An American of Finnish descent, Karhu settled in Japan in 1955 with his family, initially as a Lutheran missionary and an English teacher. A graduate of the Minneapolis School of Art, he continued to pursue his interest in painting until a Japanese friend suggested to him that his bold black lines and lively colors would be well suited to woodblock printing. He was taught to carve and print woodblocks in the traditional Japanese way by Tetsuo Yamada. He almost always creates works that describe Japan: an intimate look into a courtyard garden, a country lane, the geisha quarters of Kyoto. He is also known as a carver of Oriental signature seals, and creates humorous paintings done in a style reminiscent of Japanese sumi-e (charcoal ink pictures). He decorates the paintings with Chinese and Japanese poetry.

Karhu's art is closely related to the ukiyo-e prints of the past. His themes are inspired by places that existed then, and like the ukiyo-e artists he spares no pains in preparing the number of blocks required to achieve the results he visualizes. However, Karhu has adopted certain innovations: he has increased the size of his prints, exposing finely drawn detail, and has intensified his color sequence by overprinting an intricate line block. He has a talent for creating mood in such touches as a shaft of sunlight filtering into a courtyard or illumination falling onto a street at dusk. Karhu not only designs his prints but has mastered the meticulous cutting and printing process, whch the ukiyo-e masters left to their assistants.