Planetary Boundary Nr 9: Atmospheric aerosol loading

This is considered a planetary boundary for two main reasons: (i) the influence of aerosols on the climate system and (ii) their adverse effects on human health at a regional and global scale. Without aerosol particles in the atmosphere, we would not have clouds. Most clouds and aerosol particles act to cool the planet by reflecting incoming sunlight back to space. Some particles (such as soot) or thin high clouds act like greenhouse gases to warm the planet. In addition, aerosols have been shown to affect monsoon circulations and global-scale circulation systems. Particles also have adverse effects on human health, causing roughly 800,000 premature deaths worldwide each year. While all of these relationships have been well established, all the causal links (especially regarding health effects) are yet to be determined. It has not yet been possible specific threshold value at which global-scale effects will occur; but aerosol loading is so central to climate and human health that it is included among the boundaries.

Planetary Boundary Nr 8: Nitrogen and phosphorus inputs to the biosphere and oceans

Human modification of the nitrogen cycle has been even greater than our modification of the carbon cycle. Human activities now convert more N2 from the atmosphere into reactive forms than all of the Earth´s terrestrial processes combined. Much of this new reactive nitrogen pollutes waterways and coastal zones, is emitted to the atmosphere in various forms, or accumulates in the terrestrial biosphere. A relatively small proportion of the fertilizers applied to food production systems is taken up by plants. A significant fraction of the applied nitrogen and phosphorus makes its way to the sea, and can push marine and aquatic systems across thresholds of their own. A concrete example of this effect is the decline in the shrimp catch in the Gulf of Mexico due to hypoxia caused by fertilizer transported in rivers from the US Midwest.