“I think it means break into the old pisco stash” Edgar Boyles
‘The consequences of a collapse would likely be far-reaching’
By Sarah Kaplan
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.