A vast system of rotating ocean currents in the North Atlantic is behaving strangely – and may be approaching a tipping point.

The North Atlantic Subpolar Gyre is a system of currents located south of Greenland. (Image credit: NASA Scientific Visualization Studio)

A new analysis of shellfish shells suggests that a vast system of rotating ocean currents in the North Atlantic is behaving extremely strangely, possibly because it is approaching a tipping point.

The North Atlantic Subpolar Gyre plays a key role in transporting heat to the Northern Hemisphere and is part of a much larger network of ocean currents called the Atlantic Meridional Overturning Circulation (AMOC). However, new data suggest that the subpolar gyre has been losing stability since the 1950s, meaning its circulation could weaken significantly in the coming decades, researchers report in a study published today (October 3) in the journal Science Advances.

“This is deeply concerning,” lead study author Beatriz Arellano Nava, a research fellow in physical geography at the University of Exeter in the UK, told Live Science. “The subpolar gyre has recently been recognized as a tipping point. We still need to better understand the consequences of a sharp weakening of the subpolar gyre. But based on the few published studies, we know it will lead to an increase in extreme weather events, particularly in Europe… and also to changes in global precipitation patterns.”

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Currents in the North Atlantic Subpolar Gyre are also part of the AMOC, but the subpolar gyre can destabilize and cross tipping points independently of the AMOC.

The North Atlantic Subpolar Gyre is part of the AMOC, but it could reach a point of no return regardless of the massive network of currents. The climate impacts on Europe, in particular, would be similar to those that would occur if the AMOC were to collapse, although they could be less intense because the AMOC is much larger, Arellano Nava stated. However, “even if the impacts are not as catastrophic as if the AMOC were to collapse, the weakening of the subpolar gyre could lead to significant climate change,” she warned.

Previous research suggests that the AMOC may collapse in the near future as its main engine—the cascade of dense water flowing from the surface of the North Atlantic and Arctic Oceans to the seafloor—fails. This cascade, which until now consisted of extremely cold and salty water, is being diluted by meltwater and warmed by rising global temperatures, causing the water in some places to become insufficiently dense to sink properly. (Cold, salty water is denser than warm, less salty water.)

A similar fate awaits the North Atlantic Subpolar Gyre, which also depends on surface water sinking to the ocean floor. A cascade of dense water at the center of the gyre maintains the rotational currents, said Arellano Nava. However, the system is also partially driven by wind, so a complete collapse is unlikely, she added.

The North Atlantic Subpolar Gyre is a branch of the AMOC, so the collapse of the AMOC inevitably entails a sharp weakening of the gyre. Conversely, a weakening of the subpolar gyre does not automatically imply a collapse of the AMOC, Arellano Nava noted.

“The subpolar gyre can weaken sharply without destroying the AMOC,” she explained. “This is precisely what happened during the transition to the Little Ice Age, which occurred in the 13th and 14th centuries.”

The Little Ice Age, which lasted from approximately 1250 to the late 19th century, is one of the coldest periods in the history of the Northern Hemisphere since the end of the last ice age. Average temperatures dropped by approximately 2 degrees Celsius, causing rivers and harbors across Europe and North America to freeze over in winter, triggering agricultural crises and essentially plunging medieval society into chaos, according to The New Yorker. While factors such as volcanic eruptions and declining solar activity contributed to the onset of the Little Ice Age, the North Atlantic Subpolar Circulation is believed to have played a significant role in its intensification.

Due to climate change, conditions now differ dramatically from those of the 13th century, so scientists don't know whether another Little Ice Age is possible, Arellano Nava said. However, it illustrates some of the climate consequences we may face.

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Clues in the shellfish

For the new study, Arellano Nava and her colleagues analyzed existing datasets obtained from the shells of two species of North Atlantic clams: Arctica islandica and Glycymeris glycymeris. The clams record information about the ocean in their shells as they grow; for example, they absorb various forms of elements such as oxygen, which can provide researchers with clues about ocean processes over time.

“Thanks to the mollusc data, we can precisely date each layer,” Arellano Nava said. “They're like tree rings in the ocean.”

A close-up of the growth stripes on the shell of a blenny clam (Glycymeris glycymeris).

Researchers compiled 25 datasets to create a highly accurate picture of the North Atlantic subpolar gyre over the past 150 years. They discovered two strong instability signals. The latter is ongoing and indicates that the subpolar gyre is approaching a tipping point as a result of global warming, confirming previous observations and studies, Arellano Nava reported.

But another signal came as a complete surprise, she said. Data obtained using shellfish showed that the subpolar gyre was unstable for several years leading up to a regime shift in the North Atlantic in the 1920s. This previously described event was characterized by a strengthening of currents in the gyre. The instability of the subpolar gyre likely triggered the regime shift in the 1920s, and the chronology of events suggests that the period of instability may have been related to the recovery of the subpolar gyre after its collapse during the Little Ice Age, Arellano Nava said.

“At some point it had to regain strength, but we don't have definitive evidence of that because we haven't studied those mechanisms,” she said.

Whether or not the instability of the early 20th century was actually a signal that the subpolar circulation was returning to its full strength, the coincidence of the signal in the shellfish data and the change in the North Atlantic regime in the 1920s shows that the results are reliable, Arellano Nava said.

“If you see a loss of stability followed by a rapid change, then you can be confident that these are early warning signs of a dramatic change,” she said.