After decades of warnings, new data suggest the Atlantic’s vital circulation may withstand climate warming better than feared

See Shifting currents by Paul Voosen in Science. Excerpts:

"Climate models have long warned that global warming could weaken “deep-water formation”—the density-driven sinking that is the engine of the AMOC. The logic is straightforward: As Greenland’s ice sheets melt and sea ice formation declines, North Atlantic waters will freshen. Combined with warmer sea temperatures, the freshening makes surface waters more buoyant. The AMOC was thought to have shut down abruptly during past climate warmings, and a handful of researchers now argue such a tipping point could occur this century. A sputtering AMOC could trigger a sharp cooldown in northwestern Europe, rising seas along the U.S. east coast, and shifts in tropical rainfall."

"most climate researchers think the AMOC is more resilient than these worst case scenarios make it seem. Emerging evidence suggests the AMOC may not have actually collapsed in the warm climates following ice ages. More detailed climate models suggest it could weaken but not collapse in the current surge of warming. And studies of the AMOC’s present behavior do not yet show any clear signs of trouble. They’re also exposing new facets of the circulation that could buffer any eventual weakening."

"That stately flow actually swings wildly year to year, masking any long-term trend, the first RAPID measurements showed. Swings between apparent decline and recovery have since become a hallmark of AMOC monitoring, and a recurring source of alarm and reassessment."

"Gerard McCarthy remembers well the first time he saw an AMOC decline. It was 2011, and McCarthy, now a climate scientist at Maynooth University, had just joined the RAPID team. His first task was calculating AMOC’s strength. Beginning in 2009, it plunged. “Everyone was like, ‘The new guy made a mistake,’” he recalls. Others checked the numbers. The drop held. “We all realized that something dramatic had happened.”

What happened was not caused by climate change, but rather the weather. That winter, unusual swings in air pressure weakened the jet stream and shifted wind patterns, disrupting the AMOC’s flow. The decline likely contributed to a frigid European winter in 2009 and, by leaving more heat in tropical basins, also led to an active Atlantic hurricane season the following summer."

 "It seems the AMOC is not a single conveyor belt, but a belt of belts, each part operating semiautonomously."

"OSNAP has changed the picture in other ways, including by showing that overturning occurs not so much in the Labrador Sea, as models suggested, as it does farther north, in the Irminger and Iceland basins. Additional data suggest deep-water formation is migrating even farther north, into the Arctic Ocean, following the retreat of sea ice, Årthun says. “You’re expanding the reach of this cooling machine.” The northward migration could make the AMOC more resilient to warming"

"New climate model runs that capture more realistic melt from the Greenland Ice Sheet are less dire. In two preprints posted online in the past year—one led by Chuncheng Guo, a climate scientist at the Danish Meteorological Institute (DMI), the other led by Oliver Mehling, an ocean modeler at UU—researchers created multiple simulations where carbon emissions continued until 2250 and temperatures rose by up to 7°C. In both studies the AMOC weakened, losing about 40% of its strength. But it never collapsed. Both studies also suggest the weakening is reversible"

"that resilience persisted even in the face of catastrophic warming."

 "even if atmospheric carbon dioxide levels quadrupled, driving extreme warming, the AMOC would decline by 40% after 20 or so years—but once again, it would rebound."

"Evidence from past ice ages seemed to suggest the AMOC switched off entirely when massive pulses of freshwater from the melting of the North American ice sheet poured into the Atlantic. But new work, also presented at Ocean Sciences, suggests the AMOC may not have collapsed at all during these periods."