Climate security

North Atlantic circulation slows down

The warm, salty waters of the Gulf Stream make a northeasterly meander across the Atlantic Ocean, eventually forming the North Atlantic Current, which then funnels into the Nordic Seas. In the chill of winter, these waters cool and descend with the heavy load of their salinity. This deep convection is a key part of the Atlantic meridional overturning circulation (AMOC; Fig. 1), which can be thought of as an ocean conveyor belt that releases heat to the atmosphere above the North Atlantic Ocean before travelling through the abyssal ocean to resurface in other areas of the world.

Given the importance of the AMOC to heat exchange between the ocean and the atmosphere, the varying strength of this system is thought to have major impacts on the global climate, and has been implicated widely in some of the most remarkable and abrupt climate changes of the past2. Direct measurements of the modern AMOC flow rates show a decline in its strength in the past decade3. Reconstructions of the natural variability and long-term trends of the AMOC are needed, however, to put these recent changes in context. Two papers in Nature, by Caesar et al.4 and Thornalley et al.5, report on past AMOC variability using different approaches. Both conclude that the modern AMOC is in an unusually subdued state, but they diverge in the details of how and when the AMOC’s decline commenced.