Learn Finnish: Jos rakennamme päästöttömän voimalan, hiilijalanjälki pienenee. - If we build an emission-free power plant, the carbon footprint will decrease.

The base word is päästötön (emission-free), formed by adding the suffix -tön (without / -less) to päästö (emission). When words ending in -tön take a case ending, the stem changes to -ttömä-. Here it takes the genitive/accusative -n ending to match the object voimalan.

Why do we use the genitive-looking -n ending on voimalan instead of the partitive?

The noun voimala (power plant) is the total object of the verb rakennamme. Because the sentence expresses the intention of completing a single, whole power plant, the object takes the singular accusative case, which looks identically to the genitive ending -n.

Why is the verb pienenee used here instead of pienentää?

Finnish strictly distinguishes between verbs where something happens by itself (intransitive) and verbs where a subject does an action to an object (transitive). Pienenee means 'gets smaller' or 'decreases'. Pienentää means 'to make something smaller'. Because the carbon footprint is shrinking as a consequence rather than being directly reduced by a subject in this clause, we use the intransitive pienenee.

How exactly is the compound word hiilijalanjälki built?