The Atlantic Meridional Overturning Circulation (AMOC) is a large system of currents in the Atlantic Ocean that is driven by differences in temperature and in salinity. Cold, salty water sinks in the North Atlantic and flows southward deep in the ocean towards the equator and Antartica, and warm surface water is pulled northward through the upper layers of the ocean from the Gulf of Mexico via the Gulf Stream to the North Atlantic to replace it.
The AMOC distributes both heat and nutrients throughout the Atlantic Ocean. It facilitates the exchange of nutrients between the deep ocean and surface waters, thereby supporting marine ecosystems and biodiversity. It also contributes to the global carbon cycle by transporting carbon dioxide from the atmosphere into the deep ocean. Moreover, it moderates extremes of temperature in the coastal areas of the northern Atlantic, thereby making them more habitable and facilitating agriculture there.
Unfortunately, the AMOC is being weakened by increased freshwater input from the melting polar ice sheets and warming ocean temperatures resulting from climate change. Reduced salinity makes water lighter and thus harder to sink. This could have potentially very serious effects, including much colder temperatures in the coastal countries in northwestern Europe and a consequent drop in agricultural output. It could also damage marine biodiversity and fisheries in the North Atlantic.