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Bivalves, such as clams, oysters and mussels, record seasonal environmental changes in their shells, making them living chronicles of climate history. A new study of bivalve shells has detected two major episodes of instability in the North Atlantic Ocean's circulation systems, suggesting that the region may be heading toward a tipping point that could trigger sudden, dramatic changes in global weather patterns.

The North Atlantic Ocean has two massive current systems that are vital for regulating the climate. The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents that circulates water within the Atlantic Ocean. The Subpolar Gyre (SPG) is a part of the AMOC and is a massive, rotating whirlpool. In a paper in the journal Science Advances, researchers analyzed 25 bivalve-derived records to assess changes in stability in the subpolar North Atlantic over the past two centuries.

Specifically, the research team looked at clam shells. As clams grow, their shells create rings, like a tree. By analyzing the chemistry and width of these rings, scientists obtain precise year-on-year data on past ocean conditions, including factors such as currents, temperatures and salinity. Then, they used statistical tools to track how long the ocean system, primarily the SPG, took to recover from natural disturbances.

They discovered two periods where currents became dramatically unstable. "Clams reveal North Atlantic destabilization in the early 20th century and at present," wrote the researchers in their paper. "Our results reveal two significant destabilization episodes in the northern North Atlantic since the year 1800...the region is heading toward [a tipping point]."

The first period spanned from the 1800s to 1920. This occurred before a documented major shift in North Atlantic ocean circulation, suggesting that it was already losing balance, a classic sign of a tipping point. The second period of instability began in the 1950s and continues to this day, indicating that the ocean remains as unstable as it was before the last significant circulation change. In other words, we are heading toward another tipping point. The main source of this instability, according to the researchers, is the SPG.

So what can be done to avert this circulatory collapse? The good news is that the clams have sounded an early warning—we have not yet reached the tipping point. This study, combined with other monitoring data and climate modeling, can help governments prepare for potential regional climate changes and mitigate risks to reduce stress on the ocean.