A4 Collapse or Weakening of Circulation

Topic

Within the climate system as a whole, the Atlantic Meridional Overturning Circulation is one of the tipping points that most concerns researchers. In its current state, this circulation transports approximately eighteen Sverdrups of warm surface water northward and returns an almost identical amount—also around eighteen Sverdrups—of cold, dense water southward at depth. This northward heat transport, amounting to roughly 1.3 petawatts, is what allows Western Europe to enjoy a much milder climate than its latitude alone would dictate.

The weakening of this circulation is no longer merely a theoretical possibility: a network of oceanographic buoys deployed along the 26th parallel north recorded a decrease of nearly three Sverdrups between 2004 and 2014. However, this decline occurs alongside significant year-to-year variability, and it is precisely this variability that makes confirming a long-term weakening trend a continuing challenge for researchers.

The mechanism explaining why this circulation could potentially collapse is the Stommel salt-advection feedback, which operates as a self-reinforcing loop. The Atlantic Meridional Overturning Circulation transports salt into the North Atlantic, where evaporation further concentrates it; however, if an influx of fresh water—such as that resulting from Greenland's melting ice—reduces surface salinity in the North Atlantic, water density drops and the sinking of water diminishes. It is this reduction in sinking that causes the Atlantic Meridional Overturning Circulation itself to weaken. Once weakened, that same circulation transports less salt northward, further reducing North Atlantic salinity; this closes the loop and further weakens the Atlantic Meridional Overturning Circulation—the very system where the process began. This positive feedback loop could lead to an irreversible collapse of the Atlantic Meridional Overturning Circulation if the freshwater input exceeds a critical threshold.

Should the Atlantic Meridional Overturning Circulation collapse, the consequences would extend far beyond the North Atlantic itself: Western Europe would cool by five to ten degrees, the Intertropical Convergence Zone would shift southward, African and Asian monsoon patterns would change, and sea-level rise would accelerate along the western Atlantic coast, as the absence of this circulation causes water to pile up there rather than being redistributed to other ocean regions.

The most unsettling aspect of this instability is that its behavior is non-linear rather than gradual: the Atlantic Meridional Overturning Circulation could be nearing a tipping point without showing any obvious signs, meaning a collapse could occur without warning until it becomes imminent. Thus, from the heat transport that currently moderates Europe's climate and the salinity feedback that can weaken it, to the consequences such weakening would have on entire monsoon systems and sea levels, the Atlantic Meridional Overturning Circulation encapsulates—within a single mechanism—one of the most abrupt and unheralded changes the climate system could yet undergo.

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