Last week, Nature published a climate science study that reached a very surprising conclusion—one that other climate scientists are taking issue with. Two other scientists penned a critical response and posted it at Real Climate the same day, outlining their issues with the study's findings.

This kind of argument could be left to play out among scientists, but the BBC News covered the study without skeptical counterweight, so we thought it would be worth explaining what the arguments are about.

AMOC run amok

The study by Xianyao Chen of the Ocean University of China and Ka-Kit Tung at the University of Washington focused on the large-scale movement of water in the Atlantic Ocean. In this section of the ocean “conveyor belt” that wraps around the world, surface water is carried toward the pole before mixing downward around Greenland and heading south along the ocean floor.

That downward mixing can get jammed up, interfering with the northward movement of warmer water that plays an important role in the regional climate. (You may remember this mechanism being badly exaggerated in the disaster film The Day After Tomorrow.) Scientists have been keeping a close eye on this current system, called the “Atlantic Meridional Overturning Circulation” or AMOC.

The AMOC is generally projected to slow down as the world continues to warm and melting glacial ice from Greenland gums up the conveyor belt. Measurements have shown that it may already have slowed slightly due to global warming. But it's thought that a stoppage of the current is unlikely.

In the new study, Chen and Tung argue that The Day After Tomorrow had it precisely backward. While stalling of the current in the past has caused cold conditions around the North Atlantic, the researchers say it would be different in a warming climate. Because the Atlantic conveyor belt helps transport a large amount of heat energy trapped by our greenhouse gases into the deep ocean, slowing the conveyor belt should cause more heat energy to build up near the surface—raising global temperatures.

Most of Chen and Tung's evidence for this comes from laying Atlantic circulation data next to global temperature data and noting that it looks like temperatures rose more quickly during periods when circulation slowed. They conclude that records show little sign of human influence on the circulation, so we must be in another period of naturally low circulation that could continue for another twenty years or so—bringing faster global warming than we saw in the last couple of decades.

Feel the flow, it's circular

Ka-Kit Tung was also a co-author of a 2013 study (which we covered) that attributed most of the wiggles in global temperature to Atlantic circulation. That left human-caused global warming nearly linear going back at least 70 years, despite an increasing rate of greenhouse gas emissions.

That study attracted a lot of criticism for the way it calculated the Atlantic’s influence, somewhat circularly assuming that most of the wiggles were due to the Atlantic—sidelining other well-studied factors like sunlight-reflecting aerosol pollution, eruptions, and Pacific circulation. That same calculation appears in this new study, underlying their comparison between the strength of the Atlantic circulation and changes in the rate of global warming.

In their post at Real Climate, Penn State’s Michael Mann and Potsdam University’s Stefan Rahmstorf—both of whom have studied Atlantic circulation—take issue with this and several other aspects of the new paper.

They say the idea that slowing circulation would accelerate warming rather than cool the North Atlantic runs counter to so many studies that it enters “extraordinary claims require extraordinary evidence” territory. But, they argue, there's a complete absence of key evidence: “Chen and Tung do not show any models simulations either to provide evidence that their mechanism can actually work, neither do they discuss the various published model results that have come to the opposite conclusion.”

So should you put much stock in the new study’s forecast for the next twenty years? Probably not, argue Mann and Rahmstorf.

“It is difficult not to think of the prediction by Keenlyside et al. in Nature in 2008,” they write. “These authors made headlines around the world by predicting a phase of global cooling, ironically also largely based on a prediction of weak AMOC […] Back then the Realclimate team had solid reasons to predict that the forecast would turn out to be wrong—which indeed it did. This time, we once again do not doubt that rapid global warming will continue until we strongly reduce greenhouse gas emissions—but for reasons that have nothing to do with the AMOC.”

Sometimes a study that gets through to publication makes a bunch of researchers in that field quite grumpy—not every paper inspires a go-get-the-good-champagne toast. It's confusing for the public when these studies hit the news without clarifying context, but "grumpy" is generally a productive mode for scientists. Substantive argument forces existing ideas to be laid out clearly, which sometimes highlights a point that could be solidified by the right test.

When only one side of these debates makes the press, though, it can be hard for the public to even know there's an argument going on.

Nature, 2018. DOI: 10.1038/s41586-018-0320-y (About DOIs).