Are both the sentences expressing same thought?
The plan is environmentally disastrous.
Vs
Environmentally, the plan is disastrous.
Are both the sentences expressing same thought?
The plan is environmentally disastrous.
Vs
Environmentally, the plan is disastrous.
Close enough that in most situations they are interchangeable.
Structurally the first has the adverb acting as a modifier of "disastrous", and the second has the adverb as an adjunct of the whole clause. With different words, these structures can have different meanings:
Pat was unfortunately dressed (their clothes were poorly chosen, they should have worn different clothes, not a very natural sentence)
Unfortunately, Pat was dressed. (They should have been naked)
But in your example, I can't see any real difference.
OP's first version is the "syntactic default" for English - the adverb (environmentally) goes next to the verb (is). The second version is a "stylistic inversion" with exactly the same meaning.
Note that structurally, that second version with the "fronted" adverb looks like a whole sentence adverb. For semantic reasons that classification doesn't apply with OP's example, but it certainly could with a different adverb...
1: The plan is truly disastrous
2: Truly, the plan is disastrous
....where unquestionably truly in #2 refers to the entire assertion that follows. This leads to a small but potentially significant semantic distinction that might be more obvious in, say...
3: I didn't know he truly liked you
(I thought perhaps he was just being politely friendly to you)
4: Truly, I didn't know he liked you
(I swear I had no idea he was even "favorably disposed" to you)
TL;DR: The actual examples as given just represent different stylistic choices with the same meaning, but there are structurally similar utterances where the position of the adverb more clearly affects the meaning.