The Global Conveyor Belt
The thermohaline circulation is Earth's largest heat redistribution system. In the North Atlantic, cold winter winds chill surface waters while evaporation increases salinity. This cold, salty water becomes dense enough to sink to the abyss, driving a deep current that flows southward along the western Atlantic and eventually reaches the Southern Ocean. The return flow brings warm surface water northward, delivering roughly 1.3 petawatts of heat to high latitudes — keeping Europe 5-10°C warmer than equivalent latitudes in North America.
The Freshwater Kill Switch
The AMOC has an Achilles heel: freshwater. If enough freshwater floods the North Atlantic — from melting ice sheets, glacial lake outbursts, or increased rainfall — the surface water becomes too buoyant to sink. Deep-water formation shuts down, the conveyor stalls, and Europe plunges into bitter cold. This is not theoretical: it has happened repeatedly during the last ice age, most dramatically during the Younger Dryas 12,900 years ago.
Stommel's Bistable Ocean
In 1961, Henry Stommel showed that the thermohaline circulation has two stable states — a strong mode with active deep-water formation (like today) and a weak or off mode. The transition between them can be abrupt and hysteretic: once the AMOC collapses, removing the freshwater forcing is not sufficient to restart it. The system must cross back through a different, higher threshold, making recovery potentially take centuries to millennia.
Modern Monitoring
The RAPID array at 26.5°N has monitored AMOC strength since 2004, measuring approximately 17-18 Sv of overturning. Observations suggest a weakening trend, and paleoclimate evidence shows the AMOC was 30% weaker during the Little Ice Age. Climate projections indicate a 25-50% weakening by 2100 under high emissions, with a small but non-negligible probability of full collapse — an event that would radically alter weather patterns across the Northern Hemisphere.