Two new studies on the health of ocean currents in the North Atlantic have found that a natural decrease in the flow of these currents that began in the mid-nineteenth century has been exacerbated by modern-day climate change, resulting in the weakest the North Atlantic’s currents have been in over 1,500 years.

The Atlantic meridional overturning circulation (AMOC) plays a crucial role in the world’s network of ocean currents, where warm surface water flows through the Caribbean, northward along the Eastern Seaboard, and cuts diagonally across the Atlantic towards northern Europe. Once there, it normally cools when it mingles with waters from the Arctic, and as it cools it sinks to lower depths in the ocean, where it joins a southward current that carries the water on the next leg of its journey around the world.

But the two studies, one looking at the AMOC’s flow over the past 150 years, and the other studying a longer, 1,500-year cycle, say that this cycle is being interrupted. The short-term study has found that the AMOC has slowed by 15 percent since the middle of the twentieth century, seeing a decrease of 3 million cubic meters (106 million cubic feet) of water per second since then, roughly the equivalent of 15 times the flow of the Amazon River.

Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany says that it’s "something that climate models have predicted for a long time, but we weren’t sure it was really happening. I think it is happening. And I think it’s bad news."

The prime suspect for this slowdown is an influx of fresh water coming from melting ice sheets in the Arctic and Greenland’s glaciers. While this water is still comparatively cold, it has a much lower salinity than the water flowing from the south, and this freshness means that it is also less dense than normal seawater. That lower density makes it more buoyant, meaning it doesn’t sink as well, preventing the water from joining the lower southbound currents, and disrupting the AMOC.

This disruption comes with a number of detrimental consequences: as the AMOC is a key link in distributing heat energy around the planet, a disrupted flow prevents warmer water from reaching northern Europe, affecting weather patterns there. Warm water also pools up along North America’s Eastern Seaboard, adding energy to seasonal storms, and exacerbating sea level rise as the warmer water expands.

The study of longer-term cycles has found that the AMOC had already been experiencing a natural slowdown that started about 160 to 170 years ago, roughly when the "Little Ice Age"–a 200-year period of modest cooling in the Northern Hemisphere–ended.

"In terms of this initial drop in the AMOC, it’s very likely that’s a kind of natural process," explains University of Reading researcher Jon Robson. The onset of this long-term slowing trend happened too early to have been caused by human activity, but anthropogenic-caused global warming correlates much more closely to the 15 percent slowdown seen in the AMOC over the past few decades. "It’s very likely, based on other evidence, that human activities may have continued to suppress the AMOC, or maybe led to further weakening."

Rahmstorf predicts that the AMOC will continue to slow as climate change advances, However, he’s concerned that it might not always be a gradual process, but that the climate might reach a tipping point where the AMOC grinds to an abrupt halt, an occurrence that has happened numerous times in Earth’s past, including as recently as the end of the last Ice Age 13,000 years ago.

"I think in the long run … Greenland will start melting even faster, so I think the long-term prospect for that ocean circulation system is that it will weaken further," Rahmstorf warns. "And I think that’s going to affect all of us, basically, in a negative way."

Studies like this continue to validate the science behind Whitley Strieber, and Art Bell’s 1999 classic The Coming Global Superstorm, a prescient warning of the dangers of abrupt climate change published two decades ago.