The Planetary Boundary (PB) concept was introduced in 2009, and was aimed at defining the environmental limits within which humanity can safely operate. These scientifically prescribed thresholds are laid out in order to protect the Earth’s fragile ecosystems from major global impact. The 9 Boundaries reflect the main Earth system processes. This concept could be thought of as slightly similar to the idea of our ‘Ecological Footprint‘, which provides a way to visualise and measure the impact humans are having on the Earth, or could be described as a measure of the biocapacity budget the planet has to offer. But the PB concept is somewhat more complicated. And the consequences of crossing these boundaries are certainly quite stark.
Johan Rockström and 28 of his learned peers penned a paper defining the limits within which human life on Earth could be supported – a so-called ‘safe operating space for humanity’. Johan is an internationally recognized and respected climate scholar and scientist, who has been working on global sustainability issues for decades. His achievements, published works and the organizations he’s been advisor to are all too numerous to list, but I recommend looking him up and listening to his latest TED Talk.
The authors of this landmark paper state very clearly that these boundaries must not be crossed, lest the stability of Earth and life on her be jeopardized. The paper and concept received an update in 2015 – and they dug even deeper into the theory. It looks at 3 of the Boundaries in particular which could push Earth into a whole new state, as well as the relationship between these 3 and the remaining 6, and importantly they provide a framework so that it can actually be applied at a regional level. It must be acknowledged that none of these 9 Planetary Boundaries operate in a silo – there is deep and immovable interconnectedness between them. So now we’ll take a look at what these 9 Boundaries are and how each is measured.
Stratospheric Ozone Depletion
The stratosphere filters the UV radiation from the sun. The appearance of the Ozone Layer ‘holes’ throughout the 1980’s confirmed that the increase in anthropogenic ozone-depleting chemicals, along with the influence of polar stratospheric clouds, was having a negative impact on our planet. This combination of influences saw the lower layer of the Antarctic stratosphere become all but stripped bare of ozone. Although it appears as though there isn’t a similar global threshold for ozone, global warming will cause the formation of stratospheric polar clouds, and as such could lead to the Arctic region of stratosphere suffering from ozone ‘holes’ – which would be arguably more devastating for the far greater human population in the Northern Hemisphere.
The global actions taken as a result of the introduction of The Montreal Protocol, and subsequent amendments, have since seen the partial healing of the Stratosphere. This layer of the atmosphere is really very thin, but an incredibly important insulation around the Earth. Read more about the Ozone Layer holes here. Increased UV radiation penetration wouldn’t only lead to an increase in human skin cancers, but also great damage to the biological systems on land and in the oceans.
This Boundary is set at > 275 DU (Dobson Units). We are currently within the ‘safe’ zone for this Boundary.
Freshwater Use and the Global Hydrological Cycle
The freshwater cycle is fragile and strongly affected by climate change. Global manipulations of this cycle affect biodiversity, food and health security, and ecological functioning, such as habitats for fish recruitment, carbon sequestration and climate regulation. There are 3 categories of freshwater, and deterioration of any of the three threaten human livelihoods. These are: (i) green water – soil moisture resources, available to plants and soil biota (earthworms, microbes, soil insects); (ii) blue water – runoff and recharged supplies, available in streams and lakes; and (iii) green water flows – that which is from the soil but lost to evaporation, rather than used. Water use is increasing rapidly with a growing population and the need to feed them, so the risk of water scarcity is increasing also. By 2050 water demand is expected to increase by 55%.
The Boundary placed for freshwater is on consumptive use, and is set at ~4,000km3/year, but due to the above factors, relates closely to environmental flows of freshwater also. Crossing this Boundary will significantly increase the risk of collapse of terrestrial and aquatic ecosystems, major shifts in moisture feedback, and freshwater/ocean mixing, at regional to continental scales. With the revised paper, there have been additional basin-level PBs added, based on the amount of water required in the river system to avoid regime shifts in the functioning of flow-dependent ecosystems, and takes into account low-, intermediate- and high-flow months.
Land System Change
Land is converted to suit human use needs all over the planet, At a frightening rate. 75% of the Earth’s ice-free land has been significantly altered by human activity, and most of the remaining 25% of wilderness is contained within only 4 countries (Australia, Canada, Russia and Brazil). We have lost the majority of forests, grasslands and wetlands to agricultural land – in fact half of Earth’s habitable land is now used for this purpose. This is a leading cause of biodiversity loss across the globe, and it also impacts the water flows and the biogeochemical cycling of carbon, nitrogen and phosphorous.
The Boundary for land use was originally set at no more than 15% of the Earth’s ice-free land being used for cropland. And there was a further qualitative element to this: the land chosen should be the most productive, and damage to this level of productivity from land degradation, loss of irrigation supply, and competition for land (from urban expansion or biofuel production) must be strictly controlled. However with the revision paper came a change – rather than cropland, the control variable is now forest cover from the 3 major types of forest biomes: tropical, temperate and boreal. The biome-level Boundary set for tropical and boreal forests is 85% and for temperate forests it’s 50%.
Biogeochemical Flows
The biogeochemical cycles of nitrogen and phosphorous were the only ones originally included in this Planetary Boundary. And they have been significantly impacted by human activity – industrial and agricultural processes. But now this Boundary includes other elements, such as silicon (the carbon cycle would fit here, but instead is included in the Climate Change Boundary), and they acknowledge that the ratio between elements in the environment may have an impact on biodiversity.
