Scientists are worried the Atlantic Meridional Overturning Circulation (AMOC), a “critical aquatic conveyer belt” that drives currents in the Atlantic Ocean, is at risk of near-complete collapse.
A shutdown of the crucial circulation system 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,” the Post reports. The effects, in short, would be devastating.
“The mere possibility that the AMOC tipping point is close should be enough for us to take countermeasures,” warns Levke Caesar, a climate physicist at Maynooth University.
Scientists previously believed the AMOC would in fact weaken this century, but didn’t imagine total collapse within the next 300 years except in absolute worst-case warming scenarios.
Now, according to a new Nature study, that critical threshold “is most likely much closer than we would have expected,” said Niklas Boers, the study’s author and a researcher at the Potsdam Institute for Climate Impact Research. Any exact date, however, is still unknown.
It would take years of monitoring and data collection to officially confirm the AMOC slowdown, but there is a degree of “jeopardy” associated with waiting for that proof, scientists say. Besides, possible consequences, like a “cold blob” in the ocean south of Greenland, are already being felt.
Frighteningly, if the system does devastatingly shut down, the switch off would be irreversible in human lifetimes.
“It’s one of those events that should not happen,” said Boers. “This is a system we don’t want to mess with.”
What is the AMOC?
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.
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.
After all, there are plenty of other indications that Earth’s climate is in unprecedented territory. This summer, the Pacific Northwest was blasted by a heat wave scientists say was “virtually impossible” without human-caused warming. China, Germany, Belgium, Uganda and India have all experienced massive, deadly floods. Wildfires are raging from California to Turkey to the frozen forests of Siberia.
And the apparent consequences of the AMOC slowing are already being felt. A persistent “cold blob” in the ocean south of Greenland is thought to result from less warm water reaching that region. The lagging Gulf Stream has caused exceptionally high sea level rise along the U.S. East Coast. Key fisheries have been upended by the rapid temperature swings, and beloved species are struggling to cope with the changes.
If the AMOC does completely shut down, the change would be irreversible in human lifetimes, Boers said. The “bi-stable” nature of the phenomenon means it will find new equilibrium in its “off” state. Turning it back on would require a shift in the climate far greater than the changes that triggered the shutdown.
“It’s one of those events that should not happen, and we should try all that we can to reduce greenhouse gas emissions as quickly as possible,” Boers said. “This is a system we don’t want to mess with.” [WP]
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