He argues that field observations of the Atlantic Ocean suggest that AMOC is in fact unstable. Between mid-2009 and mid-2010, AMOC appeared to weaken, with the current carrying only two-thirds of its usual volume of water. At the same time, sea-level rise on the East Coast accelerated and Europe experienced an unusually frigid winter.

In their study, Liu and his colleagues tried to make their model more unstable. Most models, they say, do a bad job of representing AMOC. They don’t have enough salty water entering the Atlantic at the equator, and they also don’t have enough freshwater leaving it in the deep ocean.

In their experiment, they fixed this extremely crudely. Instead of fixing the underlying physics, they told the model to add much more saltwater and freshwater to the simulation. Then they doubled the amount of carbon dioxide in the simulated atmosphere, stepped back, and watched to see what would happen.

What happened is that, between year 200 and 300 of their adjusted model, AMOC rapidly collapsed.

Delworth said that even though their experiment was crude, it was revealing. “It’s a very interesting and provocative work,” he told me. “I think they are opening up this topic and saying our models may be too stable.”

“In this new study, they’ve just put a band-aid on [this stability]. They’ve said, if we alter these characteristics, the model is much less stable. But sometimes it’s really good to have these simple ad hoc techniques to address, ‘What’s the sensitivity of our models?’” he said.

The paper alone didn’t overthrow the consensus, he added, but it did suggest it should be re-examined.

Hansen, on the other hand, was more dismissive of the study’s approach. “You can’t fix the climate model simulation via ‘bias removal’—you should fix what is wrong with the model physics,” he said in an email. “They are doubling CO2, letting that change the temperature, rainfall, etc. and seeing what that does to the AMOC in their model. It’s been more than 35 million years since we had that much CO2 in the air, and sea level was more than 200 feet higher then. If we (humanity) are so stupid as to double CO2, you can count on the AMOC to shut down much faster than 300 years.”

Other climatologists, especially those who study Earth’s past, were much more positive about the paper, describing it as a necessary improvement to how we understand current climate models.

“This is an important step forward,” said Jean Lynch-Stieglitz, a professor of Earth and atmospheric sciences at Georgia Tech. “This study identifies a specific property of the climate models that would tend to make the AMOC in the models more stable than in reality.”

“Importantly, it reminds us that even if most climate model projections agree on their projections for future AMOC changes, that doesn’t necessarily mean that the projections are correct,” she added.

The instability of AMOC is one of the great open questions remaining in our understanding of climate change, one of the ongoing explorations into global warming’s “degree and extent.”