The ‘thermohaline circulation’ system of the oceans is driven by cold water sinking in the Arctic, dragging warmer surface water north to fill its place. The Gulf Stream, which has a strong warming effect on UK weather, is an integral part of this.
Influxes of fresh water from melting ice in the North Atlantic slow the thermohaline system and have caused periods of complete collapse in the past.
Most climate models indicate that as a result, the Gulf Stream will weaken during the next century but there is evidence that this is happening faster than expected. A complete shutdown of the current cannot be completely ruled out (ENDS Report 381, pp 34-36 
).
The Met Office study shows that if this were to happen, the effects would be felt well beyond the North Atlantic:
Figures for the UK show an average winter temperature after Gulf Stream shutdown of 3°C; 4°C colder than predicted with no shudown and 1°C lower than the 1961-1990 average.
The Met Office’s main climate change model suggests a fifth of UK winters will be frost-free by 2050. But without the Gulf Stream, there would be no frost-free winters and the length of the frost season would triple.
It could also accentuate some effects of climate change, including summer drying in Western and Southern Europe.
A southward shift in the intertropical convergence zone, which brings rain to tropical regions, could decrease precipitation to the north of the equator and increase it further south.
The collapse of the thermohaline circulation would increase temperatures in the deep ocean but this would be counteracted by an increase in continental ice, the study finds. Overall, it predicts an average sea level rise 12cm below that indicated by normal climate change scenarios.
But the picture is bleaker in Western Europe and North America where the reorganisation of ocean circulation could increase sea level by a further to 25-50cm.
