This is the 12127th most frequent Russian word.
изучавшее
"Изучавшее" means "that (or which) was studying or exploring."
Here, 'изучавшая' (studied) is used as a participle modifying the noun 'Книга' (book), indicating that the book is characterized by the action of studying art history.
Книга, изучавшая историю искусства, дала мне много новых знаний.
The book that studied the history of art gave me many new insights.
In this sentence, 'изучавшие' (studied) is a participle modifying 'Ученые' (scientists), showing that the scientists are described by their action of studying climate change.
Ученые, изучавшие климатические изменения, сделали важное открытие.
The scientists who studied climate change made an important discovery.
Here 'изучавший' (studied) is a participle describing 'Проект' (project), indicating that the project was characterized by the activity of studying local traditions.