Canadian Politics Today

A good example of what happens when there is a sudden change in climate is to look at Europe in the 4th and 5th centuries. The western Roman Empire collapsed. Northern Europeans began moving south and over ran the Roman frontier. Why did they move south? Crop failure due to a drop in temperature. The Roman army was unable to move because it was dependent on wheeled transport and their incredible road system was crumbling due to frost heaves. The "barbarians" were not dependent on roads.
While the collapse of the western Empire had many causes, it was the "little ice-age that brought it down. In spite of the instability and corruption at the top, the government continued to function due to an incredible professional civil service. However, even they were incapable of dealing with the cooler climate. The actual transition from civilization to dark ages took a decade or two, the collapse was likely less than five years.
That was only a minor shift in temperature, likely caused by volcanic activity, and it was confined mainly to Europe.