Getting warmer: understanding threats to ocean health

The global ocean covers 70 percent of our planet, makes Earth habitable, and contributes to economies, food supplies, and our health. Yet the ocean is increasingly threatened by the growing amount of carbon dioxide in the atmosphere.

Two Lamont-Doherty Earth Observatory scientists affiliated with the Center for Climate and Life are leading research projects that examine a few of the ways climate change affects the health of the ocean. Both researchers use the fossil remains of sea creatures as natural recorders of past climate and marine ecosystem changes. The information they obtain from these provides clues about how the future ocean and its inhabitants might be shaped by climate change.

Their studies are funded in part by the Center’s partnership with the World Surf League PURE, which enables Lamont-Doherty scientists to pursue critical research that advances understanding of climate impacts on the ocean.

Ocean acidification: The other carbon dioxide problem

Bärbel Hönisch, a marine geochemist, studies how seawater chemistry changed through time. Today, the ocean is becoming more acidic due to the rising concentration of carbon dioxide in Earth’s atmosphere, about 30 percent of which is absorbed by the ocean. While this process helps to minimize global warming, the dissolution of carbon dioxide in the ocean leads to the formation of carbonic acid. As the name implies, the addition of carbonic acid makes seawater more acidic and this ‘ocean acidification’ makes it harder for calcifying organisms such as corals, mollusks, and some plankton to build their shells and skeletons.

The current pH of the ocean is around 8.1, representing a 25 percent increase in acidity over the past 200 years. As the amount of carbon dioxide in the atmosphere continues to rise, scientists expect seawater acidity to increase another 25 percent by the end of the 21st century. This level of acidification is similar to that of the Paleocene-Eocene Thermal Maximum (PETM), which occurred around 56 million years ago. During the PETM, a sudden rise in atmospheric carbon dioxide coincided with rapid warming and seawater acidification — conditions that lasted for 70,000 years or more.

Hönisch is analyzing the shells of tiny plankton organisms called foraminifera that were preserved in deep-sea sediments during this and other time periods, which are recovered from the sea floor by deep-ocean drilling. Her goal is to quantify changes in the ocean that occurred due to past climate shifts, and to determine whether and how marine organisms adapted to these changing conditions: Did some organisms evolve and thrive? Did some go extinct?

Hönisch is using her grant from the Center for Climate and Life to answer these questions. As part of her project, she and her research team are creating a timeline that details past levels of ocean acidity and the sensitivity of foraminifera to environmental changes. This will help them determine how past ocean temperatures and acidity affected the ability of calciferous marine organisms to build and maintain their shells. Their findings may also improve predictions of the consequences of future ecosystem changes.

“Our research has shown that the two degrees Celsius warming at the end of the last ice age had a stronger effect on foraminifer species abundance and latitudinal migration than the 0.15 unit acidification of the surface ocean,” Hönisch said. “So in some ways, one might say that warming is a greater environmental stressor than acidification, at least for planktic foraminifera with this specific extent of warming and acidification. However, warming and acidification will also go hand in hand in the future and their respective effects will add up.”

Hönisch also explained that the effects of rising temperatures and acidification of seawater won’t be the same throughout the global ocean. “There will be a lot of variability in the conditions in the ocean,” she said. “There may be refuges where certain organisms can survive.”

Although observations of past ocean changes may seem grim, Hönisch noted that many marine organisms are extremely resilient to environmental change. Even the devastating impact of the Chicxulub asteroid 66 million years ago didn’t completely wipe out life in the sea, so there’s hope for the survival of marine organisms despite our rapidly acidifying oceans.

Roz Cummins, Center for Climate and Life: Columbia University, 13 August 2018. Full article.

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