Learn French: Le fait que le réchauffement climatique augmente est grave. - The fact that global warming is increasing is serious.

In French, le fait que (the fact that) usually triggers the subjunctive because it introduces a fact specifically to pass judgment on it or react to it. Here, the judgment is that the situation est grave (is serious). This evaluative weight overrides the objective certainty of the fact.

Is augmente in the subjunctive here? It looks exactly like the normal present tense.

Yes, it is in the subjunctive! For regular -er verbs like augmenter, the present subjunctive forms for je, tu, il/elle/on, and ils/elles are identical to the present indicative. Even though the spelling doesn't change, it is grammatically subjunctive due to Le fait que.

Why is global warming translated as le réchauffement climatique?

Climatique is an adjective meaning climatic or related to the climate. Like most classifying adjectives in French, it comes after the noun. So the French phrase literally means climatic warming.

What is the subject of the main verb est at the end of the sentence?

The subject is the entire noun clause Le fait que le réchauffement climatique augmente. If you strip away the subordinate details, the core structure of the sentence is simply Le fait est grave (The fact is serious).