The deep, wind-driven cold of the open ocean in midwinter, when the water’s surface temperature is only a few degrees above freezing and the wind across it removes heat quickly enough to produce visible steam rising from wave crests, is what the North Atlantic experiences in January at about 60 degrees north latitude. This is not the static cold of high altitudes. One of the most significant physical processes in the global climate system takes place here: warm, salty water that has moved north from the tropics cools at the surface until it becomes dense enough to sink, sliding down through the water column and flowing southward at depth. This completes a loop that has been going on continuously for millennia and transports massive amounts of heat, salt, and carbon through the ocean.
Freshwater inflow from Greenland’s melting ice sheet is upsetting the chemistry of density that underpins this sinking activity, which is the northern limb of what oceanographers refer to as the Atlantic Meridional Overturning Circulation. Compared to salt water, fresh water is lighter. Large amounts of meltwater that enter the subpolar North Atlantic produce a surface layer that, even after cooling, is not dense enough to sink. The system no longer needs the warm, salty water coming from the tropics to finish the shift to deep-water creation. The circle becomes slower.
Numerous sources, including as proxy reconstructions of historical AMOC behavior that use seafloor sediment cores and other geological data to infer circulation strength over centuries, provide evidence for this slowdown. According to the most widely accepted analysis, which was published in Nature Climate Change by a team that included Potsdam Institute experts, AMOC is currently at its weakest point in more than a millennium. The directional result is consistent across several independent lines of evidence, including direct mooring-based observations from the RAPID array across the Atlantic at 26 degrees north, although this is unclear because the proxy reconstructions have uncertainty ranges.
AMOC is being jeopardized by salinity dynamics that simultaneously act at several scales. The freshwater cap on the North Atlantic surface locally inhibits the vertical mixing that transports nutrients from deep to the sunlit zone, which has an impact on circulation and the marine food web. From a distance, the weakening of AMOC decreases the amount of salt exported northward from the South Atlantic, which often preserves the salinity gradient that propels the circulation. Salinity accumulates in the South Atlantic in ways that cause instability and variability in a generally more predictable area of the system when that export declines. This is referred as in oceanography as a “salinity pile-up,” and it is a feedback that may increase rather than decrease AMOC instability.
The subject of the tipping point is the one that climate scientists debate the most in-depth and that is typically covered by the media with the least accuracy. AMOC is not a switch that quickly shuts off. It’s a circulation that can gradually deteriorate over decades, losing strength in ways that have quantifiable but gradually mounting effects before possibly reaching a point where the instability becomes self-reinforcing. With published estimates ranging from quite near-term to much more distant depending on the model, the emissions scenario, and the assumptions made about ice sheet dynamics, it is truly unclear where that threshold is and how close the current track leads the system to it. The threshold is sensitive to the rate of warming rather than merely the overall amount of warming, as recent modeling work has made clear. The critical point is lowered at a faster rate because the ocean has less time to adapt.
Significant AMOC deterioration, short of complete collapse, has different effects on different parts of the world. Ireland and Britain are currently much warmer than equivalent latitudes in Canada due to AMOC’s substantial warming effect relative to latitude, which moves heat northward. While greenhouse gas warming persists elsewhere, a weakening AMOC reduces that heat transmission, resulting in regional cooling in northwestern Europe.
AMOC weakening eliminates a physical influence that presently reduces sea levels along the Atlantic coast of North America, potentially speeding up the pace of sea level rise seen in cities from Boston to Miami. Changes in AMOC have been linked to rainfall patterns in tropical and subtropical regions; the intertropical convergence zone’s location and the behavior of the monsoon systems in West Africa and South Asia are sensitive to North Atlantic circulation in ways that could impact agricultural productivity in some of the world’s most densely populated areas.
