There is only one goddess Gaia and Al Gore is her only prophet...

[kandrathe]

While the untimely death of a person is a tragedy, the death of a large number is a natural selection process. It is to a species as an amputation is to an individual -- unpleasant but potentially life saving. Besides, compared to the population migrations of the first and early second millennium, those of the 1800s were only a small percentage of the populations involved.

Maybe. If you are assuming the ones that got killed were the less adapted ones. When geology or climate is fairly stable for decades, or centuries, then suddenly changes catching a large population by surprise, their death is tragic and not necessarily good for the species. Generally, those who are more highly specialized to their environment suffer the most.

Figuring 25 years as a generation, in less than 3 generations the Visigoths moved from the Caucasus to the central Iberian peninsula. A bit more than a week's walk per generation. They were not the only ones during that period (mid first millennium). The spreading of the Vikings was also pretty much series of long jumps interspersed with long periods of static settlement. Our knowledge of the movements of large numbers in Africa and the Americas is limited because these movements were not recoded in a manner that has come down to us. While the spread of humans from the Bering bridge to Tierra del Fuego may well have been at the rate you claim, there are some indications of rapid changes, especially in Central America. In Asia, we have the rapid movement of the Mongols from Central Asia as far West as the Balkans and as far East as a failed invasion of Japan.

There were certainly those more mobile than others, and perhaps more motivated by hunger or hostile neighbors chasing them.

Not at all comparable. Perhaps it was foolish to build a city on the slopes of a dormant volcano, but that has been done (and is being done) many times throughout history. From when the volcano went active till Pompeii was destroyed was, effectively, instantaneous compared to climate change.

Not really that incomparable. Climate change would be fairly normal, then some extreme event would make it unlivable. A storm, a cold snap, a summer without a harvest, etc. You can look at human population centers most anywhere and say, "perhaps it was stupid to build so near a coastline, or in a desert, or in a cold climate, or in a river valley, or below sea level."