The AMOC: what is it, how is it changing and why does it matter?

What is the AMOC?

AMOC stands for ‘Atlantic Meridional Overturning Circulation’. This is a giant conveyor belt moving warm, salty surface water northwards in the Atlantic and returning it southwards at depth. The circulation is driven by water sinking in the North Atlantic as it gets cooler and denser, aided by surface winds. The conveyor belt helps to move heat northwards from the tropics and keeps western Europe much warmer than other locations at the same latitude, like Canada.

How might the AMOC change?

We know that the AMOC has varied significantly in the past, with evidence that it could have collapsed to a fraction of its current strength previously, including around 8,200 years ago. Under climate change, scientists expect the North Atlantic to become warmer and less salty due to an influx of meltwater from melted sea ice and the Greenland ice sheet. This is very likely to weaken the AMOC.  

Additionally, scientists believe that there may be a feedback mechanism leading to instability and stronger decline in the AMOC. The mechanism revolves around the AMOC also naturally transporting saltier water northwards, which sinks more easily as it cools, because the salt increases the density of the water. If the AMOC becomes weaker, the salt supply to the North Atlantic also weakens and so it gets harder for the water to sink. This makes the conveyor belt weaker, leading to a potential self-reinforcing ‘feedback loop’, where a weakening AMOC leads to less water sinking which weakens the AMOC further, causing a tipping point to occur, past which the AMOC may continue to weaken even without further global warming, reaching a new stable state with a much weaker circulation.

Computational climate models support this conclusion, with most models showing a decrease in the strength of the AMOC over the next century due to climate change, but the level of this decline varies greatly between models. Some models show the sinking of dense water stopping completely, leaving only a very weak AMOC pushed by the wind, bringing very little heat northwards. Others show the AMOC as more stable, with only small decreases of 10 or 20% and minor impacts on the climate. Importantly, all these models show AMOC changes occurring over the course of decades, and impacts will unfold over these timescales, with further impacts continuing beyond the end of the century.

The observational evidence for AMOC weakening in the last hundred years is debated. The AMOC has only been directly observed since 2004 and although the data show a small weakening, this is very difficult to separate from natural changes and studies debate whether it is statistically significant. Using indirect data to suggest how the AMOC may have changed on a longer timescale (such as through surface temperature changes), some studies suggest the AMOC may have weakened over the last few centuries, while others suggest it may not have done.

What could a large shift in the AMOC mean?

Since a large change in the AMOC has not been observed while humans have been directly measuring the climate, the impacts of a large shift are uncertain. As the AMOC moves large quantities of heat northwards, any decrease would be expected to lead to a cooling around the North Atlantic, and potentially a small amount of warming elsewhere. The cooling could reach 5°C in the UK and 2–3°C degrees over Western Europe more broadly, although estimates for this vary significantly and greater cooling cannot be ruled out.

A large cooling of the North Atlantic would also lead to reductions in rainfall over Europe, with significant consequences for agriculture, and stronger wind and storms. However, the impacts would likely extend beyond Europe, with a shift in the global distribution of heat moving the bands of rain around the equator, making the northern hemisphere drier and the southern hemisphere wetter on average. This could lead to reduced precipitation and disruption to the West African and Indian monsoons, causing potential damage to agriculture and food security. The wider impacts of a large shift in the AMOC on national economies is highly uncertain, although research on this topic is ongoing.

What could be done?

The primary driver of an AMOC change is global warming caused by humans emitting growing quantities of greenhouse gases. Every fraction of a degree of global warming increases the risks of these impacts, and some change is likely already locked in. Therefore, actions to reduce emissions, principally through a move away from burning fossil fuels, will be important to lessening the pace of global warming and its impact on the AMOC.

Intervention through methods such as solar geoengineering has also been suggested, where cooling of the ocean surface would potentially help water to sink and strengthen the AMOC. However, very few studies have been conducted on the overall impact of geoengineering on the AMOC and further knock-on effects from such methods are not well understood.

In some regions, the changes due to climate change may compound those from AMOC, with both causing increased drying in the Mediterranean, for example. In these areas, the emphasis on precautionary adaptation to these impacts is increased beyond that already present due to climate change. In some regions, a weaker AMOC may lead to cooling rather than the warming expected with global climate change, leading to large uncertainty in potential impacts. In these areas there may still be ‘no-regret’ actions, which are good preparation under all outcomes, such as investment in insulation for buildings and increasing the resilience of infrastructure.

Overall, the AMOC is likely to decline over the 21st century, adding uncertainty to climate change and potentially causing large impacts to some regions. These changes will take place over decades and, while not a cause for short-term panic, should be a strong motivation for faster reductions in greenhouse gas emissions and investment in robust climate change adaptation.

This Explainer was written by Jon Rosser and Ruth Chapman and edited by Georgina Kyriacou.