Atlantic Meridional Overturning Circulation (AMOC)

See nytimes/atlantic-ocean-climate-change (fantastic graphics).

As the polar ice caps are melting into the ocean the cold water is interrupting the northern portion of the massive Atlantic ocean spanning loop that circulates warm water and air out of the tropics and up into northern latitudes. It's basically a massive planet-spanning heat pump. If it stops circulating, then parts of Europe may temporarily get colder and the tropics could get much hotter.