AMOC Time?: Study Suggests Ocean Shift Could be Close

September 11, 2021

AMOC is the Atlantic Meriodonal Overturning Circulation – a key driver of global oceanic currents that is known to have flipped direction suddenly a number of times in the not so distant past – leading to global changes that would have powerful consequences should they happen in today’s world.

The idea was the inspiration for the 2004 movie “The Day After Tomorrow” – a very much over-the-top and science fiction version of global catastrophe. But real consequences could impact food and agriculture in some of the world’s most populace economies – including the US and Europe.
Above, I discussed the ramifications with J. P. Steffens, a key expert in ice core historical records in Greenland and Antarctica, who has mapped similar changes in the past.

Washington Post:

Human-caused warming has led to an “almost complete loss of stability” in the system that drives Atlantic Ocean currents, a new study has found — raising the worrying prospect that this critical aquatic “conveyor belt” could be close to collapse.

In recent years, scientists have warned about a weakening of the Atlantic Meridional Overturning Circulation (AMOC), which transports warm, salty water from the tropics to northern Europe and then sends colder water back south along the ocean floor. Researchers who study ancient climate change have also uncovered evidence that the AMOC can turn off abruptly, causing wild temperature swings and other dramatic shifts in global weather systems.

Scientists haven’t directly observed the AMOC slowing down. But the new analysis, published Thursday in the journal Nature Climate Change, draws on more than a century of ocean temperature and salinity data to show significant changes in eight indirect measures of the circulation’s strength.

These indicators suggest that the AMOC is running out of steam, making it more susceptible to disruptions that might knock it out of equilibrium, said study author Niklas Boers, a researcher at the Potsdam Institute for Climate Impact Research in Germany.

If the circulation shuts down, it could bring extreme cold to Europe and parts of North America, raise sea levels along the U.S. East Coast and disrupt seasonal monsoons that provide water to much of the world.

“This is an increase in understanding … of how close to a tipping point the AMOC might already be,” said Levke Caesar, a climate physicist at Maynooth University who was not involved in the study.

Boers’s analysis doesn’t suggest exactly when the switch might happen. But “the mere possibility that the AMOC tipping point is close should be motivation enough for us to take countermeasures,” Caesar said. “The consequences of a collapse would likely be far-reaching.”

The AMOC is the product of a gigantic, ocean-wide balancing act. It starts in the tropics, where high temperatures not only warm up the seawater but also increase its proportion of salt by boosting evaporation. This warm, salty water flows northeast from the U.S. coastline toward Europe — creating the current we know as the Gulf Stream.

But as the current gains latitude it cools, adding density to waters already laden with salt. By the time it hits Greenland, it is dense enough to sink deep beneath the surface. It pushes other submerged water south toward Antarctica, where it mixes with other ocean currents as part of a global system known as the “thermohaline circulation.”

This circulation is at the heart of Earth’s climate system, playing a critical role in redistributing heat and regulating weather patterns around the world.

As long as the necessary temperature and salinity gradients exist, AMOC is self-sustaining, Boers explained. The predictable physics that make dense water sink and lighter water “upwell” keep the circulation churning in an endless loop.

But climate change has shifted the balance. Higher temperatures make ocean waters warmer and lighter. An influx of freshwater from melting ice sheets and glaciers dilutes North Atlantic’s saltiness, reducing its density. If these waters aren’t heavy enough to sink, the entire AMOC will shut down.

It’s happened before. Studies suggest that toward the end of the last ice age, a massive glacial lake burst through a declining North American ice sheet. The flood of freshwater spilled into the Atlantic, halting the AMOC and plunging much of the Northern Hemisphere — especially Europe — into deep cold. Gas bubbles trapped in polar ice indicate the cold spell lasted 1,000 years. Analyses of plant fossils and ancient artifacts suggest that the climate shift transformed ecosystems and threw human societies into upheaval.

“The phenomenon is intrinsically bi-stable,” Woods Hole Oceanographic Institution President Peter de Menocal said of the AMOC. “It’s either on or it’s off.”

But is it about to turn off now?

“That’s the core question we’re all concerned about,” said de Menocal, who was not involved in Boers’s research.

In its 2019 “Special Report on the Ocean and Cryosphere in a Changing Climate,” the U.N. Intergovernmental Panel on Climate Change projected that the AMOC would weaken during this century, but total collapse within the next 300 years was only likely under the worst-case warming scenarios.

The new analysis suggests “the critical threshold is most likely much closer than we would have expected,” Boers said.

The “restoring forces,” or feedback loops, that keep the AMOC churning are in decline, he said. All the indicators analyzed in his study — including sea surface temperature and salt concentrations — have become increasingly variable.

It’s as though the AMOC is a patient newly arrived in the emergency room, and Boers has provided scientists with an assessment of its vital signs, de Menocal said. “All the signs are consistent with the patient having a real mortal problem.”

Below, I discussed the issue with Michael Mann, who had just published a study looking at decreased flow in the North Atlantic.

5 Responses to “AMOC Time?: Study Suggests Ocean Shift Could be Close”

  1. redskylite Says:

    Excellent video – an expert making the complex understandable to the layman, and shaking our perception of time and complacency. I fear for a time when food is in extreme short supply due to ocean circulation impacts on agriculture, with associated loss of law and order. Will we look to our armies to control the mayhem, will they be up to the task ?


    Troops stationed in Mali as part of a United Nations’ peacekeeping force have been unable to use their communication devices until the evening, when the temperature cools off.

    • redskylite Says:

      Funded by the National Environmental Research Council (NERC) and the National Science Foundation (NSF), the Rapid Climate Change-Meridional Overturning Circulation and Heatflux Array started in 2001 and was fully deployed in 2004 provides continuous observations of the gulf stream which is measured by an array of instruments along 26°N.

      No dramatic forecasts yet as a relatively short time on time series data, but should be well-placed to make forecasts in the near future, and act as an early warning provider from observational data.


      “In the light of AMOC estimates from this work, the RAPID time-seres brought a number of surprises:”

      4) Gradual AMOC decline: Over decade of 26.5°N observations, the AMOC has been declining at a rate of about 0.5 Sv per year, 10 times as fast as predicted by climatemodels. It is too early to say whether this is an on-going trend caused by global warming, or whether it is part of the decadal variability known as the Atlantic Multidecadal Oscillation. ”

      I understand from a recent paper the “Atlantic Multidecadal Oscillation”
      is in doubt as a regular oscillation now.

      “After 20 Years, Researcher Now Says the Atlantic Multidecadal Oscillation Doesn’t Really Exist”

  2. rhymeswithgoalie Says:

    We’re dancing around so many tipping points (AMOC, permafrost melt acceleration, collapse of ocean food chains) people have no clue how quickly things can change. Still, we have to rah-rah the effort because of how much worse it can get.

    And, for the record:

  3. Anthony William O'brien Says:

    We knew the Deepwater formation had slowed down, but are surprised when the AMOC is affected. Turn off the engine and be surprised when the car slows.

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