Atlantic Circulation Collapse: New Clues on the Fate of a Crucial Conveyor Belt
Among the many reasons why we ought to cut climate-warming greenhouse gas emissions as quickly and sharply as possible, the weakening of a system of
ocean currents known as the Atlantic Meridional Overturning Circulation, or AMOC, ranks high indeed. Several key scientific papers over the last couple of years have put this long-percolating climate concern back on the front burner. It’s hard to overstate how widespread and calamitous the impacts could be if this conveyor belt were to collapse – and it’s a process that could begin in the next several decades, if new work is on target.
The AMOC is a vast oceanic loop that carries warm water northward through the uppermost Atlantic toward Iceland and Greenland, where it cools and descends before returning southward. It ferries colossal amounts of heat from the tropics toward the polar region, helping balance Earth’s climate machine just as it’s done on and off for millions of years.
‘On’ and ‘Off’: The AMOC’s Speed Settings
At its heart, the AMOC is a heat redistribution device. It takes some of the excess warmth that builds up at Earth’s sun-drenched tropical latitudes and spreads it poleward across the North Atlantic. It’s part of a global conveyor belt that extends throughout the world’s interlinked oceans. This is sometimes called the global thermohaline circulation because of the variations in temperature (“thermo-”) and salinity (“-haline”) that drive the system.
As water flows north across the Atlantic, evaporation from the surface makes it progressively saltier. The current then chills as it flows from the tropics toward the far North Atlantic off Greenland. Since colder, saltier water is denser than warmer, fresher water, the flow sinks and returns southward at depth.
For North America and Europe, the Gulf Stream is by far the most crucial part of the AMOC. Not only does the Gulf Stream’s warmth keep northern Europe far milder than other locations at such high latitudes, but the flow itself serves as a type of protective seawall for North America’s Atlantic coast. Because of the way ocean height and ocean currents interrelate, sea level can be several feet lower toward the U.S. East Coast, on one side of the Gulf Stream, than it is toward the central North Atlantic, on the other side.
Like most weather and climate features, AMOC varies on timescales ranging from the brief (hours and days) to the geologic (thousands of years). But a simple climate model developed more than 60 years ago was the first to show that AMOC also has two favored long-term speed settings: essentially, “on” and “off” (or “strong” and “weak” modes).
An AMOC collapse is a shift from the “on” to the “off” mode.
What Makes the AMOC Fizzle?
Meltwater poring from the Arctic into the far North Atlantic in massive amounts seems to be capable of triggering AMOC collapse. It does so by overtopping and displacing saltier water from the AMOC with fresher meltwater that floats atop the ocean rather than joining the AMOC’s descending branch. The result: stopping up the AMOC’s south-to-north flow, just as a stalled car can lead to a miles-long traffic jam upstream.
This basic idea – that the AMOC can hum along for many thousands of years, then collapse and stay weak for hundreds of years before a gradual recovery – has proven durable.
It shows
