What is the difference in meaning between 'apparire' and 'sembrare'?
Questo fenomeno appare davvero inquietante.
Questo fenomeno sembra davvero inquietante.
Please, explain in reference to the difference, if any, between 1. and 2..
Appear and seem are the respective exact translations. There is almost no difference in meaning of those two sentences, but the two verbs are not interchangeable in every context, just like in English.
Apparire refers more to "becoming manifest, come into view"; sembrare implies more a personal opinion behind the statement: it seems (sembra - to the speaker or to a collective group) expresses some interpretation of what is happening.
For instance, the sentence:
Una nuvola appare all'orizzonte
cannot be written with "sembrare".
Moreover, "sembrare" is widely used in a probabilistic fashion with the subjunctive mood:
Sembra che stia per piovere (it looks like it's going to rain - it seems to me/to the ones watching the sky, but it's not certain-)
IMO there is no difference between 1) and 2), let's wait for a native speaker to confirm it (or not).
Possible translations to english:
This phenomenon appears to be really disturbing.
This phenomenon seems to be really disturbing.
The Treccani dictionary says that ‘sembrare’ is synonymous with ‘parere’. In my opinion
questo fenomeno appare davvero inquietante
seems to be modeled after English (or French) rather than being proper Italian. But, apart from this, I see no real difference between the two sentences.