全球变化知识资源中心

Anthropogenic nutrient flows exceed the planetary boundaries. The boundaries and the current excesses vary spatially. Such variations have both an ecological and a social facet. We explored the spatial variation using a bottom-up approach. The local critical boundaries were determined through the current or accumulated flow of the preceding five years before the planetary boundary criteria were met. Finland and Ethiopia served as cases with contrasting ecology and wealth. The variation in excess depends on historical global inequities in the access to nutrients. Globally, the accumulated use per capita is 2300 kg reactive nitrogen (N_r) and 200 kg phosphorus (P). For Finland, the accumulated use per capita is 3400 kg N_r and 690 kg P, whereas for Ethiopia, it is 26 kg N_r and 12 kg P. The critical N boundary in Finland is currently exceeded by 40 kg cap⁻¹ a⁻¹ and the accumulated excess is 65 kg cap⁻¹ a⁻¹, while the global current excess is 24 kg cap⁻¹ a⁻¹ and there is space in Ethiopia to increase even the accumulated flow. The critical P boundary is exceeded in Finland and (although less so) in Ethiopia, but for contrary reasons: (1) the excessive past inflow to the agrifood system in Finland and (2) the excessive outflow from the agrifood system triggered by deficits in inflow and waste management in Ethiopia. The critical boundaries set by Finnish marine systems are lower and those set by freshwaters are higher than the planetary boundaries downscaled per capita. The shift to dominance of internal loading in watercourses represents a tipping point. We conclude that food security within the safe boundaries requires global redistribution of nutrients in residues, soils and sediments and of rights to use nutrients. Bottom-up assessments reveal local dynamics that shed new light on the relevant boundary criteria and on estimates and remedies.