On the boundary between man-made and natural emissions: Problems in defining European ecosystems

Man-made as well as natural sources contribute to the occurrence of trace c onstituents in the atmosphere. A strict distinction between these source ty pes is needed to support international agreements on emission reductions. H owever, the nomenclatures in existing emission inventories treat several so urce sectors ambiguously, such that certain emissions are considered man-ma de in one system and natural in the other. An investigation of the situatio n for Europe reveals that there are three such sources sectors that contrib ute significantly (more than 10%) to the total emissions of at least one po llutant: animal husbandry, forests, and possibly soils. Using qualitative p arameters for distinguishing between natural and cultivated systems, it may be concluded that, at least under European conditions, both animal husband ry and soils as emission sources are controlled by humans in nearly all asp ects. Forests, even if managed, as is the European practice, retain many na tural characteristics. However, even for forests, emissions are certainly v ery different to those existing before human influence became dominant beca use of extensive deforestation and replanting (often of nonnative species). A "colonization" theory approach suggests that just such a difference betw een the natural situation and the conditions as caused by man are attribute d to anthropogenic emissions. Still, a complete assessment of the respectiv e contributions cannot be provided because of a lack of a "baseline" natura l situation. Therefore the best solution, in practice, will be to formulate international agreements in terms of specific and unambiguous source secto rs.