Planetary Boundaries

Planetary Boundaries is a framework for expressing the resilience of the Earth as a system, and the risk that human activities are destabilizing the system. The Earth system has been remarkably stable for thousands of years. Losing that stability would be very harmful to humans and society.

The diagram shows which Earth system processes are being pushed to dangerous levels, and which processes are still at relatively "safe" levels with low global risk.

Select a wedge to find out more about a process and how we're monitoring it.

Find out more about the Planetary Boundaries framework through the tabs above, and watch our introductory video.

Earth-system processes

Life on Earth is part of a huge system of processes and connections. Living creatures affect each other and impact the chemistry and even the climate of the world around them, and the effects go both ways in a great feedback loop. Humans have the capacity to alter this entire Earth system through our own activities. We also have the capacity to help maintain the Earth system in a healthy state even as we grow and advance; to maintain it, we need to monitor the system's "health". The Planetary Boundaries framework was designed to help us do this.

The processes of the Earth system behave in ways that we can understand, observe, and even predict. The Planetary Boundaries framework starts with the physical, chemical, and biological processes by which we humans are changing the Earth system.

Control variables

Each wedge in this diagram represents one of these processes. We monitor its "status" using measurements such as concentrations or flow rates. These measurements are called "control variables" because they represent key aspects of the processes they're used to monitor.

Some processes are monitored using more than one control variable; for example, the status of Biogeochemical Flows uses both nitrogen flow and phosphor flow.

The processes that make up the Earth system are tightly connected. This means a change in one process will change other processes -- which will in turn change the first process again, and so on.

All human activity will affect Earth system processes in some way. Because the processes are so connected, if our activity changes them too much, we risk destabilizing the entire Earth system.

However, because we know how the Earth system has changed in the past and we can predict the impacts of what we do, we can figure out how much is "too much". Then we can set boundaries for ourselves to maintain the "health" of the Earth system. If a variable we're monitoring crosses a boundary, we know that process is at a high risk of serious problems on a global scale.

In the diagram, markers that are outside of the "safe limits" circle show variables that have crossed a boundary.

A red marker ("definitely high risk") means the variable is so far past its boundary that we know the process can destabilize the system.

An orange marker ("probably high risk") means the variable has crossed its boundary and is likely to destabilize the system, but it might not be quite far enough yet to do that. This hopefully gives us some warning.

A green marker ("low risk") means the variable has not crossed the boundary we have chosen, so that system is likely to remain stable. In other words, the Earth system and human society can likely adapt to any changes taking place and remain healthy overall.

The boundaries framework is very similar to the way we monitor our own health. Think about having your blood pressure checked. There's a "healthy range" of blood pressure; if yours is outside of that range, you need to find out what's causing it and (generally) do something about it. If it's outside of the healthy range by a lot, it's probably already having an impact on your health, and you need to fix the problem immediately or risk serious consequences. If it's outside by only a little, it might still be impacting your health, but it might not cause any further problems as long as you correct the cause soon.

The "boundary" for blood pressure is a value that medical professionals have chosen. It's chosen so that it's close to the levels where serious problems are likely, but far enough that it can act as a warning before things get bad.

Climate Change

All other Earth-system processes are affected by changes to the global climate, including temperature, weather patterns, and more. This is what makes climate change one of the core processes.

Control variables

The climate change process is monitored using carbon dioxide concentrations in the atmosphere; CO2 is the most important of the greenhouse gases.

In order to maintain a stable climate, we want to keep carbon dioxide concentrations in the atmosphere at or below .

If the Earth receives more energy from the sun than it releases into space, the temperature will increase. This imbalance is measured as a difference in intensity.

For more information on energy imbalance, see the KCVS applets Planetary Climates and The Earth's Radiation Balance.

The Earth's climate is governed by feedback cycles with other systems. When the climate changes, this shifts the balance in other systems, which can further affect the climate. Some of these feedbacks drive more change, while others can counter the changes and help to stabilize the Earth system.

The Earth's surface gets most of its energy from the sun. Much of that energy is reflected or re-emitted back into space. Greenhouse gases in our atmosphere, such as carbon dioxide, trap some of this energy so that it warms up the planet instead of escaping. Although we need to trap some energy to maintain a climate suitable for life, too much energy causes global temperatures to rise. This affects the biosphere, water cycles, atmosphere, and other processes.

