Learn French: Il est probable que le glacier fondra. - It is probable that the glacier will melt.

In French, il est probable que (it is probable that) expresses a strong likelihood, so it is treated as a near-certainty and takes the indicative mood. This is the opposite of il est peu probable que (it is unlikely that) or il est possible que (it is possible that), which express doubt and require the subjunctive.

If the sentence were negative, would we still use the indicative?

No. When you make the phrase negative (il n'est pas probable que) or turn it into a question (est-il probable que ?), you are introducing doubt. This triggers the subjunctive, so the sentence would become: Il n'est pas probable que le glacier fonde.

Why use the future tense fondra instead of the present tense?

Because the indicative mood allows you to use whichever tense matches the reality of when the event happens. The glacier melting is a future event, so the future tense (fondra) is used. If this phrase had required the subjunctive, you would generally just use the present subjunctive (fonde) to refer to a future event.

Could I start this sentence with C'est probable que instead of Il est probable que?

In casual spoken French, yes, you will hear c'est probable que. However, il is the grammatically correct dummy pronoun for formal impersonal expressions, so il est probable que is the standard you should use in writing or careful speech.