We asked climate scientists at the National Centre for Atmospheric Science to explain the Atlantic Meridional Overturning Circulation (AMOC), and what it means for our planet, and how climate change may affect ocean currents and temperatures.
What is the AMOC and why is it important?
The AMOC is an important component of Earth’s climate, and plays a key role as an ocean conveyor belt.
The AMOC powers the poleward transport of heat and nutrients from the tropics to the Northern Hemisphere and across the equator – and draws down heat and carbon to the deep ocean.
Once in the Northern Hemisphere, the water brought up by AMOC from the tropics becomes cooler and then denser. The denser water sinks and then travels south, where it will eventually warm and resurface again.
This ocean conveyor belt enables heat to be distributed around our planet, and is important for regional and global climate, as well as changes in weather around the world.
The AMOC accounts for around 90% of the total northward ocean heat transport across the Atlantic Ocean.
How can AMOC change?
The AMOC is a large system and is known to vary on a range of time scales and due to a range of natural and human factors.
One of the main driving forces of AMOC is the marked temperature and density difference between the cold North Atlantic Ocean and the warm tropics.
Warming of North Atlantic waters, due to natural variability and human-caused climate change, has the potential to reduce the temperature difference with the tropics and the density of the North Atlantic. This temperature-driven weakening could lead to sudden disruption, and slowing down, of major ocean current systems.
Dr Dan Hodson, a climate scientist at the National Centre for Atmospheric Science and University of Reading, adds: “While the waters of the North Atlantic are warming, they are also mixing with melting ice and becoming less salty and so less dense. Lighter water means that the sinking of ocean water takes longer or happens less, and the AMOC slows down as a result.”
Since 1993 the melting of Greenland’s ice sheet, due to human-caused global warming, has added around 5000 km3 more freshwater into the subpolar North Atlantic Ocean. The increase in freshwater input has decreased the concentration of salt, and in turn, ocean water in this region is becoming less dense.
What are the impacts of a weaker AMOC?
A slower AMOC circulation – or a collapse in the currents – caused by warmer temperatures and lower salinity, could affect different places and processes around the world.
“The strength of the AMOC is understood to vary on different timescales due to a range of processes and factors, which means making predictions is very complex. There are large uncertainties involved in predicting if and when an abrupt weakening of AMOC could occur, “ explains climate scientist Professor Jon Robson at the National Centre for Atmospheric Science and University of Reading.
If the AMOC undergoes a significant and abrupt weakening, which there is significant evidence pointing towards, this would have severe consequences globally, and for the UK, for rainfall, winds and sea level.
Professor Jon Robson at the National Centre for Atmospheric Science and University of Reading
The AMOC not only pulls heat and nutrients northwards, but it also tugs the Intertropical Convergence Zone, also known as the ITCZ, in that direction. The ITCZ is a band of low pressure that encircles the Earth, and is linked to frequent thunderstorms and heavy rain in the tropics.
The AMOC currently contributes to milder climates experienced in Europe, and also brings colder water to the eastern coastline of North America.
A slowing down of AMOC would have a ripple effect across these regions.
For example, a slower weaker circulation would lead to shifts in rainfall patterns and increased likelihood of droughts and floods, with significant impacts on agriculture and food security.
A weakening AMOC is linked with less rainfall across Europe, Asia, Africa and central and northern parts of America, and more rainfall over Australia, South Africa, and the Amazon rainforest in South America.
A weakened AMOC could also result in rising sea levels at the USA’s eastern coast, which could escalate the likelihood of coastal flooding in places like Boston and New York, and lead to intensified impacts of hurricanes and related storm surges.
Other impacts of a weakened AMOC include pooling of heat in the South Atlantic Ocean, altered areas of sea ice at both poles, disruption to land and marine ecosystems and biodiversity, and socio-economic pressures related to climate resilience and responses to hazardous weather.
How has AMOC changed in the past?
The last time scientists can tell that AMOC stopped and restarted, via proxy evidence from ice cores, was during the Ice Ages. About 115,000 to 12,000 years ago, this was most likely due to ice covering the oceans.
The AMOC has only been regularly monitored since 2004. The lack of direct observations and measurement data leaves big questions to be answered by scientists, about the long term trends of AMOC and the reasons for its cycles of decline.
What does climate change mean for AMOC in future?
As global temperatures continue to rise, due to human-created greenhouse gas emissions, the AMOC could experience an abrupt weakening in the foreseeable future.
Computer models that focus on forecasting AMOC changes are currently unable to capture all of the important ocean and climate processes involved – and often exclude Greenland ice melt – but they do indicate that climate change poses a real threat to the Atlantic’s conveyor belt of heat and nutrients.
Professor Jon Robson warns of the risks ahead:
“It is clear that the warning lights are flashing on for the North Atlantic climate system and further greenhouse gas emissions are only going to increase the likelihood of abrupt changes in the North Atlantic”.
Due to the limited observations of the AMOC, and the available data that can be incorporated into computer models simulating the AMOC, scientists cannot be certain how fast AMOC is declining or when exactly it could weaken or even switch off.
But it is clear that climate change is impacting ocean circulation, due to factors such as ice melt, but to what extent needs further observations and investigation.
“The exact predictions that the AMOC will shut down entirely need to be taken with some scepticism,” Professor Robson states.
He goes on to describe what scientists need to do for monitoring climate change impacts on AMOC: “Given the slow progress in reducing greenhouse gas emissions, it’s crucial that we continue to bring a range of observations and models together in order to make more accurate predictions of the potential for rapid North Atlantic climate change.”
How is the Gulf stream related to the AMOC?
As waters in the AMOC flow northward in the North Atlantic they travel through the Gulf Stream – and so the Gulf Stream could be thought of as a component of the AMOC.
But, the amount of water transported by the Gulf Stream is larger than the AMOC, and the Gulf Stream itself is mainly driven by the impact of winds on the North Atlantic Ocean and the effect of the Earth’s rotation. The Gulf Stream will continue to flow as long as the winds keep blowing and the Earth keeps spinning.
A key difference between the Gulf Stream and the AMOC, is their respective roles in moving heat around the climate system.
The Gulf Stream is often thought to be a key contributor to warming western Europe, as it moves warm water from the Gulf of Mexico into the Atlantic Ocean.
Observations also show that it is the AMOC that dominates how much heat is transported to more northerly latitudes. Although the Gulf Stream will keep flowing even in the event of an AMOC collapse, we would still see substantial climate impacts both regionally and globally.