The mid-Pliocene, from 2.6 to 5.3 million years ago, may be the closest the Earth has ever been\u2014climatologically speaking\u2014to where we are now.<\/p>\n
With an atmospheric carbon dioxide level about the same as today\u2019s, \u201cthe Pliocene is the last time we had a stable, warm climate globally,\u201d says Catherine Davis, an oceanographer at North Carolina State University. The average global temperature was about 2 \u00b0C to 3 \u00b0C warmer than it is now, she says. The Pliocene had something else climate experts are predicting for our future: massive oxygen minimum zones (OMZs)\u2014largely lifeless stretches of ocean severely lacking in oxygen.<\/p>\n
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\u2019s 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\u2019s surface areas.<\/p>\n
During the Pliocene, however, OMZs were much more widespread. In recent research<\/a>, 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\u2019t speculate as to where OMZs could develop in the future, but it\u2019s certainly suggestive.”<\/a><\/p>\n Yet with our modern climate changing to look more and more like the Pliocene, Davis says, this \u201ccould give us a glimpse of what Earth\u2019s oceans might look like 100 years from now.\u201d<\/p>\n Tim Dudeck, a biologist at the Leibniz Centre for Tropical Marine Research in Germany who wasn\u2019t 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\u2014which can trigger areas often called dead zones\u2014the large OMZs have slow and subtle effects.<\/p>\n The consequences of growing OMZs could be far-reaching, says Sarah Cooley, a climate scientist at the nonprofit Ocean Conservancy.<\/p>\n \u201cChanging oxygenation of the oceans will favor some species that can tolerate lower oxygen conditions,\u201d she says. \u201cWhether they are the species humans currently depend upon remains to be seen, but it\u2019s likely that key ecologically, economically, or culturally important species won\u2019t thrive in lower oxygen conditions.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":" The mid-Pliocene, from 2.6 to 5.3 million years ago, may be the closest the Earth has ever been\u2014climatologically speaking\u2014to where we are now. With an atmospheric carbon dioxide level about the same as today\u2019s, \u201cthe Pliocene is the last time <\/p>\n