rapid climb vs ascent

rapid climb and ascent both are nouns.

Word	Noun	Adjective	Verb	Adverb
rapid climb	Yes	No	No	No
ascent	Yes	No	No	No

As nouns, ascent is a hypernym of rapid climb; that is, ascent is a word with a broader meaning than rapid climb:

Other hypernyms of rapid climb include ascension, rise, rising.

rapid climb (noun)	ascent (noun)
a rapid rise	the act of changing location in an upward direction
	a movement upward; rise above the ground
	an upward slope or grade (as in a road)