Some examples of connections to other Earth-system processes:

Climate change and biosphere integrity interact to set the "state" of the Earth-system as a whole, determining what sort of ecosystems will thrive and what will struggle. One way this is seen is in "parallel evolution" and in “convergent evolution” where species in different parts of the world develop similar traits independently in response to similar climates.

Ocean acidification decreases the ocean’s ability to act as a carbon sink because organisms that act as biological carbon pumps are less efficient due to the changing conditions and the decreased availability of carbonate ions which is essential in forming calcium carbonate shells. The decreased capacity of oceans to be a carbon sink then increases the effects of climate change.

Changes in drought-flood cycles, because of climate change, can impact biogeochemical flows through erosion and nutrient cycles (Lade et al., 2020).

Historically, the changes in the state of the Earth-system can be seen in geological records of biogeochemical processes. For example, the great oxidation event during the Paleoproterozoic era allowed iron to be oxidized from a soluble form into an insoluble form which settled out and formed iron rich sediments.

Examples of STEM curriculum

Energy Systems; Types of Electromagnetic Radiation; Gas Laws; Kinetic Molecular Theory; Infrared Spectroscopy; Molecular Vibrations; Earth’s Energy Balance; Positive Feedback Loops; Carbon Capture and Storage; Geoengineering; Atmospheric Residence Time; Gas Properties; Gas Global Warming Potential; Black Body Curve; Isotopes (in ice cores to measure climate)

Biosphere Integrity

All life processes depend on each other in a delicate balance. An example of this balance can be found in the relationships between predators and prey. For example, sea otters control the sea urchin population through predation. Without the sea otters, the sea urchin population can grow uncontrollably which then decimates kelp forests. This balance of life processes can be disrupted by environmental change from anthropogenic sources. The loss of biosphere integrity not only affects the functioning of ecosystems but also its resilience to further physical changes. Biosphere integrity also provides ecosystem functions which support other earth systems. The loss of biosphere integrity is influenced by habitat loss, pollution, climate change and over harvesting of natural resources (Rafferty, 2019).

Control variables

Genetic diversity provides the potential for species to better adapt to new conditions brought about by changes in the other Earth system processes. It is measured as number of extinctions per million species per year.

In order to have a healthy planet, all species need to fill specific roles in the ecosystem. If certain functions are not fulfilled, the integrity of the biosphere is lost.

Life processes depend strongly on each other -- consider predators and prey as a simple example -- so harm is felt throughout the biosphere.

Some examples of connections to other Earth-system processes:

Climate change can affect biosphere integrity by changing the environment and ecosystems rapidly, making it difficult for organisms to adapt quick enough. For example, an increase in water temperatures due to climate change can cause coral bleaching, compromising the habitat for many organisms. Another example is how climate change decreases the amount of polar ice sheets affecting a wide array of organisms including polar bears (Rockström et al., 2009). The reduction of biosphere integrity reduces the productivity of ecosystems. This makes ecosystems less able to sequester carbon which affects climate change (Lade et al., 2020). Climate change can affect biosphere integrity by changing the environment and ecosystems rapidly making it difficult for organisms to adapt quickly enough. For example, an increase in water temperatures due to climate change can cause coral bleaching, reducing the habitat for many organisms. Another example is how climate change decreases the amount of polar ice sheets affecting a wide array of organisms including polar bears (Rockström et al., 2009).

Examples of STEM curriculum

Diversity; Adaptations; Genetics; Zoology; Population Ecology; Evolution; Genetically Engineered Organisms; Biomes; Conservation Biology

Land-System Change

Land system changes are defined as the amount and pattern of land system change in all terrestrial biomes: forests, woodlands, savannas, grasslands, shrublands, tundra, and so on. The land system change boundary includes a focus on the biophysical processes in land systems that directly regulate climate, exchange of energy, water, and momentum between the land surface and the atmosphere.

Control variable

Area of forested land globally as percentage of original forest cover.

Some examples of connections to other Earth-system processes:

When land is cleared for large monocrop plantations the biodiversity decreases thereby diminishing the biosphere integrity (Lade et al., 2020). Clearing land is often done using fire, which releases the stored carbon as carbon dioxide, the main control variable for climate change. This process also destroys the natural carbon sinks which in turn affects climate change (Rockström et al., 2009). Climate change could also decrease the productivity of agricultural land which may cause more land to be cleared to compensate for the decrease in yield (Lade et al., 2020). Tropical forests also impact climate through evapotranspiration which can change when forests are cleared (Lade et al., 2020). An increase in agricultural land requires more freshwater to irrigate crops.

