This whole discussion makes me remember the 2004 movie 'The Day After Tomorrow' depicting a sudden slowing or stopping of the thermal currents that bring warm air and ocean water from south of the equator north keeping the upper part of the planet temperate. When the currents slow, Europe and the upper part of the U.S. starts freezing quickly. People begin fleeing southward towards the warmth, but cannot get far enough because of the U.S. built wall which stops them. Watching it even then, it made me think hard about that possibility.