The Gulf Stream, the ocean current that brings warm temperatures and mild climates to northwestern Europe, is slowing down. New research shows that the current is now 15% weaker than in it was in around 400 AD. It also reveals that much of the weakening has taken place since 1850 and the beginning of industrialization, strongly indicating that human-caused global warming, through the burning of fossil fuels, is the culprit. If the Gulf Stream were to be disrupted, there would be severe consequences for the climates of British Isles, France and the Nordic countries.

The Gulf Stream is a warm Atlantic Ocean current. It originates in the Gulf of Mexico and flows northeastwards, following the eastern coastlines of the United States and Newfoundland before heading out across the Atlantic Ocean. There it splits in two: the Canary Current turns south while the North Atlantic Drift streams towards Europe. The current splits again, with one branch, the Norwegian current, flowing past Scotland and up the Norwegian coast. The Gulf Stream brings warmer temperatures to these lands than would otherwise be the case at these high latitudes.

The Gulf Stream and its northern extensions form part of the Atlantic Meridional Overturning Circulation (AMOC). This is a conveyor-belt like system that carries warm water northwards towards the north pole and back again. When the warm water reaches Arctic regions, it cools, becomes denser and sinks. It then flows southwards as a deepwater current. This cooling has in recent years been severely hampered by global warming, which has caused water temperatures in the Arctic to rise.

Satellite images show that summer sea ice in the Arctic has undergone dramatic retreat since records began in 1979—a loss of almost 30%. Continued retreat is likely in future years. At the same time, melting ice from the Greenland ice cap tips fresh water into the sea. This is less dense than salty seawater, so the disruption to AMOC is even greater.

What would happen if AMOC shut down altogether? Scientists predict that this would cause a sudden collapse of the Gulf Stream—something that might occur over just a few years. Northwestern Europe would be deprived of mild southwesterly winds so would suffer far more extreme winters. Sea levels would also rise markedly along the eastern seaboard of the United States, bringing severe flooding.