Examples of STEM curriculum

Ecology; Types of Ecosystems; Biospheres; Carbon Sinks; Food Production

Freshwater Use

Freshwater use is blue water from rivers, lakes, reservoirs, and renewable groundwater stores. Disruptions and/or changes in the environment can impact freshwater use.

Control variable

Globally, maximum amount of consumptive blue water use.

Some examples of connections to other Earth-system processes:

Freshwater is connected to land use change because an increase in agricultural land usually requires more water to irrigate crops.

Freshwater can be used at a rate that reduces biosphere integrity. For example, to meet the freshwater demand rivers are dammed which can cause a loss of biodiversity. When dams are built large areas of land and organic material become submerged underwater. This organic material undergoes decomposition in an anaerobic environment which produces methane, a powerful greenhouse gas which contributes to climate change. Climate change also changes the patterns of rainfall, causing droughts and increased severity of storm events which can compromise the amount of freshwater that can be safely extracted (Rockström et al., 2009).

Examples of STEM curriculum

Water Cycle; States of Matter; The Remarkable Properties of Water (Specific Heat Capacity, Enthalpy Change of Vaporization, Change of Density with Temperature, Boiling Point, Surface Tension); Intermolecular Forces

Biogeochemical Flows

Biogeochemical flows are pathways between living organisms and the environment. Specifically here the focus is on how phosphorus and nitrogen flow into the environment. The flow of phosphorus and nitrogen is primarily from fertilizer application. Both the regional-level phosphorus and nitrogen boundaries have been transgressed due to a few agricultural regions of very high application rates.

Control variables

Industrial and intentional biological fixation of nitrogen.

Flow of phosphor from freshwater systems into the ocean globally, and flow from fertilizers to erodible soils regionally.

Some examples of connections to other Earth-system processes:

Excess nutrient inputs, mostly from agricultural sources, can affect both freshwater availability and biosphere integrity (Steffen et al., 2015). Nitrogen and phosphorus species can enter water through agricultural run off, a nonpoint source pollutant. These additional nutrients can cause algae blooms. The decomposition of algae requires oxygen which depletes the dissolved oxygen in the water creating an anoxic environment that is harmful to fish and other organisms. The decomposition of algae blooms also releases greenhouse gases contributing to climate change.

Nitrogen fixation through the Haber-Bosch process is energy intensive and currently requires methane as a feedstock. The process emits large amounts of carbon dioxide which contributes to climate change.

Nitrous oxide is also a powerful greenhouse gas that can contribute to climate change. Nitrous oxide is released through agricultural processes (especially fertilizer), energy use, and industrial processes.

Ammonia released by fertilizers reacts with nitrogen or sulfur oxides to form aerosols.

Land use change to agricultural land usually requires both an increase in freshwater use and nutrient inputs disrupting the biogeochemical flow processes.

Examples of STEM curriculum

Reaction Thermodynamics; Reaction Kinetics; Dynamic Chemical Equilibrium; Haber-Bosch Nitrogen Fixation; Catalysts; Le Chatelier’s Principle; Activation Energy; Atom Efficiency (green chemistry); Nitrogen Speciation; Plant Life Cycles; Nitrogen Cycle; Phosphorus Cycle; Types of Rocks; Erosion; Gases; Chemical Reactions; Biochemistry; Bioavailability; Chemical Reactions; Chemical Change; Enthalpy Change

Ocean Acidification

The ongoing decrease in pH in the ocean is linked to the CO2 concentrations. The increasing concentration of free H+ ions in the surface ocean is a result of increased atmospheric CO2. This in turn effects carbonate chemistry, and lowers the saturation state of aragonite, a form of calcium carbonate formed by multiple marine organisms. These changes can interfere and make it more challenging for marine organisms to form shells and skeletons, while existing shells may begin to dissolve.

