Atlantic thermohaline circulation

Freshwater from the melting Arctic may cause warm currents (including the gulf stream) to weaken significantly in the coming 100 years. Within 200 years it might even shut down completely. Meltwater lowers the salt concentration in the North Atlantic so that water becomes lighter. This slows the deep water currents that carry cold water to the south, thereby causing even the warm northward Gulf Stream to shut down.

Caption: Atlantic thermohaline circulation (THC)
Credit: Thermohaline circulation | Globaïa & NASA/Goddard Space Flight Center Scientific Visualization Studio