The warm gulf stream is a current that brings warm water from the tropics to the poles, where it cools, sinks and returns southwards. It weakens because 1) warming makes water less dense and more buoyant and 2) fresh and cold water from the melting Greenland ice sheet disrupts the flow.
The warm gulf stream has weakened about 15% since 1950. Past collapses of the gulf stream have caused western Europe to descend into freezing winters. A significantly weakened system is associated with more – maybe much more – severe storms in Europe, faster sea level rise on the east coast of the US, increasing drought in the Sahel and collapsing deap sea ecosystems.
The system is said to be highly non-linear and has been associated with abrupt changes in temperature when disturbed, such as winter temperatures changing up to 10C within three years in some places.
Counterintuitively, the cooler water close to western Europe that is entailed in a weaker gulf stream helps warm air to flood into Europe from the south, thereby also possibly increasing summer heatwaves.