Man-made as well as natural sources contribute to the occurrence of trace constituents in the atmosphere. A strict distinction between these source types is needed to support international agreements on emission reductions. However, the nomenclatures in existing emission inventories treat several source sectors ambiguously, such that certain emissions are considered man-made in one system and natural in the other. An investigation of the situation for Europe reveals that there are three such sources sectors that contribute significantly (more than 10%) to the total emissions of at least one pollutant: animal husbandry, forests, and possibly soils. Using qualitative parameters for distinguishing between natural and cultivated systems, it may be concluded that, at least under European conditions, both animal husbandry and soils as emission sources are controlled by humans in nearly all aspects. Forests, even if managed, as is the European practice, retain many natural characteristics. However, even for forests, emissions are certainly very different to those existing before human influence became dominant because of extensive deforestation and replanting (often of nonnative species). A colonization theory approach suggests that just such a difference between the natural situation and the conditions as caused by man are attributed to anthropogenic emissions. Still, a complete assessment of the respective contributions cannot be provided because of a lack of a baseline natural situation. Therefore the best solution, in practice, will be to formulate international agreements in terms of specific and unambiguous source sectors. ¿ 1999 American Geophysical Union