The Atlantic Meridional Overturning Circulation (AMOC) is one of our planet's key circulation systems and, historically, it has caused dramatic changes in climate. Now, it is experiencing an unprecedented slowdown and may be less stable than thought. The consequences can potentially be severe, and humanity does not have a silver bullet to reverse these changes.
What is the AMOC and why is it so important to the climate? The name itself is a good description of the main characteristics of this system of ocean circulations.
“It flows through the Atlantic in a meridional (north-south) direction and is defined by an overturning of water masses. That is, it carries warm, saline surface waters from the South towards the subpolar North Atlantic and returns cold, deep-water southwards,” says Levke Caesar, Post-Doctoral Researcher at Maynooth University.
“The amount of water turned over in this way is measured in Sverdrup (Sv), where 1 Sv equals 1 million cubic metres of water per second. The average strength of the AMOC (commonly also known as Gulf Stream System) is about 20 Sv, which equals about 100 times the amount carried by the world’s largest river, the Amazon.”
Essentially, the AMOC redistributes heat from the tropics and even the Southern Hemisphere by transporting it to the mid-latitude and subpolar North Atlantic where the heat is released to the atmosphere. The exchange of warm and cold water that happens because of the AMOC is responsible for up to 90% of the northward heat transport in the Atlantic and plays a role in the fact that the Northern Hemisphere is on average 1-2 degrees warmer than the Southern Hemisphere.
“As a result, it ameliorates the climate over Western Europe and the European sector of the Arctic by increasing temperatures. Associated with this are reduced storminess over the Atlantic and increased tropical rainfall in the Sahel,” explains Sybren Drijfhout, Professor in Physical Oceanography and Climate Physics at the University of Southampton.
“It also transports nutrients from the nutrient-rich Southern Ocean into the Atlantic, feeding marine ecosystems. It really is crucial for many marine ecosystems and fisheries,” he says.
The AMOC also facilitates the subpolar North Atlantic to be the area of largest anthropogenic carbon uptake in the world ocean, keeping atmospheric CO2 concentrations more moderate.
Global weather patterns are changing at a worrying pace and ocean currents such as the AMOC are no exception. A study from 2021 suggested that the currents are already at their slowest point of the last 1,600 years, whilst a different study from the same year found “an almost complete loss of stability over the last century” of AMOC and that it may be nearing a shutdown. The latter study suggests most of the weakening is the result of the burning of fossil fuels.
That said, Drijfhout warns against sounding the alarm too soon. Although a weakening of the AMOC has indeed been observed and the current is predicted to weaken further in the future, the observational records are too short to link the observed weakening to climate change.
The AMOC is a delicate and finely wired system. As Caesar explains, one of the drivers of the AMOC is the so-called deep-water formation: as the warm, saline waters of the AMOC flow north, they are cooled by the atmosphere and thus become denser and heavier. If the water at the surface is denser than the water in the ocean layers below, it sinks. Climate models predict that this process will slow down as a result of global warming leading to a significant weakening of the AMOC.
The researcher says there are multiple possible reasons for that: an increase of precipitation over the high latitudes of the North Atlantic as well as the melting of Arctic sea ice and the Greenland Ice Sheet adding freshwater to the northern Atlantic, lowering the salinity of the upper ocean. This, along with increased warming of the ocean surface, reduces the density of the upper water masses and, therefore, suppresses deep water formation.
While the full list of consequences of a slowdown still must be investigated, Caesar says it has already been linked to a sea-level rise on the east coast of the US, increased storminess in north-western Europe and an increased risk of drought in the Sahel region.
As for Antarctica, Drijfhout explains, poleward shifting winds appear to facilitate more upwelling of relatively warm water (partly originating from the North Atlantic and transported by the deep currents of the AMOC to the south) onto the Antarctic continental shelves, where they melt the floating ice tongues surrounding the Antarctic ice sheet. This will be the main cause of rising sea levels for many centuries to come, even if we manage to stop global warming today.
How much time do we have left then? There’s good news and bad. On the one hand, Caesar says that scientists don’t expect “a sudden stop in the sense that the current will slow down significantly overnight”. Indeed, the AMOC is believed to be “a tipping element in the climate system and can therefore reach a point where a further slowdown will be very hard or impossible to prevent but it will still take time to slow down,” she adds.
On the other hand, Caesar and most other scientists want it to be very clear: once the system slows down significantly there are no innovations or technologies that could “fix” it again.
“There is not much we can do to control climatic changes in the ocean as they respond slower to climate change in the atmosphere and continue to change after atmospheric changes become small. The only thing that could really help is reducing carbon emissions as fast as possible and limiting global warming,” says Drijfhout.