Is there any plausible scenario whereby the Viking Age could begin several hundred years earlier? If so, what might the impact be?
Specifically, my goal is to have the Viking Age begin in the 1st or 2nd century CE. An environmental cause would be ASB (although that could be interesting in its own right). Another possibility would be an earlier contact between the Romans and Scandinavia.
The obvious first target of any Viking raids would be Roman Britain. Depending on how much damage they did (and who was Emperor at the time), this would probably either push the Romans to reinforce Britain, leading to a more Romanized Britain, or lead to the Romans abandoning Britain as not worth it, in which case we might get a Norse England rather than a Saxon one. Beyond that, the Norse would raid into northern Gaul, Iberia, and eventually the Mediterranean.
Once the Norse started to threaten mainland Europe, the Romans would take action. They might invest in a stronger navy. Certainly they'd build coastal defenses. There would also likely be an ill-conceived punitive expedition against Scandinavia (fat chance anyone gets back from THAT one).