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The mid-Pliocene, from 2.6 to 5.3 million years ago, may be the closest the Earth has ever been—climatologically speaking—to where we are now.
With an atmospheric carbon dioxide level about the same as today’s, “the Pliocene is the last time we had a stable, warm climate globally,” says Catherine Davis, an oceanographer at North Carolina State University. The average global temperature was about 2 °C to 3 °C warmer than it is now, she says. The Pliocene had something else climate experts are predicting for our future: massive oxygen minimum zones (OMZs)—largely lifeless stretches of ocean severely lacking in oxygen.
OMZs are common in the ocean. They occur in its deeper reaches, typically between 200 and 1,000 meters below sea level, where oxygen concentrations plummet from their typical level of around six to eight milligrams per liter nearer the surface to less than two milligrams per liter. At these depths, explains John Cigliano, a biologist at Pennsylvania’s Cedar Crest College, microbes deplete the oxygen in the water as they decompose sinking organic material. The lack of oxygen, Cigliano says, restricts many animals to the ocean’s surface areas.
During the Pliocene, however, OMZs were much more widespread. In recent research, Davis looked for traces of a specific foraminifera in records of seafloor sediments from around the world. This tiny shelled organism only lives in low-oxygen water, giving Davis a way to calculate the extent of Pliocene OMZs. “Most of the North Atlantic Ocean hosted an OMZ in the Pliocene, whereas there are virtually no Atlantic OMZs today,” she says. “I can’t speculate as to where OMZs could develop in the future, but it’s certainly suggestive.”
Yet with our modern climate changing to look more and more like the Pliocene, Davis says, this “could give us a glimpse of what Earth’s oceans might look like 100 years from now.”
Tim Dudeck, a biologist at the Leibniz Centre for Tropical Marine Research in Germany who wasn’t involved in the work, says that rising water temperatures favor the expansion of large, stable OMZs. In contrast with the shorter-lived, often deadly low-oxygen events triggered by things like excessive nutrient runoff—which can trigger areas often called dead zones—the large OMZs have slow and subtle effects.
The consequences of growing OMZs could be far-reaching, says Sarah Cooley, a climate scientist at the nonprofit Ocean Conservancy.
“Changing oxygenation of the oceans will favor some species that can tolerate lower oxygen conditions,” she says. “Whether they are the species humans currently depend upon remains to be seen, but it’s likely that key ecologically, economically, or culturally important species won’t thrive in lower oxygen conditions.”