Learn Russian: Глобальное потепление усиливается, и эта угроза всё серьёзнее. - Global warming is intensifying, and this threat is increasingly serious.

This is a set grammatical structure. When the particle всё is placed right before a comparative adjective or adverb (like серьёзнее), it indicates a progressive comparison. It translates to "more and more" or "increasingly."

Why do we use the short comparative серьёзнее instead of более серьёзная to match the feminine угроза?

When a comparative is the predicate of a sentence (meaning it comes after the implied verb "is"), the short form ending in -ее is standard and does not change for gender or number. You would use более серьёзная if it were an adjective modifying the noun directly, like in the phrase "a more serious threat" (более серьёзная угроза).

Why does усиливается end with the reflexive particle -ся?

The base verb усиливать is transitive and means "to strengthen [something else]". Because global warming is intensifying on its own rather than acting on a direct object, we add the reflexive -ся to make it intransitive.

How is the noun потепление constructed?

It comes from the root тепло (warmth) or тёплый (warm). Adding the prefix по- and the suffix -ение creates a verbal noun that describes the ongoing process of becoming warm.