The result echoes previous studies finding early warning signals in the circulation, says Stefan Rahmstorf, who studies the AMOC current system at the Potsdam Institute for Climate Impact Research. “As always in science, a single study provides limited evidence, but when multiple approaches lead to similar conclusions, this must be taken very seriously, especially when we’re talking about a risk that we really want to rule out with 99.9 percent certainty,” says Rahmstorf. “The scientific evidence now is that we can’t even rule out crossing a tipping point already in the next decade or two.”
Still, scientists don’t agree about whether sea surface temperature is a good indicator of the health of this massively consequential circulation. “Fundamentally, I am deeply skeptical that SST is actually a proxy of AMOC,” says climate scientist Hali Kilbourne, who studies the current system at the University of Maryland Center for Environmental Science. “But there’s certainly a school of thought of people who think it’s the best thing going—and it may be the best thing going right now. I don’t think we have a good alternative, which is why people are using it.”
“I really question whether [SST] is an adequate proxy for AMOC itself,” agrees Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research. “But the trouble is there aren’t really adequate measurements.”
The core of the issue is that sea surface temperatures are just one component of the AMOC system; other factors also help determine Atlantic temperatures. Warm waters flowing north have an effect, but so does the atmosphere touching the water. “There’s a lot of what we call air-sea interactions—the heat exchange between the atmosphere and the ocean,” Kilbourne says. “And that’s not at all related to ocean circulation.”
“This SST fingerprint, although sensitive to the AMOC, is not solely driven by it, so these changes may be overestimated,” agrees Goes, the oceanographer from the University of Miami and NOAA. “Current climate models do not give a strong probability of the collapse of the AMOC this century.”
The beauty of the SST dataset is that it stretches back 150 years, so scientists can see longer-term trends in temperatures. However, those early shipboard measurements were made by people hauling buckets of water aboard and sticking a thermometer in—not exactly the precision that modern science demands. “It is not ideal, but it’s the best we can do,” says Peter Ditlevsen, “since we need measurements to go back to the pre-industrial era to assess the natural state of the AMOC, before it began slowing down toward the collapse.”
Satellite measurements of SST began in the late 1970s, providing much better coverage across oceans. And it wasn’t until 20 years ago that scientists deployed a dedicated AMOC sensor array, known as RAPID, which also measures current velocities and salinity—another factor that influences the density of water. By comparing this modern data to the historical SST data, Peter Ditlevsen says, they can compensate for the influence of the atmosphere on the sea surface, isolating the signal of the AMOC system.