This means higher levels of UV radiation reach ground level. The appearance of the Antarctic ozone hole was proof that increased levels of man-made ozone-depleting chemical substances, interacting with polar stratospheric clouds, had passed a threshold. Fortunately, because of the actions taken as a result of the 1989 Montreal Protocol, we appear to be back on track to staying within this boundary.
Loss of biosphere integrity results in the loss of local and regional biodiversity, which makes ecosystems more vulnerable to changes in climate and ocean acidity. Currently, the extinction rate is used as a boundary measure for loss of biosphere integrity. Today, the global extinction rate far exceeds the rate of speciation.39 If the current extinction rate is sustained, an undesired system change is highly likely.
This includes microplastics, pesticides, heavy metal compounds and radioactive materials. Persistent organic pollution, for example, has caused dramatic reductions in bird populations and impaired reproduction and development in marine mammals.
This is measured by CO2 concentration in the atmosphere, with a suggested boundary of 350 parts per million (ppm) above the pre-industrial level.41 We’ve now surpassed 390 ppm CO2 in the atmosphere. The loss of summer polar sea-ice is almost certainly irreversible. This is one example of a well-defined threshold that, when breached, gravely impacts the Earth system.
This is a reduction in the ocean’s PH due to CO2 absorption: around one-quarter of our CO2 emissions dissolve in the ocean.43 This makes it difficult for essential marine life to survive. Unlike most other human impacts on the marine environment, which are often local in scale, this boundary has global ramifications. It is also an example of how tightly interconnected the boundaries are, as atmospheric CO2 concentration is the underlying variable for both the climate change and ocean acidification boundaries.
This is measured in terms of ‘blue’ and ‘green’ water. Blue water is the freshwater held in surface reservoirs. Green water is the fraction of rainfall that is absorbed by soil to feed plants. The freshwater cycle is closely linked to climate change and its boundary mirrors that of the climate boundary. A water boundary related to consumptive freshwater use and environmental flow requirements has been proposed to maintain the overall resilience of the Earth system.
This is driven primarily by agricultural expansion and intensification. Humanity may be reaching a point where further agricultural land expansion at a global scale may seriously threaten biodiversity and undermine the regulatory capacities of the Earth system. The Planetary Boundaries framework proposes that no more than 15% of global usable land should be converted to cropland.
Nitrogen and phosphorus are both essential elements for plant growth, but activities like agriculture, poor wastewater management and fossil fuel use convert more atmospheric nitrogen into reactive forms than all of the Earth’s terrestrial processes combined. A significant fraction of these nutrients make their way to the sea, and can push marine and aquatic systems across ecological thresholds of their own,46 while impacting human health.
This is impacted by GHG emissions and land-use change that releases dust and smoke into the air. Shifts in climate patterns and monsoon systems have already been seen in highly polluted environments, giving a quantifiable regional measure for an aerosol boundary.
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