8th century Norsemen, whose raids terrified Europe, were not that much more developed than Iron Age Germanic tribes and were not the first pirates of the North Sea-Saxons and Franks before them also raided coastal areas. What made them different from predecessors was their naval tech. Norse were not coast-huggers, they seemed to appear out of nowhere from perspective of raided people and that made them so terrifying.
So lets assume, that Norse naval tech improvements came earlier, during Roman Warm Period (say 2nd-3rd century AD). Thus there are Norse raids of Britain and Gaul, Faroe Islands, Iceland and Greenland are settled. What would be impact? Would Romans abandon Britain earlier due to Norse pressure? Roman naval tech improves too as response? And earlier settlement of Iceland and Greenland gives Norse better chance to estabilish permanent presence in New World?
So lets assume, that Norse naval tech improvements came earlier, during Roman Warm Period (say 2nd-3rd century AD). Thus there are Norse raids of Britain and Gaul, Faroe Islands, Iceland and Greenland are settled. What would be impact? Would Romans abandon Britain earlier due to Norse pressure? Roman naval tech improves too as response? And earlier settlement of Iceland and Greenland gives Norse better chance to estabilish permanent presence in New World?