is one of the serious side effects of a warming atmosphere and a major threat to marine life," said NCAR’s Matthew Long, the study’s lead author, in a report in AtmosNews, a publication of Colorado-based NCAR.
Scientists have known that, over time, a warming climate probably would sap the ocean of oxygen, leaving fish, crab, squid, sea stars and other marine life struggling to breathe, NCAR says. A separate study, reported by Ocean Scientists for Informed Policy, a group associated with the Scripps Institution of Oceanography, says that in the tropical Atlantic, blue marlin and tuna experienced a 15 percent reduction in vertical habitat from 1960 to 2010 because of growing oxygen-depleted zones.
The NCAR study shows that ocean warming is reducing the amount of oxygen dissolved in the southern Indian Ocean and parts of the eastern tropical Pacific and south Atlantic basins, and it projects that between 2030 and 2040 more widespread deoxygenation may occur, including waters off the U.S. West Coast.
Until now it has been difficult to determine whether the oxygen drain was having a noticeable impact and whether the deoxygenation was attributable to climate change or to natural variations in winds and temperature at the water surface.
The NCAR study, authored by Long, Curtis Deutsch of the University of Washington and Taka Ito of Georgia Tech and published in the journal Global Biogeochemical Cycles, says the effects of climate change are being felt in parts of the world now; others should be evident across large regions of the ocean between 2030 and 2040. Yet in other ocean areas, including off the east coasts of Africa, Australia and Southeast Asia, deoxygenation effects are not expected until after 2100.
The ocean — from the depths to the shallows — gets its oxygen from the surface, either directly from the atmosphere or from phytoplankton, which release oxygen into the water through photosynthesis. Warming surface waters absorb less oxygen, the scientists say, and the oxygen that is absorbed often can’t penetrate very deeply because as water heats it expands, becomes lighter than the water below and is less likely to sink.
Using a supercomputer, the scientists ran simulations to study dissolved oxygen in, or projected to be in, the oceans from 1920 to 2100 to find out how concentrations of oxygen varied naturally in the past and when deoxygenation from climate change is likely to become more severe than those natural variations.
The study’s authors found that they could distinguish between oxygenation patterns caused by natural weather phenomena and patterns caused by climate change.
The range of some marine species already is shifting because of changes in oxygen content, according to OSIP. One study shows that decreases in mid-depth oxygen content along the Japanese continental slope during the past 60 years have resulted in Pacific cod shifting their distribution to shallower depths. Another reports that off the U.S. West Coast, the Humboldt squid has greatly expanded its range, and that expansion coincides with areas of significant oxygen declines, according to OSIP.
OSIP scientists say some marine life, such as jellyfish and squid, are highly tolerant of low levels of dissolved oxygen, but other groups, such as fish and crustaceans, require more oxygen and are highly vulnerable to oxygen declines.
Off Southern California, midwater fishes, which are vital to the ocean food web, declined 63 percent between periods of high and low oxygen, they say. The vulnerability of fish and crustaceans is evident in the large die-offs that have occurred during low oxygen events off Oregon and in the Gulf of Mexico.
“Coastal waters are experiencing significant reductions in oxygen due to nutrient pollution, but these impacts are also exacerbated by climate change,” OSIP says on its website. “Dead zones, areas where most organisms cannot live due to oxygen limitation, are now reported for more than 479 systems, and since the 1960s their numbers have doubled approximately every decade.”