The Gulf Stream, also known as the North Atlantic Conveyor is the warm current flowing North from the tropics to the Arctic. This warm current is responsible for the temperate climate enjoyed by Europe and the US. England without the Gulf Stream would have the same temperatures as Newfoundland (at the same corresponding latitude). The current carries 1 million billion watts of heat (1 petawatt).
Understanding the mechanism driving this current is the key. Warm saline water flowing North meets the ice cold freshwater in the Arctic causing a sudden temperature drop. Because saline water is heavier than freshwater it drops deep down below the surface freshwater. The effect of this sudden plunging of now cooled saline water is to create a draw at the surface which ensures a steady flow of warm water northward. This conveying of warm surface waters northward and cool waters deep in the ocean southward is also known the Atlantic Conveyor or thermohaline circulation.
The Gulf Stream has been flowing since the end of the last ice age and whole temperate ecosystems have evolved in the Northern hemisphere in response to the warmer climate. As climate changes these ecosystems will vanish.
And things are already starting to change. As Global Warming takes effect the Arctic ice melts in increasingly large quantities causing a deluge of cool freshwater flowing South. The result; the warm Gulf Stream waters are now mixing with the freshwater flows much further South causing the warmer waters to no longer reach the higher latitudes.
The latest research shows that The Gulf Stream has already reduced in size by 10%. Professor Schlesinger's research (University of Illinois) suggests that there is 50% chance of it completely switching off if worldwide CO2 emission rates continue to rise. This would result in a temperature drop of at least 5°C.
If we are able to significantly reduce emissions in line with a maximum 2°C temperature rise this probability would drop to 25%.
The North Atlantic Conveyor last slowed in 1300AD. It changed the climate enough to create a "little ice age" which lasted for 550 years. It last stopped 8200 years ago. Northern Europe was left under 0.5 miles of ice for 100 years.