The world’s oceans are heating up. And like a bottle of pop left out in the sun, they’re going flat. Except instead of carbon dioxide, they’re losing oxygen. As with most liquids, water’s ability to dissolve oxygen largely depends on temperature. The warmer the water, the less oxygen it can hold.
Though oxygen-starved or hypoxic zones have always existed, warmer waters may be causing these zones to expand, according to new research published in the journal Science. The study, led by Lothar Stramma at the University of Kiel in Germany, addressed changes in different regions of the oceans, and found that oxygen levels in tropical oceans hundreds of feet below the surface have declined over the past 50 years.
These low oxygen areas are generally found in the Pacific, Atlantic and Northern Indian Ocean, according to coauthor Gregory C. Johnson, an oceanographer with the federal Pacific Marine Environmental Laboratory in Seattle. Though located deep within the ocean, these oxygen-minimum zones have the ability to affect coastal areas.
“Along California, there are undercurrents that carry these low oxygen waters forward,” Johnson says. “These oxygen-depleted zones are spilling onto the continental shelf off the coast of California and are starting to [expand] near the coast of Peru.”
The low oxygen levels suffocate some species while driving out others. “As these areas expand, certain species’ habitats become more and more limited,” Johnson says.
More significantly, surface warming increases stratification, creating a barrier between the lighter warm water and denser cold water in the ocean. It’s like putting maple syrup at the bottom of a container and lighter fluid at the top, explains Jack Sobel, a scientist at the nonprofit Ocean Conservancy. The two can’t mix. “There’s a stronger contrast in density between warm waters above and cool waters below,” Johnson says.