Part 3.4: Atlantic Meridional Overturning Circulation (AMOC)
The Atlantic Meridional Overturning Circulation (AMOC) is a current system in the Atlantic Ocean that brings warm water from the tropics to Europe. It is part of a broader network of global ocean circulation patterns that transports heat around the world.
The illustration above shows that the AMOC consists of multiple parts. The Gulf Stream and the North Atlantic Current (red line) transport warm salty water in the upper layers of the ocean from south to north. As the warm water flows north, it cools and some of it evaporates, making the water saltier. In the high latitudes of the North Atlantic, the low temperature and high salinity make the water denser, and this dense water sinks deep into the ocean. The cold, dense water slowly spreads southwards, several kilometres below the surface (blue line). Eventually, the water gets pulled back to the surface and warms through a process called “upwelling”. The Gulf Stream and the North Atlantic Current transport the warm water back into the North Atlantic, and the cycle is complete. This whole process ensures that the ocean is constantly mixed and that heat and energy are distributed around the Earth. This in turn contributes to the climate we experience today.
Why is the AMOC a tipping point?
Increased precipitation and melting of continental ice near the Greenland Ice Sheet is bringing new fresh water into the North Atlantic. The fresh water mixes with the ocean’s salt water, making it less salty and thus less dense.This lighter water is less able to sink even at cold temperatures. As the sinking of the dense, cold water in the North Atlantic is what drives the AMOC, the whole circulation slows down. Slower circulation can bring less salty water from the tropics to the North Atlantic. This means that the proportion of fresh water from rain and the Greenland ice sheet in the North Atlantic increases, and the water can sink even less. This whole process is a self-reinforcing system.
Research indicates that the AMOC already slowed down 15% since mid-20th century, which is consistent with climate model projections. The question is, at what point does the circulation completely stop?
Where is the tipping point of the AMOC?
There is still a lot of uncertainty about exactly where this tipping point is, and it is a topic of active research. A rough estimate is 3–4°C above pre-industrial levels, and climate models suggest that a complete AMOC shutdown is unlikely in the next 100 years.
What would happen if we crossed the tipping point?
Crossing the tipping point could lead to a shutdown of the AMOC. This means the AMOC could transition to a new stable shutdown state for thousands of years or eventually recover — both of which have been observed in different models. But if you’re only interested in the next few hundred years, it doesn’t make any difference, because if the AMOC shuts down, it will stay off for quite a long time.
Since the AMOC plays a critical role in transporting heat from the tropics to the north, a shutdown would lead to widespread cooling (up to 5°C) in the northern hemisphere, especially in western Europe and the east coast of North America. This would lead to reduced evaporation in the North Atlantic and a change in precipitation patterns. The knock-on effects would be significant. One study suggests that this would lead to a “widespread cessation of arable farming” on the island of Great Britain.
At the same time, the Southern Atlantic would heat up, which could trigger other tipping points like the dieback of the Amazon rainforest or the shrinking of the West Antarctic Ice Sheet (WAIS). It would also significantly alter nutrient conditions in the Atlantic Ocean.
These are just a few examples of what could happen if we cross the AMOC tipping point. Because Earth’s climate is a very complex system, this scenario is subject to very large uncertainties, so we cannot predict exactly what would happen. What we can say, however, is that crossing this tipping point would be pretty bad for us.