LET’S DISCUSS: The 9 Planetary Boundaries

Image © J. Lokrantz/Azote based on Steffen et al. 2015.

The boundaries indicate limits within which the human race can continue to thrive for future generations, and crossing them risks irrversible environmental change and damange...

In 2009, Johan Rockström led a group of 28 internationally renowned scientists to identify 9 key processes that regulate the stability and resilience of our planet. They proposed quantitative ‘planetary boundaries’ within which humanity can continue to develop and thrive for generations to come. Crossing these boundaries increases the risk of generating large-scale abrupt or irreversible environmental changes.

Let’s break things down…


What are the 9 boundaries?

1. Atmospheric Aerosol Loading – Particle Pollution

This is not yet quantified, though aerosol particles that enter our atmosphere can be very damaging to human health.  The boundary was proposed primarily because of the influence of aerosols on Earth’s climate system. Through their interaction with water vapour, aerosols play a critically important role in our water cycles, affecting cloud formation and global / regional patterns of atmospheric circulation, such as the monsoon systems in tropical regions. They also have a direct effect on climate, by changing how much solar radiation is reflected or absorbed in the atmosphere.

2. Biochemical Flows – Nitrogen and Phosphorus

Nitrogen and phosphorus are both essential elements for plant growth, so fertilizer production and application is the main concern.  Human activities now convert more atmospheric nitrogen into reactive forms than all of the Earth’s terrestrial processes combined. Much of this new reactive nitrogen is emitted to the atmosphere in various forms rather than taken up by crops. When it is rained out, it pollutes waterways and coastal zones, a threshold we are already crossing.

3. Land System Change

Forests, grasslands, wetlands and other vegetation types are primarily being converted to agricultural land by humans. This activity is undoubtedly behind the serious reductions in biodiversity, and it impacts on water flows and on the biogeochemical cycling of carbon, nitrogen and phosphorus and other important elements. Scientists say that a boundary for human changes to land systems needs to reflect not just the absolute quantity of land, but also its function, quality and spatial distribution.

4. Freshwater Use

The consequences of human modification of water bodies include both global-scale river flow changes and shifts in vapour flows arising from land use change. These shifts in the hydrological system can be abrupt and irreversible. Water is becoming increasingly scarce – by 2050 about half a billion people are likely to be subject to water-stress.

5. Ocean  Acidification

25% of the CO2 that humans emit into the atmosphere is ultimately dissolved in the oceans. Here it forms carbonic acid, altering ocean chemistry and decreasing the pH of the surface water.  Beyond a threshold concentration, this rising acidity makes it hard for organisms such as corals and some shellfish and plankton species to grow and survive, and losing various species would greatly impact on marine ecosystems.  CO2 concentration is the underlying controlling variable for both the climate and the ocean acidification boundaries.

6. Climate Change

We have sadly already passed this planetary boundary due to the levels of CO2 in the atmosphere. We are already losing summer polar sea-ice, and weakening carbon sinks, so really the question now is: how long we can remain over this boundary before large, irreversible changes become unavoidable?

7. Novel Entities – Chemical Pollution

Emissions of toxic and long-lived substances such as synthetic organic pollutants, heavy metal compounds and radioactive materials represent some of the key human-driven changes to the planetary environment. These compounds can have potentially irreversible effects on living organisms and on the physical environment.  At present, we are unable to quantify a single chemical pollution boundary, although the risk of crossing Earth system thresholds is considered sufficiently well-defined for it to be included in the list.

8. Biosphere Integrity – Biodiversity Loss

The demand for food, water and resources are the main drivers for change. The current high rates of ecosystem damage and extinction can be slowed by efforts to protect the integrity of living systems (the biosphere), enhancing habitat, and improving connectivity between ecosystems while maintaining the high agricultural productivity that humanity needs.

9. Ozone Depletion

The stratospheric ozone layer in the atmosphere filters out ultraviolet (UV) radiation from the sun. If this layer decreases, increasing amounts of UV radiation will reach ground level. This can cause a higher incidence of skin cancer in humans as well as damage to terrestrial and marine biological systems. Fortunately, because of the actions taken as a result of the Montreal Protocol, we appear to be on the path that will allow us to stay within this boundary.


All information sourced from the Stockholm Resilience Centre.

Posted by Claire Edwards 29 July 2021

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