Hot and cold

Interruption of the Gulf Stream may lead to large cooling in Europe

A repeat of this scenario is still a possibility in the future, it seems:

The investigated time interval, called the Eemian, occurred before the last Ice Age and was characterized by warmer-than-present temperatures in large parts of the globe. The Eemian climate evolution can therefore serve as an analogue for a future warmer climate.

The study of fossil remains, such as plants and insects, preserved in geological deposits in northern Finland revealed an abrupt climatic cooling event that happened in an otherwise warmer climate. During this event the temperatures dropped 2–4°C and remained low for a period of  500–1000 years. Comparison with seafloor sediment records from the Norwegian Sea and the North Atlantic indicates that the rapid cooling was associated with a sudden slowdown in North Atlantic deep water
formation and a reduction in the northward extension of the Gulf Stream that transports heat to northern Europe.

The new evidence shows that the last time when temperatures were significantly warmer than today, climate instability occurred.

"This may have been caused by melt water coming from the Greenland Ice Sheet, disrupting the North Atlantic Ocean circulation. While the exact mechanism behind the sudden cooling still remains uncertain, the study illustrates the potential for major climatic instability in and around the North Atlantic regionunder future global warming", says Karin Helmens at the Department of Physical Geography, Stockholm University.

I am assuming, by the way, that a 4 degree drop in present temperatures for centuries would be disastrous for present day northern Europe.  On the other hand, if it comes after the region has warmed 2 or 3 three degrees, I'm not sure.

Furthermore, although Hansen has made the point that a future industrial society can readily and cheaply produce enough warming gases that would hold off  a new Ice Age, there would like be much controversy about using them for a situation of only one part of the world needing it, while the rest of the world is already too hot.

But continue with your geo-engineering dreams, libertarians!