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24 September 2014
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The Gulf Stream

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.

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 oceanThe Gulf Stream brings warmth to the UK and north-west Europe and is the reason we have mild winters. Without this steady stream of warmth the British Isles winters are estimated to be more than 5C cooler, bringing the average December temperature in London to about 2C.

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.

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 IPCC believe it is very likely that the Gulf Stream will slow down during the 21st Century but very unlikely it will undergo a ‘large abrupt transition’. The average reduction predicted by the various models used is 25%. This slowing will have a cooling effect but the temperature will still increase in the region overall.

It suggests that the British Isles, especially western regions, will see a significantly smaller temperature increase than other areas of land mass.

Sea level rises
Glaciers and ice sheets

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