The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that carry warm water from the tropics northwards into the North Atlantic.
It acts like a conveyor belt, driven by differences in temperature and salt content – the water’s density. As warm water flows northwards it cools and some evaporation occurs, which increases the amount of salt. Low temperature and a high salt content 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. Eventually, it gets pulled back to the surface and warms in a process called “upwelling” and the circulation is complete.
This global process makes sure that the world’s oceans are continually being mixed, and that heat and energy are distributed around the earth. This, in turn, contributes to the climate we experience today.
Oceanographers have been continuously measuring the AMOC since 2004. These measurements have shown that the AMOC varies from year to year, and it is likely that these variations have an impact on the weather in western Europe. However it is too early to say for sure whether there are any long term trends.
Before 2004 the AMOC was only measured a few times, and to go back further into the past we need to look at indirect evidence (for example from sediments on the sea floor). The indirect evidence doesn’t always agree on the details, but it seems likely that there have been some large, rapid changes in the AMOC in the past (for example around the end of the last ice age).
Some scientists believe the changes to this ocean circulation poses a huge risk for Europe. The devastating droughts last summer were caused by the AMOC being at its lowest point in 1200 years, and now they say it is the weakest it has been in the last 1600 years. Watch the video which explains findings from recent research.
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However, according to the UK Met Office, climate models suggest that the AMOC will weaken over the 21st Century as greenhouse gases increase. This is because as the atmosphere warms, the surface ocean beneath it retains more of its heat. Meanwhile increases in rainfall and ice melt mean it gets fresher too. All these changes make the ocean water lighter and so reduce the sinking in the ‘conveyor belt’, leading to a weaker AMOC. So the AMOC is very likely to weaken, but it’s considered very unlikely that large, rapid changes in the AMOC, as seen in past times, will happen in the 21st Century.
Educators need to be careful not to exaggerate the impact of such processes as teachers should try to keep to the facts and data, as AMOC has only been measured regularly since 2004 – we simply do not know, nor can we yet predict with any accuracy, what the future will bring. But it is an important issue that we ought to teach about and its impact on the development of previous ice ages for example as polar ice melted.