Learn Finnish: Asukasluku tulee kasvamaan huomattavasti. - The population size will grow considerably.

While Finnish normally uses the present tense combined with context clues to express future actions, this analytical future construction (tulla + 3rd infinitive illative) is used to express a strong, inevitable prediction or a calculated forecast. It is very common in formal contexts like statistics, weather forecasts, or economic trends, which is exactly what a population change is.

What is the exact breakdown of the compound word asukasluku?

It is made up of asukas (resident or inhabitant) and luku (number or figure). It literally means the number of residents. You might also see the word väkiluku, which uses väki (people or crowd) and means the exact same thing in this context.

How do we get the adverb huomattavasti from its root verb?

It originates from the verb huomata (to notice). From that, we get the adjective huomattava (noticeable or considerable). To turn that adjective into an adverb, you add the standard adverbial suffix -sti, resulting in huomattavasti (considerably or noticeably).

If I wanted to just use the standard present tense for the future here, how would the sentence change?