Control variables

Ocean acidification is measured in terms of the average global ocean saturation state of carbonate ions with respect to aragonite, as a percentage of pre-industrial aragonite saturation state. To maintain ocean ecosystems, we need to stay above of the pre-industrial aragonite saturation state of the mean surface ocean. Currently this level is about of preindustrial values and is decreasing as we further transgress beyond the climate change boundary of CO2.

Some examples of connections to other Earth-system processes:

Ocean acidification reduces the amount of dissolved calcium carbonate available in the water which is needed by many marine organisms such as shellfish and coral (Steffen et al., 2015). Coral reefs are a diverse ecosystem which supports many other organisms affecting biosphere integrity (Rockström et al., 2009).

Climate change is directly linked with ocean acidification, as an increase in carbon dioxide in the atmosphere increases the amount of carbon dioxide dissolved in the ocean (Steffen et al., 2015). Land systems change releases carbon dioxide (see land use change processes tab) which in turn affects ocean acidification.

Examples of STEM curriculum

Acid-Base Chemistry; Solutions Chemistry; Equilibrium; Solubility; pH Dependent Speciation; Gas properties; Henry's law; Concentration Calculations; Kinetic-Molecular Theory of Gases; Buffer Solutions; Precipitation; Complexation

Atmospheric Aerosol Loading

Aerosols are small particles suspended in the atmosphere. These can be pollutants that condense into small droplets, as well as smoke and dust that we release into the air. Aerosols affect cloud formation, as well as reflect or absorb solar radiation, depending on the type of particle, which can affect our climate.

Control variables

Atmospheric aerosol loading is measured by the overall aerosol concentrations in the atmosphere. Although we can measure these, scientists are still working on how to set boundaries on this process. The interactions between aerosols, climate, biosphere, and other systems make this a challenging task.

Technically, an aerosol is any fine particles suspended in air or some other gas (such as CFCs). These particles could be liquid droplets or solid. Fog, dust, and volcanic ash are examples of naturally-occurring aerosols. Human activity releases aerosols such as smoke and sulfates.

Fine aerosols can reach the most delicate parts of our lungs can cause serious damage to humans and other animals. Aerosols can also affect weather patterns, by altering cloud formation for example.

Researchers are studying the mechanisms by which aerosols affect both health and climate. We will need to understand these complex mechanisms and interactions in order to set a planetary boundary here.

Some examples of connections to other Earth-system processes:

By affecting cloud formation, aerosols disrupt the natural water cycles affecting the freshwater earth system. The disturbance in the natural water cycles, caused by aerosols, changes the environment affecting biosphere integrity (Lade et al., 2020) (Rockström et al., 2009). Aerosols alter weather patterns and therefore change erosion which alters biogeochemical flows.

Aerosols reflect incoming solar radiation. This reduces the amount of energy the earth absorbs which mitigates the effects of climate change. Aerosols can also reflect outgoing radiation back to the earth’s surface. However, some aerosols such as black carbon, can also settle on high albedo surfaces, such as ice, causing more radiation to be absorbed instead of being reflected (Baird et al., 2012).

Examples of STEM curriculum

States of Matter; Properties of Matter; Light and Optics; Albedo

Stratospheric Ozone Depletion

Ozone (O3) in the stratosphere filters out ultraviolet radiation that is harmful to biological systems. Certain chemicals that we release into the atmosphere, such as chlorofluorocarbons (CFCs), cause ozone molecules to break apart, depleting the ozone layer.

Control variables

For monitoring this process in terms of its boundary, ozone levels are measured over mid-latitudes (away from the poles), since that is where most humans and human activity are found. To keep out enough ultraviolet radiation, our goal is to keep the concentration of ozone in the stratosphere at or above .

Ozone (O3) in the stratosphere filters out ultraviolet (UV) light that is harmful to biological systems. We're familiar with UV light causing sunburns; without the ozone "layer" it would be much more intense. It could hurt animals and plants, including food crops, and affect animal behaviour and other aspects of ecosystems.

Some of the novel entities that we've released into the atmosphere, particularly CFCs (chlorofluorocarbons), destroy ozone by breaking the molecules apart. Weather patterns cause these materials to collect near the poles, especially the south pole, and the unusual conditions of Antarctic winter and spring enable these materials to do serious damage to the stratospheric ozone. In the mid-1980's, it was discovered that the ozone over the Antarctic had become so depleted it was called a "hole".

