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.

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Planetary Boundary Nr 7: Land system change

Land is converted to human use all over the planet. Forests, wetlands and other vegetation types are converted primarily to agricultural land. This land-use change is one driving force behind reduced biodiversity and has impacts on water flows as well as carbon and other cycles. Land cover change occurs on local and regional scales but when aggregated appears to impact the Earth System on a global scale. A major challenge with setting a land use-related boundary is to reflect not only the needed quantity of unconverted and converted land but also its function, quality and spatial distribution.

Planetary Boundary Nr 6: Freshwater consumption and the global hydrological cycle

The freshwater cycle is both a major prerequisite for staying within the climate boundary, and is strongly affected by climate change. Human pressure is now the dominating driving force determining the function and distribution of global freshwater systems. The effects are dramatic, including both global-scale river flow change and shifts in vapour flows from land use change. Water is becoming increasingly scarce and by 2050 about half a billion people are likely to have moved into the water-stressed category. A water boundary related to consumptive freshwater use has been proposed to maintain the overall resilience of the Earth system and avoid crossing local and regional thresholds ‘downstream´.

Planetary Boundary Nr 5: Ocean acidification

Around a quarter of the CO2 humanity produces is dissolved in the oceans. Here it forms carbonic acid, altering ocean chemistry and decreasing the pH of the surface water. Increased acidity reduces the amount of available carbonate ions, an essential building block used for shell and skeleton formation in organisms such as corals, and some shellfish and plankton species. This will seriously change ocean ecology and potentially lead to drastic reductions in fish stocks. Compared to pre-industrial times, surface ocean acidity has increased by 30%.

The ocean acidification boundary is a clear example of a boundary which, if transgressed, will involve very large change in marine ecosystems, with ramifications for the whole planet. It is also a good example of how tightly connected the boundaries are, since atmospheric CO2 concentration is the underlying controlling variable for both the climate and the ocean acidification boundary.

Planetary Boundaries Nr 4: Climate Change

Image: Jennifer Renee / FreeDigitalPhotos.net

We have reached a point at which the loss of summer polar ice is almost certainly irreversible. From the perspective of the Earth as a complex system, this is one example of the sharp threshold above which large feedback mechanisms could drive the Earth system into a much warmer, greenhouse gas-rich state with sea levels metres higher than present. The weakening or reversal of terrestrial carbon sinks, for example through the ongoing destruction of the world´s rainforests, is another such interdependent tipping point. Recent evidence suggests that the Earth System, now passing 387 ppmv CO2, has already transgressed this Planetary Boundary. A major question is how long we can remain over this boundary before large, irreversible changes become unavoidable.

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