besorgniserregend
concerning
Describing a situation or development that gives cause for concern and is unsettling.
Der rapide Rückgang der Insektenpopulation ist besorgniserregend.
The rapid decline in the insect population is concerning.

concerning / alarming
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concerning
Describing a situation or development that gives cause for concern and is unsettling.
Der rapide Rückgang der Insektenpopulation ist besorgniserregend.
The rapid decline in the insect population is concerning.

The word is derived from the fixed expression 'Besorgnis erregen' (to cause concern). It is the present participle form and literally means 'causing concern'.
Die Lage im Krisengebiet ist besorgniserregend.
The situation in the crisis region is concerning.
'Besorgniserregend' (concerning) can also be used as an adverb to intensify another adjective or adverb. It then describes an unsettling degree or extent.
Die Arbeitslosenzahlen sind besorgniserregend hoch.
The unemployment figures are worryingly high.
This adjective is often used to describe abstract nouns such as 'Entwicklung' (development), 'Zustand' (condition), 'Nachricht' (news), or 'Tendenz' (trend).
Die Ärzte beobachten eine besorgniserregende Entwicklung bei dem Patienten.
The doctors are observing a concerning development in the patient.
critical
'kritisch' and 'besorgniserregend' can both describe a serious or problematic situation. 'kritisch' often sounds stronger and means that a situation is dangerous or decisive and can quickly get worse. 'besorgniserregend' mainly emphasizes the feeling of concern: something makes people uneasy or gives cause for concern. Learners may confuse the words because both fit negative developments. In practical terms: if a situation is really at a dangerous point, 'kritisch' fits better. If you want to say that something is worrying, 'besorgniserregend' is more natural.

Die steigenden Mietpreise in der Stadt sind für viele Familien besorgniserregend.
The rising rents in the city are concerning for many families.
Wegen der starken Blutung war die erste Stunde nach der Operation kritisch.
Because of the heavy bleeding, the first hour after the operation was critical.