Corals Reveal Changes in Ocean Circulation, Prove Climate Modelers Correct

coral ocean temperature photo

Photo via Ohio State University, credit Andrea Grottoli

Climate modelers have predicted that a shift in global temperatures will slow the ocean's natural circulation of warmer waters at the surface and colder waters near the sea floor, and it looks like that is exactly what is happening according to researchers at Ohio State University and the University of Toronto. They've found that corals in the western tropical Pacific Ocean, specifically Gorgonians shown above, reveal a shift in thermocline within their layers of growth, similar to how tree trunk rings showing drought years. After going about a century deep in coral rings, the researchers have evidence that the ocean's natural mixing system seems to be slowing as predicted. According to Ohio State University, the slowing of circulation between warmer water at the surface, where changes in temperature occur on a daily basis, and colder water in the ocean's depths that stays at a more stable temperature, has been expected as global temperatures rise. That modeling is shown to be playing out as researchers study the growth of corals over the last 100 years and more specifically, over the last few decades.

The boundary layer between warm surface water and cool deeper water is called the thermocline, which naturally shifts in depth over time depending on the different weather and seasonal conditions, and different areas of the ocean. However, climate modelers predicted that the thermocline would become ever more shallow -- and thus the change from warmer to cold water ever more stark -- with global warming.

The Gorgonian corals studied have growth areas that shift with currents and provide a way to analyze how the boundary between water levels has changed over time. By taking samples from varying depths and studying the exoskeletons and the nutrition levels in their layers (in the same way researchers study tree rings to gain an ecological history of an area) the scientists found that the thermocline has gotten shallower particularly since the 1970s.

"Climate modelers looking at how the Pacific might respond to global warming have predicted that the atmospheric patterns in the tropical Pacific would weaken, and if that happened, you would expect the thermocline to get shallower in the western tropical Pacific," said Branwen Williams, one of the members of the research team. "Our data are some of the first proxy data to support what the modelers have been predicting."

By expanding their study to corals in other areas of the world, the scientists will be able to know for sure if the thermocline is shifting to a shallower level on a global scale. Ruling out local changes is the next step in their research, and they're set to move their research eastward across the Pacific.

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