Study: Key Atlantic Ocean circulation could shut down by mid-century

A key element driving the Atlantic Ocean may dramatically slow down — and then turn off completely — during the next few decades, a new study finds.

Why it matters: The circulation, known as the Atlantic Meridional Overturning Circulation (AMOC), includes the Gulf Stream.

• Part of a "global conveyor belt" that moves water around the world, the AMOC helps to regulate everything from the rate of sea level rise on the East Coast to Europe's average temperatures.

• A sudden shutdown of this ocean current, which the new study shows could occur during midcentury, or between 2025 and 2095, was part of the plot of the 2004 disaster film “The Day After Tomorrow.”

Zoom in: The research, published Tuesday in the journal Nature Communications, uses statistical methods to tease out early warning signals of an AMOC collapse, including a slow-down in the current and increasing variability.

• The findings also suggest the shutdown may happen sooner than previously thought, and perhaps within many people's lifetimes.

Threat level: A breakdown would likely be potentially irreversible and would mark a major “tipping point" — critical thresholds in Earth’s climate system that, when crossed, will usher in severe changes.

• Climate models indicate that an abrupt stop could trigger extreme cooling across parts of the Northern Hemisphere. Rainfall patterns around the world would be disrupted and major floods and storms would increase, leading to further sea level rise in the North Atlantic basin among other effects.
• While the precise effects of a collapse aren’t fully understood, scientists believe it would have vast socioeconomic consequences for tens of millions of people, acutely impacting coastal cities, agriculture, fish populations and marine ecosystems.

Reality check: Detailed observations of this circulation pattern via modern instruments are relatively short, the study notes. The scientists extended records further back in time by using sea surface temperatures to shed light on trends.

• The authors also noted they couldn't rule out overlooked or undiscovered mechanisms not reflected in their assessment.

How it works: The AMOC is a system of ocean currents that transport water and heat between the Southern Ocean and the far North Atlantic Ocean in a years' long cycle that influences weather patterns in both hemispheres.

• The currents are driven by differences in water density regulated by changes in temperature and salt concentration.

• As warm water flows north along the ocean's surface, it becomes denser as evaporation increases salinity levels and temperatures decrease, making it sink deeper.
• It then slowly spreads southwards before rising from the ocean’s depths in the Southern Ocean in a process known as “upwelling,” thereby completing the circuit.

The intrigue: Previous studies have shown that the already slow-moving series of ocean currents is likely further weakening due to human-caused climate change, which is accelerating ice melt in Greenland.

• This increases the volume of lighter freshwater entering the North Atlantic.

Yes, but: The study’s conclusions run contrary to the most recent report from the U.N. Intergovernmental Panel on Climate Change, which found an AMOC shutdown is “very unlikely” to occur this century.

What they’re saying: Michael Mann, a climate scientist at the University of Pennsylvania who has published studies on the slowing AMOC but was not involved in the new research, said he questions some of the methods the new study uses. Nevertheless, he finds its conclusions plausible.

• “I think the authors in this case are on to something real,” he told Axios via email. “We could be talking decades rather than a century.”