The verb BE is necessary here because the clause uses a present continuous construction.
- She is talking
- Technological advances are beginning ...
In the second example we see the verb are agreeing with the plural Subject technogicaltechnological advances.
We could rephrase the Original Posters sentence like this:
- Technological advances are beginning to offer hope that wind power will come to be accepted as a reliable and important source of electricity only now.
The word only often means something like not apart from or, — as in this case — not before. When only has this meaning, if we move the only phrase to the beginning of the sentence then we need to change the sentence. We need to invert the Subject and the auxiliary verb. We put the auxiliary before the Subject:
Only after considering all the consequences [should] [you] make risky decisions.
Only now [are] [technological advances] beginning to offer hope that wind power will come to be accepted as a reliable and important source of electricity.
In the examples above we see the auxiliary verbs should and are occurring after the Subjects technological advances and you instead occurring after the auxiliary verbs instead of before them.