Learn Farsi: تغییر اقلیم یک بحران زیست‌محیطی است. - Climate change is an environmental crisis.

It is a compound word. It combines zist (life or living) with mohit (surroundings). Together, zist-mohit forms the noun for "environment." Adding the suffix -i at the end turns it into the adjective zist-mohiti, meaning "environmental."

I usually use âb o havâ to talk about weather. Why is eqlim used here?

While âb o havâ is the everyday term for weather and local climate, eqlim is a more formal, scientific term for broad climate zones. The established term for the global phenomenon of "climate change" is specifically taghyir-e eqlim.

Where does the Ezafe go in the phrase yek bohrân-e zist-mohiti?

The Ezafe links the main noun to its adjective, so it goes right after the noun: bohrân-e zist-mohiti (environmental crisis). You never put an Ezafe after yek (a/one) because numbers precede the noun they count without needing a connector.