The Atlantic meridional overturning circulation (AMOC for short) is a system of ocean currents in the North Atlantic that act as a conveyor belt to move water and air, thus creating weather systems and redistributing heat on our planet. Warm, salty water flows north in the upper layers of the Atlantic ocean, while colder water flows south in the deeper reaches. It transports heat from the Southern Hemisphere and Tropics to the north.
Changes in AMOC’s circulation impact the global climate system, from monsoon rainfall in Africa and India to hurricanes in North America and Western Europe.
According to a study published in the journal Nature, AMOC has slowed by about 15 percent since the mid-20th century – the slowest in 1,500 years – cooling the subpolar Atlantic ocean and warming the Gulf Stream region.
Researchers took measurements from a 5.2-million-square-kilometer (2-million-square-mile) patch of cold water in the North Atlantic – known as the “cold blob” – to measure circulation patterns and seasonal temperatures on the ocean’s surface. These changes are likely in response to increasing carbon dioxide in the atmosphere paired with warmer global temperatures observed in the last 150 years.
“Continued global warming is likely to further weaken the AMOC in the long-term, via changes to the hydrological cycle, sea-ice loss and accelerated melting of the Greenland Ice Sheet, causing further freshening of the northern Atlantic,” said the paper’s authors.
The slowdown is intensifying and, according to a second study published in Nature, it opens the doors for a potential complete shutdown of the ocean’s current that would be a “tipping point”. The authors warn such a collapse is at least a few decades away but would be catastrophic nonetheless.
You might be asking yourself: What does global warming have to do with all of this? Well, a few things.
First of all, warmer water makes it harder for cold, more dense water to sink and turn over. As ice sheets and glaciers in Greenland melt, freshwater pours into the ocean where this churning process would normally happen. Here the water gets less dense and less salty, decreasing its ability to sink. Increased precipitation in the Northern Hemisphere and more evaporation in the Southern further compounds this.
Secondly, the wind also impacts how current systems move and can change currents significantly when combined with freshwater. Reduced wind force over the ocean can cause “immediate weakening” of vertical salinity and changes the stratification of the ocean, while also triggering a slowdown of thermohaline circulation.
Some researchers say these projections aren’t sufficiently supported by enough data yet.