Slowdown of North Atlantic circulation caused sudden cold spells lasting hundreds of years

A new study from the University of Helsinki in Finland reports that sudden cold spells lasting hundreds of years took place in the middle of the warm Eemian climate period, about 120 000 years ago, as a result of the slowdown of the North Atlantic circulation. Evidence presented in other studies suggests that the circulation system of the North Atlantic Ocean is currently in a weakened state that is unprecedented in the past 1 600 years.

These cold intervals saw a fall in temperature of a few degrees, and replacement of forests by tundra at the study site in northern Finland. The Eemian, which took place before the last Ice Age, had a climate generally warmer than present.

According to the researchers, the sudden shifts of Eemian climate are connected to disturbances in the North Atlantic circulation, which happened during that time. Today, the warm oceanic currents of the North Atlantic maintain a relatively temperate climate in Europe. The future development of this oceanic circulation has been hard to predict, however, and possible disturbances have not been ruled out.

“These results strongly suggest that the North Atlantic circulation is sensitive to disturbance, with major effects in Northern Europe,” says University of Helsinki postdoctoral researcher Sakari Salonen, who headed the study1.

The results are based on a geological deposits studied in Sokli, Northern Finland. At this site, thick geological layers have been preserved in a deep depression, allowing them to survive through the following Ice Age.

“The Sokli site is unique in the northern parts of the world, which has made the site invaluable in the study of past, long-term climate change,” says Karin Helmens, a researcher from Stockholm University and a long-time coordinator of the studies at Sokli.

To understand the importance of this study, it’s important to mention several other studies.

Among these, two published in Nature which present evidence suggesting that the circulation system of the North Atlantic Ocean is in a weakened state that is unprecedented in the past 1 600 years.2

The warm, salty waters of the Gulf Stream make a northeasterly meander across the Atlantic Ocean, eventually forming the North Atlantic Current, which then funnels into the Nordic Seas.

In the chill of winter, these waters cool and descend with the heavy load of their salinity. This deep convection is a key part of the Atlantic Meridional Overturning Circulation (AMOC), which can be thought of as an ocean conveyor belt that releases heat to the atmosphere above the North Atlantic Ocean before traveling through the abyssal ocean to resurface in other areas of the world.

Given the importance of the AMOC to heat exchange between the ocean and the atmosphere, the varying strength of this system is thought to have major impacts on the global climate and has been implicated widely in some of the most remarkable and abrupt climate changes of the past.

Direct measurements of the modern AMOC flow rates show a decline in its strength in the past decade. Reconstructions of the natural variability and long-term trends of the AMOC are needed, however, to put these recent changes in context. 

One group of researchers found that the strength of the AMOC was relatively stable from about ad 400 to 1850, but then weakened around the start of the industrial era. This transition coincides with the end of the Little Ice Age — a multicentennial cold spell that affected many regions of the globe.

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Another study, published in Science in 2016, examined ice cores from Greenland and Antarctica and showed that there was a period during the last ice age, when our planet experienced dozens of abrupt temperature shifts. Plummeting and rising again every 1500 years or so, those sudden temperature shifts wreaked havoc on the ecosystems. 

The study3 offered the first evidence that the ocean’s overturning circulation slowed during every one of those temperature plunges – at times almost stopping. 

Featured image credit: NOAA

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