A bright orange float bobs up from 2,000 meters below the Atlantic surface at one point, somewhere off the coast of the Canary Islands. It is insignificant in comparison to the vastness of the sea. However, the scientists on board the RRS Discovery keep an eye out for it with the level of focus typically associated with something much more dramatic. Two years’ worth of ocean measurements are carried by that float. Every data point feels important right now.
For more than 20 years, the RAPID array, a string of ten instrument-filled moorings positioned across the Atlantic at a latitude of 26 degrees north, has been silently gathering readings. Its goal is to monitor the Atlantic Meridional Overturning Circulation, or AMOC, an ocean current system that works somewhat like a massive, slow pump. Warm water flows to the north. It cools. It sinks. It eventually turns around after flowing south along the ocean floor. One of this system’s most well-known features is the Gulf Stream.
The data from those moorings is beginning to show that the deep ocean’s behavior has changed. The temperature has changed. Behavior has changed in ways that were mostly theoretical until recently. At the scale of decades, it’s still unclear exactly what this means. However, scientists are not taking it lightly.
These worries are given more historical weight by recent research that was published in Nature Communications. Researchers from UCL examined microscopic fossil shells that were preserved in seafloor mud off the east coast of Canada. These organisms are known as foraminifera, and during their lives, they used their shells as a biological ledger to record the salinity and temperature of the surrounding seawater. The team discovered something startling. The Gulf Stream moved hundreds of miles northward and approached the Nova Scotian coast approximately 12,900 years ago during the Younger Dryas, an abrupt cold period. There, the temperature of the waters increased by about four to five degrees Celsius. Marine ecosystems were momentarily disrupted.
The Younger Dryas itself was a terrible period; over the course of less than a century, global mean temperatures fell precipitously, with annual averages in some regions of the Northern Hemisphere falling by as much as ten degrees Celsius. It didn’t happen gradually. The system-wide adjustment was jarring. Furthermore, it seems that the Gulf Stream’s northward shift was a feature of the sequence rather than an afterthought.
The fact that the shift had been anticipated for years by climate models lends significance to this particular finding. Until now, no one had observed direct geological evidence of it. One of those moments that tends to calm a room is witnessing science validate a model prediction using sediment cores extracted from the ocean floor. The components of the Atlantic circulation system are interconnected and subject to sudden changes, according to lead researcher Fangjingcheng Zhu. That observation is no longer theoretical.
The current concern is whether melting ice, especially from Greenland, and rising ocean temperatures could start to lower the density of North Atlantic waters, weakening the sinking that powers the entire system. RAPID has been keeping a close eye on that mechanism. Furthermore, the more cautious framing—that the AMOC could weaken significantly without collapsing—is a significant statement in and of itself, even though the majority of researchers do not anticipate a dramatic collapse. Cooler temperatures throughout northwest Europe, higher seas along the east coast of the United States, and altered rainfall patterns in the tropics are all likely consequences of a weaker AMOC.
The long-held belief that warm, fresh water causes less sinking and slower AMOC isn’t holding up as neatly as anticipated, according to Susan Lozier of the Georgia Institute of Technology. It turns out that models are still learning to take into account the buffers and feedbacks found in the ocean. That serves as a reminder that the system is more intricate than any clear narrative would imply, rather than being comforting.
Observing all of this, it seems as though oceanographers are in the uncomfortable position of being halfway between early warning and true uncertainty.
