[1]: Forward I  Between 1998 and 2001 he carried out PhD research at KU Leuven University, Belgium, in which he investigated the role of human and natural processes in land degradation in the Ethiopian highlands.

Promoters of this research were professors Jean Poesen and Seppe Deckers (both at KU Leuven), Jan Moeyersons at the Royal Museum for Central Africa in Tervuren) and Mitiku Haile of Mekelle University in Ethiopia.

[1][4] Nyssen's research contributes to the identification and quantification of changes in the coupled system "humans-environment" – with a focus on slope processes, hydrogeomorphology, land degradation and soil conservation.

Nyssen's research showed that high levels of soil loss in the Ethiopian highlands are caused by a combination of erosive rainfall, steep slopes, and impacts of deforestation, overgrazing, an agricultural system where the open-field system dominates, and the aftermath of poverty induced by a long history of feudalism in Ethiopia.

[5][9] Nyssen's research in Belgium is also related to the consequences of human activities on geomorphic processes: the reintroduction of beavers, spoil tips of the derelict coal mining industry, and the age-old agriculture in the Pays de Herve region, which led to the occurrence of lynchets or cultivation terraces, by analogy to a common practice in current Ethiopia;[10] The beaver dams have drawn his attention because they contribute to conserving water in the rivers' headwaters.