Fertilizer production and application is the largest problem – because nitrogen and phosphorous are important for plant growth. But most of the nitrogen is converted to reactive forms in the atmosphere, rather than being taken up by plants, so ends up polluting waterways and coastal zones, and accumulating in the biosphere. Similarly, most of the phosphorous (which is not a natural element, rather a mined and finite fossil mineral) intended for improving plant growth finishes up in aquatic systems. The affected waterways could then see a large algal bloom, which in turn results in a heavy bacteria load to consume the algae – leading to higher oxygen uptake from the water, therefore decreased oxygen availability for marine life.
The limits for the other elements included in this PB are yet to be set, but will likely be required. The Boundary for phosphorous has a two level approach. The global-level Boundary is set at a flow of 11 Tg/year from freshwater systems to the oceans; and the regional-level Boundary has been set to a flow of 6.2 Tg/year from fertilizers to erodible soils. The nitrogen boundary for eutrophication* of aquatic ecosystems is 62 Tg/year. The authors have calculated an N:P ratio, which is 11.8, highlighting the close relationship between these two elements.
* The over abundance of nutrients in the water stimulating aquatic plant life growth and subsequently leading to decreased dissolved oxygen presence.
Atmospheric Aerosol Loading
This Boundary exists primarily due to the impact aerosols have on Earth’s Climate System, which is based on two things: their interaction with water vapour and subsequently their role in the hydrological cycle affecting cloud formation; and their ability to change the amount of solar radiation which is either reflected or absorbed. But due to aerosols referring to all particles in the atmosphere, including carbon, soot, dust, smoke and emissions from nitrates and sulfates, there are also strong implications for public health from excessive volumes of them.
This Boundary is measured in Aerosol Optical Depth (AOD) and there have been conservative levels applied for several reasons: the complexity of aerosol type, source and seasonal variation makes this boundary application difficult; and the power of the aerosols is such that it can decease monsoon activity dramatically, with only a small rise. So the Boundary is set at 0.25 AOD, with a zone of uncertainty of 0.25-0.50 AOD.
Introduction of Novel Entities
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, and as such can cause irreversible damage to the planet and living organisms on it.
Due to the number of substances (over 100,000 major compounds used in commerce – but many more if the list included nanomaterials and plastic polymers, which degrade to microplastics), and the relatively poorly understood effects from many of these compounds, no Boundary has yet been applied for chemical pollution.
Ocean Acidification
The absorption of carbon dioxide by the oceans means that this is dissolved and forms carbonic acid in the water. The result is alteration in water chemistry – lowering the pH, or making the water acidic. We covered the importance of carbon sequestration in the oceans here and the negative impacts of ocean warming (including acidification) here. But as a quick recap: rising acidity makes it very difficult for some corals, shellfish and plankton to survive, based on the reduction of aragonite saturation state (a type of calcium carbonate) being formed – and this has huge implications for the other marine life. There has been an increase in ocean acidity (presence of H+ ions) by 30% and a 16% decrease in carbonate concentrations since pre-industrial times, due to increase in atmospheric CO2.
Aragonite (Ωarag) saturation state must therefore be maintained at a level of 80% or higher, compared with pre-industrial average levels.
Changes in Biosphere Integrity
The increasing demand on our planet for food, water and natural resources have been placing dangerous amounts of pressure on the biodiversity on our planet. The current rates of extinction and biodiversity loss are very high and must be brought down to limit the risk of these decreases being irreversible and to limit the knock-on effects to entire ecosystems – and it’s predicted that this current level of loss will be the 6th mass extinction event on Earth (and the first one to be linked directly to human activity).
To accurately provide boundaries for biodiversity loss further research is needed to capture reliable data – because global extinction rates are often measured inaccurately and with a time lag. The authors suggested an interim label of Biodiversity Intactness Index (BII) and have placed a preliminary Boundary of 90% on this.
Climate Change
Unsurprisingly, the predominant aspect of this Boundary is carbon dioxide concentration. We understand that this is the major driver of climate change as it’s a powerful and very long lasting greenhouse gas. But the other thing to consider, and which has therefore had a limit placed on it is Radiative Forcing. This is the amount of change to the solar radiation reaching the Earth as influenced by natural or anthropogenic factors. The change, therefore the radiative force, can be either negative (having a cooling effect on the Earth’s surface) or positive (having a warming effect). Each of the identified climatic factors which have a radiative force have their forcing values calculated between 1750 and present day. It is of course undeniable that we are in the midst of experiencing devastating climate change effects: sea level rise; increased frequency and severity of heat waves; many regions of the world are experiencing storms, extreme rainfall and even rain bombs; increased frequency and length of droughts in some regions of the world; more fierce and larger wildfires; and the massive loss of ice from Antarctica and Greenland. But things will get worse unless we stop our momentum of passing this Planetary Boundary.
The CO2 Boundary has been set at 350 parts per million (ppm), which we have already exceeded. The Radiative Forcing Boundary has been set at +1.0 W m-2 (ie., positive force of 1 Watt of solar radiation per square meter) – which we have also exceeded.
So now that we understand a little more of the complexity and very fine balance which has been in place on Earth for hundreds of thousands of years – and how quickly the human race has upset that – we might see the urgency to change from another new perspective. Mitigating the risk of worsening effects of climate change is not just about trying to save some far flung place from rising sea levels: it’s about global public health, and extinction of wildlife who will find the ‘new’ world inhospitable. Stopping the release of harmful chemicals and aerosols isn’t just about having healthier waterways: it’s about removing further pollutants from the air for the sake of public health and reducing global warming. Maintaining biodiversity isn’t just about saving the animals because we like animals: it’s about the knife-edge balance of all life being in some way dependant on another. Every one of the 9 Planetary Boundaries is linked to at least one other, and so maintaining an even keel for all is vital for a symbiotic and balanced and productive human existence on Earth.
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