As for the climate change argument this movement of people groups is not uncommon. Rome up until the arrival of the Goths had been dealing with "barbarian" incursions for a thousand years.
Rome invade Gaul, Germany, and Britannia which people seem to omit. There is no argument that climate change forced them too. They did it because they could. The "barbarian" tribes invaded because they could. Both sides in this conflict are human and have human motivations.
So my guess would be that there were several very cold years in a row, which caused famines, and the populations understood that their best survival chance was to migrate south, where they had to compete with the roman empire for land.
[1] https://fr.wikipedia.org/wiki/Fichier:2000_Year_Temperature_... [2] https://fr.wikipedia.org/wiki/Fichier:2000_Year_Temperature_... [3] https://en.wikipedia.org/wiki/Great_Famine_of_1695%E2%80%931...
It has saddened me to see this viewpoint associated in the American cultural wars with one party's "enemy camp", where discussion of climate as a problem is automatically associated with all kinds of unrelated policy positions. People are unaware this historical explanation was posited by hoary old (and largely long-dead) twentieth-century men who had no dog in the contemporary political divide.
Their longboats made them amphibious. Their ships were durable enough for the open ocean, but still able to land on any beach, or sail fair upriver.
For example, the Roman Republic had adapted to constant warfare and the steady stream of casualties caused by it. And then the Late Republic got into trouble, because Rome had become too good at war, and the required casualties didn't happen.
They had too many births before the potato famine -- it's the reason why it was such a disaster in Ireland compared to the rest of Europe -- and they continued up until a few decades ago. This happened during a time when it was unusually easy to move to "empty" land and colonize it.
Anytime they had a population explosion, younger sons would have to go A Viking.