The Gulf Stream (Conveyor Belt)

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The world's oceans move constantly. Ocean currents flow in complex patterns and are affected by the wind, the water's salinity and temperature, the shape of the ocean floor, and the earth's rotation.
How does it work?

The gulf stream is one of the strongest ocean currents in the world. It is driven by surface wind patterns and differences in water density. Surface water in the north Atlantic is cooled by winds from the Arctic. It becomes more salty and more dense and sinks to the ocean floor. The cold water then moves towards the equator where it will warm slowly. To replace the cold equator-bound water, the gulf stream moves warm water from the Gulf of Mexico north into the Atlantic.

The gulf stream brings warmth to the UK and north-west Europe and is the reason we have mild winters. The average annual temperature of north-west Europe is about 9C above the average for our latitude.
The past

At the end of the last Ice Age, when the ice sheet covering North America melted, the sudden increase in fresh water reduced the salinity of the north Atlantic surface water and therefore less 'dense water' sank and moved towards the equator. This reduced, or even shut-down completely, the warm gulf stream. Temperatures in north-west Europe fell by 5C in just a few decades
The present

Recent observations have shown that since 1950 there has been a decrease of 20% in the flow of cold water in the Faeroe Bank channel between Greenland and Scotland. This is one source of cold dense water that drives the density-based component of the gulf stream. There may be an increase in flow from other cold water sources, but, if not, it could be the start of the slow down of the gulf stream.
The future

Today, the Greenland ice sheet is more stable, but an increase in global temperatures (and therefore melting sea ice) and precipitation may add more fresh water to the north Atlantic, similar to what happened at the end of the last Ice Age.

Current climate models predict that if greenhouse gas emissions continue to increase, the component of the gulf stream driven by the differences in water density is likely to decrease by 25% in the next 100 years. As the gulf stream becomes weaker, it may become less stable and therefore be more likely to shut down completely in the future.
Implications

A reduced gulf stream would mean that less heat is brought to north-west Europe and therefore harsher winters. However, current climate model predictions are confident that the increase in temperatures resulting from an increase in greenhouse gas emissions is much greater than the potential cooling effect, so a cooling of the UK climate is unlikely this century.

Water:
Sea level rises
Glaciers and ice sheets
Flooding


http://www.bbc.co.uk/climate/impact/gulf_stream.shtml