Learn Italian: Spero che l'energia rinnovabile diminuirà la siccità. - I hope that renewable energy will decrease the drought.

You are right that sperare (to hope) usually triggers the subjunctive. However, when the action in the dependent clause (decreasing the drought) happens in the future compared to the main clause (hoping right now), Italian uses the future indicative instead. We use diminuirà to clearly show this future action.

Is rinnovabile masculine or feminine in this sentence?

It is feminine here to match l'energia, but its form doesn't actually change between masculine and feminine. Adjectives ending in -e in the singular are used for both genders. In the plural, they change to end in -i, so the plural would be le energie rinnovabili.

Can diminuire take a direct object?

Yes! Diminuire can be used without an object (for example, if the drought itself just decreases) or with a direct object as it is here, where the renewable energy is actively decreasing something else (la siccità).

What does the accent mark on siccità tell us?

The accent on the final vowel means you must put the stress of your voice on that last syllable. Also, as a handy rule, feminine nouns ending in an accented vowel do not change their spelling in the plural. If you wanted to say the droughts, it would simply be le siccità.