The concept of planetary boundaries was developed to quantify and monitor the broadest areas of environmental impact that would affect human habitability on Earth, and with that, define and quantify the limits within which human activity could safely be conducted.
So far, nine boundaries have been defined, with more open to being added. Three of these are thought to have already been breached: climate change, loss of biodiversity and biogeochemical flow.
The biogeochemical flow boundary refers to the phosphorus and nitrogen cycles. Specifically, the nitrogen cycle boundary has been breached, due to the heavy use of fertilisers that have leaked into water bodies and the wider environment, as well as nitrogen emissions by factories. This has led to eutrophication affecting sea life e.g. shrimps in the Gulf of Mexico, and to a growing disruption to the nitrogen cycle that exceeds that experienced by carbon in the carbon cycle.
Biodiversity loss refers to the accelerating extinction of species from Earth, which serves to indicate the overall habitability of the planet, as well as the stability and prosperity of existing biological communities.
Climate change is another Earth-wide factor that determines the health of the biosphere, the limits on safe human activity, and the habitability of the planet.
Other planetary boundaries include land use, freshwater use, ozone depletion, and ocean acidification. These are deemed to be in check at present, within the safe boundary. Ozone depletion was a critical issue in the past, which has been successfully reversed partially through the ban of toxic aerosols (CFCs, chlorofluorocarbons).
The depletion observed in 2010 is projected to reverse by 2028 to the levels of ozone present in 1992, and continue to improve beyond. Ozone in the atmosphere shields against the Sun’s dangerous, if excessive, UV (ultraviolet) radiation which can harm organisms.
Land use refers to the human use of land for agriculture, rearing livestock, urbanisation, etc. These uses impact the carbon cycle, biodiversity and freshwater use. Evidently, these planetary boundaries are all interconnected. Ocean acidification is caused by increasing amounts of carbon dioxide dissolving in it, due to its increased concentration in the air. High acidity can be harmful to aquatic life by interfering with metabolic activity and immune function, and causing coral bleaching.
Freshwater forms only a tiny percentage of water available on Earth, and is defined by water that does not have a high concentration of solutes like salt in it. Most plants and mammals that form the basis of agriculture rely on freshwater. The contamination of freshwater with toxic chemicals, such as in the case of oil spills, can reach huge areas of the environment and deem its water poisonous, leaving life for dead.
The final two planetary boundaries, chemical pollution and atmospheric aerosol loading, have not yet been quantified to determine where they lay across the safe boundary. Chemical pollution refers to any type of toxic, high concentration or radioactive chemical that is dumped into the environment, such as heavy metals and radionuclides. These can kill organisms, damage their fertility or cause illness.
Atmospheric aerosol loading refers to the fine solid or liquid droplet particles found in air. There are natural ones such as fog, and artificial ones like smog. Human products that end up in the atmosphere can have cooling effects on the planet by reflecting sunlight, or an opposite heating effect such as in the case of soot in the atmosphere which can create a greenhouse effect, trapping heat. It is thought that on the balance, aerosol loading is having a cooling effect, but there are not enough details to establish where within the safety boundary these activities lie, or where the boundary could be.
A possible tenth boundary was proposed as the total primary production by plants. This could be used to indicate the overall function of ecosystems.