But it wasn't from global warming. It was global cooling during the Dark Age that caused the Huns to seek new land southwest of where they had been, which forced the various gothic tribes to move southwest too, which forced the Anglos out of their territory (modern day Germany area). The Anglos went north into Scandanavia (going north when it's already too cold where you're at to grow crops makes life even worse). We know that the Huns moved there too, we're not sure if pressure from the Huns is why the Anglos moved again -- this time west to Brittania (modern day England).
Something to think about whenever we hear the horrors of global warming -- historically it's the centuries long global cooling years that make most civilization commit atrocities against others.
Life during the Modern Warm Period is so nice that our government pays farmers to avoid growing too much food. That would have been unheard of when this country was formed during the Little Ice Age, growing food was a yuge challenge and many people starved to death.
The Huns moved west because of a gigantic volcanic eruption which blotted out the sun. The local grass stopped growing and the Huns had horses which needed grass for food. Therefore, the Huns went west from one pasture to another. And of course any local village along the way would be raided for human food. When the Huns got to The Roman Empire, the Pope gave 500 pounds of gold to the Huns to spare the Romans.