In the same year he took part in the First Polish West Greenland Expedition, organized by Aleksander Kosiba, which provided him with enough material for his PhD dissertation.

[1] Jahn survived the Nazi occupation of Poland by working as a feeder of lice at Rudolf Weigl's typhus research institute in Lwów.

He also conducted research in Siberia, Alaska and other parts of Scandinavia which made him one of the foremost polar geomorphologists in the world.

[2] In 1968, as a rector of Wrocław University, he made the decision to support the student strikes against Communist censorship.

[1] Likewise, during the martial law in Poland in 1982, he spoke out against the policies of the Jaruzelski government and was consequently removed by the authorities from his position as chair of the Committee on Polar Research of the Polish Academy of Sciences.