The pronoun "it" is the subject. It refers to a subject which, although not explicitly stated, is clear from the context. In this case, "it" = "general circumstances".

In this sentence, "it" is functioning as a dummy subject, so the correct option is 2: "it" is the subject, not referring to anything, functioning as a dummy subject. The sentence is structured as a conditional sentence, where "it" is used as a placeholder subject to introduce the condition. The real subject of the sentence is "here" or "this place," which is the location being referred to in the sentence. So, the meaning of the sentence can be paraphrased as "If I opened the window, this place would be too cold."

It - would be the subject/personal pronoun. It would be too cold in here - main subject.

The second option is correct. "It" is just holding the place of the subject. This happens often in English when we talk about weather, time or distance.