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Oceanography
Marine critters record global warming

Most climate scientists agree that Earth’s warming has accelerated, creating debate about the extent to which humans have driven that change. While people have been putting their instruments to work, layers of fossilized marine creatures, resting undisturbed offshore of Southern California, have acted as an independent record of ocean temperature for millennia. Now, data from such layers is mirroring the same warming trend that instruments have shown — suggesting humans are contributing to global warming.

David Field, an oceanographer at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., and his colleagues studied species of plankton known as foraminifera, or forams, which are highly sensitive to temperature changes in the ocean. Various species of the microscopic creatures inhabit oceans around the world, and when they die, their calcite shells collect on the seafloor.

The team was not “being imprudent” by relating their findings to human-caused warming.

Peter deMenocal,
Lamont-Doherty Earth Observatory

Looking at different layers of ocean sediments containing shells, Field’s team found that warm-water tropical species took hold in oceans off Southern California during the last century in levels unseen throughout the entire 1,400-year span of the core sample. The entire sample spanned centuries-long events such as the Little Ice Age and the Medieval warm period, but variability on the scale of decades was “more dominant” in the cores, Field says, and the last several decades stand out as anomalous.

The most “compelling” part of the study, which was published in the Jan. 6 Science, is that unlike previous studies of ocean temperatures, Field’s team’s research “puts the modern world within the context of the world before people were really present in large numbers,” says Peter deMenocal, a paleooceanographer at Lamont-Doherty Earth Observatory in New York. And, he says, the study shows that “whatever has happened in the last century is really unique within the context of the last 15 centuries.”

Field and colleagues studied 1- to 3-meter-long cores from the Santa Barbara Basin that contained annual layers of sediments and forams. In the basin, low oxygen levels have kept animals from venturing to the seafloor and disturbing the layers, allowing for well-preserved annual layers of sediments with forams, similar to the rings on a tree.

Field counted species within two-year intervals for about the last 260 years and then species within five-year intervals back about 1,400 years. The counts, he says, revealed that tropical and subtropical species increased in abundance after 1925 and then dramatically increased in the mid-1970s. Abundances of different foram species that prefer cool water dropped during those same times.

Known natural variability in climate on the scale of decades and centuries does not account for the sudden changes observed in the core measurements, Field says. “The best explanation,” he says, is that greenhouse gas emissions are affecting marine ecosystems.

If the study stood alone, without support from other studies showing the same trend, then “one might be a little bit skeptical,” deMenocal says. He notes that changes in nutrients, runoff from development or other non-greenhouse-gas-related human influences could affect the ecosystem and not be tied to global warming. The timing and nature of the transition, however, deMenocal says, are consistent with a number of other studies, most of which were focused on land.

“This is the ocean part of that story,” deMenocal says. Field and colleagues make a good case that post-Industrial Revolution climate changes are anomalous within the context of a longer period, deMenocal says, and the team was not “being imprudent” by relating their findings to human-caused warming.

Kathryn Hansen

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