accumulate
v.v. to collect or gather more and more of something over a long period of time. You use this when things grow in number or amount slowly.
v. to gather or acquire an increasing number or quantity of something; to amass over time. Transitive when referring to the act of gathering, but also functions intransitively when describing a gradual increase.
Dust will accumulate on the shelves if you do not clean them.
Over the course of his long career, the professor managed to accumulate a massive collection of rare manuscripts.
As carbon dioxide continues to accumulate in the atmosphere, scientists warn that global temperatures will rise at an accelerating rate, leading to irreversible ecological shifts.
First attested c. 1487; from Middle English accumylaten, borrowed from Latin accumulātus, perfect passive participle of accumulō (“to amass, pile up”) (see -ate (verb-forming suffix) and -ate (adjective-forming suffix)), formed from ad (“to, towards, at”) + cumulō (“to heap”), from cumulus (“a heap”) + -ō (first conjugation verb-forming suffix). Cognate with French accumuler.
The verb is both transitive and intransitive. When used intransitively, it describes a natural or passive buildup.