Why Scientists Are Clashing Over the Atlantic’s Critical Currents

So much on this planet depends on a simple matter of density. In the Atlantic Ocean, a conveyor belt of warm water heads north from the tropics, reaching the Arctic and chilling. That makes it denser, so it sinks and heads back south, finishing the loop. This system of currents, known as the Atlantic Meridional Overturning Circulation, or AMOC, moves 15 million cubic meters of water per second.

In recent years, researchers have suggested that because of climate change, the AMOC current system could be slowing down and may eventually collapse. A paper published yesterday in the journal Nature Communications warns that the collapse of the AMOC isn’t just possible, but imminent. By this team’s calculations, the circulation could shut down as early as 2025, and no later than 2095. 

That’s a tipping point that would come much sooner than anyone thought. “We got scared by our own results,” says Susanne Ditlevsen, a statistician at the University of Copenhagen and coauthor of the new paper. “We checked and checked and checked and checked, and I do believe that they're right. Of course, we might be wrong, and I hope we are.” But there’s vigorous debate in the scientific community over just how quickly the AMOC might decline, and how best to even figure that out.

It’s abundantly clear to researchers that the Arctic is warming up to four and a half times faster than the rest of the planet. Arctic ice is melting at a pace of about 150 billion metric tons per year, says Marlos Goes, an oceanographer from the University of Miami and NOAA's Atlantic Oceanographic and Meteorological Laboratory who was not involved with the new paper. Greenland’s ice sheet is also rapidly declining, injecting more freshwater into the sea. That deluge of freshwater is less dense than saltwater, meaning less water sinks and less power goes into the AMOC conveyor belt. 

The consequences would be brutal and global. Without these warm waters, weather in Europe would get significantly colder—more like that of similar latitudes in Canada and the northern United States. “In model simulations, the collapse of the AMOC cools the North Atlantic and warms the South Atlantic, which may result in drastic precipitation changes throughout the world,” Goes says. “There would be changes in storm patterns over the continental areas, affecting the monsoon systems. Therefore, a future AMOC shutdown could bring massive migration, impacting ecological and agricultural production, and fish population displacement.” 

Ditlevsen did her team’s calculation by using measurements of Atlantic sea surface temperatures as a proxy for the AMOC. These readings go all the way back to the 1870s, thanks to measurements taken by ship crews. This meant researchers could compare temperatures before and after the start of the wide-scale burning of fossil fuels and the ensuing changes to the climate. 

Because the AMOC system involves warm water heading north from the tropics, if the circulation is slowing down, you’d expect to find cooler temperatures in the North Atlantic over time. And indeed, that’s what Ditlevsen’s group found, once they compensated for the overall warming of the world’s oceans due to climate change. “When it is established that the sea surface temperature record is the fingerprint of the AMOC, we can calculate the early warning signals of the forthcoming collapse and extrapolate to the tipping point,” says University of Copenhagen climate scientist Peter Ditlevsen, coauthor of the new paper. (The Ditlevsens are siblings.)

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