Ocean Sciences News

Scientists Discover Evidence of Long “Ocean Memory”

Measurements from a 19th century scientific expedition have revealed that the deep Pacific waters are cooling from lower global temperatures centuries ago.

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The world’s first oceanography expedition, which set sail in 1872, is still uncovering new insights today.

In a study published in January in Science, researchers analyzed thousands of measurements from the HMS Challenger expedition, the scientific voyage that sailed around the globe from 1872 to 1876. The data revealed that the deep Pacific Ocean is still cooling from a dip in global temperatures that chilled surface waters several centuries ago.

Waters take so long to reach the depths of the Pacific that “they are still responding to the cooling trend that marked the entry into the Little Ice Age,” said first author Jake Gebbie at the Woods Hole Oceanographic Institution in Woods Hole, Mass. The findings could improve climate models and may offer a clue into how future oceans will respond to modern global warming.

Data Taken by Hemp Rope

Understanding the ocean’s history through past changes in climate can help scientists pinpoint just how much heat is stored in the oceans, a hot topic of current research.

Earth’s climate cooled globally between the 14th and 19th centuries in what’s known as the Little Ice Age, causing temperatures to dip roughly half a degree Celsius by some estimates. Past research has uncovered evidence of the Little Ice Age across the globe, cropping up in ice cores, tree rings, corals, sediments, and cave formations.

In the latest study, the researchers pulled data from the Challenger expedition, which “marks the beginning of modern oceanography,” said Gebbie. The scientific expedition took top to bottom measurements from the world’s oceans for 4 years, often using thermometers tied to hemp ropes. The new research compared the temperatures with modern-day measurements and ran an independent model using 2,000 years of climate records to search for cooling trends.

HMS Challenger under sail in 1874
Credit: National Oceanic and Atmospheric Administration/Department of Commerce

A Chilling Trend

Gebbie and his collaborators found a signal of cooling in the Pacific in both the measurements and the model. The model calculated that the Pacific deep waters have cooled by 0.02°C over the last century. The data match up “pretty closely” with what the model predicted, said Gebbie.

Taken together, the results reveal that the deep Pacific is still slowly being replaced by waters from the Little Ice Age, which are causing the deep to cool down. Although the decrease in temperature may not seem like much, Gebbie explained, “when you sum up that temperature change over such a big volume of the Pacific Ocean, it actually adds up to a lot of energy.” The cooling trend is enough to offset one quarter of the heat gained in the surface of the ocean during the 20th century.

This offset is not enough to overwhelm the steadily increasing global warming signal in the surface ocean, he notes, but will help researchers better constrain how much heat has been taken up by the ocean.

Gebbie believes that the latest results could be helpful for climate modelers. “Most comprehensive climate models are started from equilibrium at some time near the beginning of the industrial revolution,” Gebbie noted. “What this study shows is that there’s still some influence from ancient signals that originated from the surface before the industrial revolution.”

“Ocean Memory”

Timothy DeVries, an assistant professor at the University of California, Santa Barbara, who was not involved in the study, said that these results are important because “it reminds us of the long memory of the ocean.” Scientists who measure ocean temperature trends will “take note of these results” and begin scouring their field measurements for these signals, said DeVries.

“Long memory in the climate system is both a fascinating phenomenon and also a major difficulty in understanding the ocean state today,” said Carl Wunsch at Harvard University who is acknowledged in the paper. “This paper is, to my knowledge, the first one to document direct, plausible evidence that the deep ocean ‘remembers’ long-ago climate states.”

The latest results hint at future consequences of present-day climate change. “The signal of modern warming will more quickly overwhelm the previous signals,” Gebbie said. Once the present-day warming reaches the depths, he added, “it will take several hundred to even a thousand years for that signal to be removed.”

“Whatever we’re doing today at the surface will have consequences for a long time,” Gebbie said.

—Jenessa Duncombe (@jrdscience), News Writing and Production Intern

Citation: Duncombe, J. (2019), Scientists discover evidence of long “ocean memory”, Eos, 100, https://doi.org/10.1029/2019EO115717. Published on 07 February 2019.
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