Soon after this discovery, researchers identified the cause, and nations around the world agreed to find ways to stop the release of CFCs and other ozone-harming substances into the atmosphere. The Antarctic ozone "hole" is still there, in part because these substances remain in the atmosphere for a long time, but it is no longer growing. In fact, recent evidence (Reiny 2018, Strahan 2018) suggests it is starting to recover.

Some examples of connections to other Earth-system processes:

Stratospheric ozone depletion is caused by the introduction to the stratosphere of certain novel entities, as discussed in the novel entities process tab. These substances include CFCs, which are used as refrigerants. In this way aerosol loading can affect ozone depletion (Lade et al., 2020).

Because of the harm that UV radiation can do to plants and animals, ozone depletion lets more UV radiation through the atmosphere, impacting biosphere integrity.

Ozone depletion is connected to biogeochemical flows because nitrous oxide is an ozone depleting substance. Main sources of nitrous oxide are from agricultural processes, and energy and industrial applications.

Tropospheric ozone is a greenhouse gas which affects climate change, however as many ozone depleting substances in the stratosphere are greenhouse gases in the troposphere, the relationship of ozone and climate change is complex (Lade et al., 2020). Climate change and the increase of carbon dioxide in the atmosphere cools the stratosphere because more heat is trapped in the lower levels of the atmosphere. The cooling of the stratosphere slows the rate of ozone depleting reactions (Lade et al., 2020).

Examples of STEM curriculum

Gas Laws; Chemical Reactions; Types of Electromagnetic Radiation; Parts of the Atmosphere; Bonding; Molecular Structure; Free-Radical Reactions; Catalytic Processes; Chemical Change

Novel Entities

Novel entities are materials introduced by human activity, including chemical pollution, organisms, microplastics, and many others.

Control variables

Researchers are working on how best to quantify such a broad process, but there is not yet a scientific consensus on what a single control variable should be. One important aspect of this working to find criteria we can use to identify novel entities that are likely to cause global problems, before we release them into the environment.

Humans have recently developed many powerful and useful new substances, most of which eventually end up in our environment on a large scale. This can be harmless, and even when pollution causes serious damage it may be in a particular place. Sometimes the novel entities we release have serious effects on global systems, which can risk destabilizing the Earth system as a whole.

Some examples of connections to other Earth-system processes:

A well-known example of this process is the release of CFCs (chlorofluorocarbons), used primarily in air conditioning and refrigeration systems; these were first thought to be harmless, but caused serious damage to the stratospheric ozone layer, including the seasonal formation of a "hole" over the Antarctic. CFCs along with their replacements HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons) are also potent greenhouse gases which influence climate change.

Acid rain damages ecosystems on a large scale, reducing the biosphere integrity. Acid rain is caused by sulphur dioxide and nitrogen oxides produced by transportation and industrial activity.

Both examples are to a large extent success stories. By changing the way we use and handle CFCs, and finding alternative substances, we've stopped the growth of the "ozone hole" and there are signs that it's starting to recover (Merzdorf 2020). Policies in a number of countries have been largely effective at nearly eliminating acid rain in many places, though it's still a serious problem in some countries (Ogden 2019). Government programs to neutralize acidified lakes and rebuild the ecosystems are starting to show success, helped by the biosphere itself as microbes in lake sediments are actively neutralizing the acidic inputs to lakes (Rudd et al. 1986).

Microplastics are another major threat to biosphere integrity. Microplastics can accumulate in aquatic environments where they can be ingested by organisms. These microplastics can then disrupt the organism’s digestion tract and release chemical pollutants into the organism’s tissues (Baird et al., 2012). These plastics and chemical pollutants accumulate up the food chain through biomagnification (Baird et al., 2012).

Another well known example of a novel entity is a group of compounds called persistent organic pollutants (POPs). POPs are also known as “forever chemicals” as they do not breakdown in the environment and can bioaccumulate in organisms due to their lipophilic properties decreasing the biosphere integrity. Many POPs are transported to polar regions due to atmospheric circulation and the volatility of the compounds. Climate change and an increase in temperature can affect the transportation of POPs by allowing the POPs to further concentrate in polar regions.

Examples of STEM curriculum

Pollution; Waste Management; Bioaccumulation; Persistent Organic Pollutants; Toxicology; Invasive Species; Genetically Engineered Organisms; Microplastics; Photochemical Smog