When the particle つつ ‘while doing’ is combined with ある ‘to be’, it indicates that someone or something is in the ongoing process of (A). (A) will be a verb in its ます-stem form. This formal grammar structure primarily highlights that (A) is slowly but surely continuing or changing. This ‘change’ is often in regard to an increase, decrease, expansion, contraction, or something similar.
This pattern can be translated simply as ‘to be (A)ing’.
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毎年（まいとし）、年金（ねんきん）の受給額（じゅきゅうがく）が減（へ）りつつあるので、生活（せいかつ）に困（こま）っている高齢者（こうれいしゃ）が増（ふ）えてきている。
Every year, more and more elderly people are having trouble making ends meet because their pension benefits are decreasing.
日本（にほん）では物価（ぶっか）が上（あ）がりつつある。
Prices in general are rising in Japan.
最近（さいきん）はインターネットを使（つか）って買（か）い物（もの）ができるので、わざわざお店（みせ）に行（い）って買（か）い物（もの）をする人（ひと）が減（へ）りつつある。
These days, people can shop using the Internet, so fewer people are going out of their way to go to stores to shop.
Caution - Compared to ている,
つつある tends not to be used in relation to physical motions, and only highlights ‘(A)ing’ in regard to actions that indicate an ongoing state of change, rather than simply ‘continuing’.