.
In this case, the meanings are the same. I cannot guarantee that they are always synonymous, however, or that both can always be used-- note that they require different sentence structures.
.
since the 1980s – since is a preposition. from the 1980s onwards– onwards is an adverb
However, now that I consider it, onwards is irrelevant to the comparison: there is no relation in meaning between since and onwards. The adverb is an unnecessary addition, merely serving as emphasis: from the 1980s is sufficient, and is synonymous with since the 1980s.
.