A study just published in Nature Climate Change reports an "exceptional" weakening of the Gulf Stream system across the twentieth century, and suggests that the entire system of the Atlantic Meridional Overturning Circulation is in jeopardy. But the lack of measurements means that the degree to which this is true is unclear. The weakening of the system is the probable cause of the recent harsh winters in North America, as the polar vortex has ceased to be held in position by warm air that is normally driven northward by the current. While there was some recovery in the 1990s, the authors of the study believe that the melting of Greenland is a key factor causing the decline of the current, and that this will increase in the future.
The Gulf Stream system has not been this weak in the past millennium, according to the authors. In the study, they comment, "using a multi-proxy temperature reconstruction for the AMOC index suggests that the AMOC weakness after 1975 is an unprecedented event in the past millennium. Further melting of Greenland in the coming decades could contribute to further weakening of the AMOC."
At present, the decline is affecting both North America and Europe. It is believed to be responsible not only for the instability of the polar vortex, but also for a generalized decline in air circulation. This could be contributing to extreme pollution events now taking place in Europe. For example, as this is being written, the most polluted city on Earth is not Bejing or New Delhi, but Paris.
The decline of the Gulf Stream has been predicted for some time in Unknowncountry’s Climate Watch, and is a primary trigger of sudden climate change in Whitley Strieber and Art Bell’s book Superstorm. The scenario in the book depicts sudden, violent change, and if the Gulf Stream were to stop completely, this would happen. However, a more likely scenario is that it will continue a slow decline as the enormous momentum of the current ceases to be driven by sufficient temperature differentials between the northern and southern Atlantic.