The planetary boundaries are nine areas that represent a “safe operating space” for humanity. We have crossed six of them. This page offers a basic breakdown of each boundary to help those not trained in earth science understand what they mean.
- What are the planetary boundaries?
- Boundaries we have passed
- Climate change
- Radiative forcing
- CO2 concentration
- Biosphere integrity
- Genetic diversity
- Planetary function
- Land system change
- Freshwater change
- Blue water
- Green water
- Biogeochemical flows
- Phosphorous (P)
- Nitrogen (N)
- Novel entities
- Boundaries at immediate risk
- Ocean acidification (WIP)
- Boundaries in the safe zone
- Atmospheric aerosol loading
- Stratospheric ozone depletion
What are the planetary boundaries?
The physical status of the Earth since the last ice age has allowed human beings to flourish. Because the average global temperature was relatively stable for those 10,000 years, we were able to begin to settle down and grow our food rather than moving from place to place to hunt and gather it. It was this ability to settle that encouraged the growth of knowledge, ideas, and technology. However, once we figured out how to extract and burn fossil fuels and industrialize agriculture, humans became a major driving force of change on our planet.
In 2009, a group of 29 scientists came together to propose that nine key areas ensure a safe operating space on Earth—the planetary boundaries. The idea behind the boundaries was to look at the safe and stable Earth systems that had existed for the last 10,000 years but were now being thrown out of equilibrium by human activities.
The scientists from that 2009 paper argued that humanity was risking irreversible and catastrophic changes if we continued down our current path and that we had already crossed the boundary for three of those systems. Today, all nine of the boundaries have been quantified and analyzed, and we have now passed six of them.
- 2009: Planetary Boundaries: Exploring the Safe Operating Space for Humanity
- 2015: Planetary boundaries: Guiding human development on a changing planet
- 2023: Earth beyond six of nine planetary boundaries
Boundaries we have passed
- Boundary: +1 watts per square meter (W/m2)
- Where we are: +2.91 W/m2
Radiative forcing boils down to the difference between energy coming from the sun, versus how much energy leaves the Earth and escapes into space. Higher radiative forcing means that more of the sun’s energy is getting stuck in our atmosphere. Greenhouse gases (GHGs) prevent heat from escaping back into space, so as we have increased GHGs by burning fossil fuels, we have also increased radiative forcing.
Imagine a pot of boiling water. If you keep the lid on, it traps the heat, and the water boils faster. If you leave the lid off, some of that heat escapes into your kitchen, and the water doesn’t warm as quickly. By increasing the GHG concentration in our atmosphere, we have put the lid on our pot.
How do we measure it?
Each GHG absorbs and releases heat differently. Basically, we measure the concentration of each gas in the atmosphere, and then we calculate the warming influence of that gas. Once we add them all together, we have our radiative forcing.
- Boundary: 350 parts per million carbon dioxide (ppm CO2)
- Where we are: 418 ppm CO2 (as of Sep 15, 2023)
CO2 concentration ties into radiative forcing in that it is a potent GHG, but how do we know it is increasing and that it comes from fossil fuels?
How do we measure CO2 concentration?
There is a volcano in the middle of the Pacific Ocean called Mauna Loa, where CO2 in the atmosphere is measured every day. Mauna Loa is so remote and tall that there isn’t contamination from direct CO2 sources, so it is a great place to measure CO2 in real-time in the atmosphere. You can actually see the Northern Hemisphere “breathing” in the record—CO2 dips during the summer when plants are actively growing and rises in the winter when they aren’t. This record is called the Keeling Curve, and it has been measured since 1958.
What is being measured is not technically a concentration, because measuring concentration would depend on external factors like pressure and temperature, and would make the measurement inconsistent. Mauna Loa is actually counting the number of CO2 molecules that are found in a million molecules of dry air (so after any water vapor is removed). It is generally called a concentration because that is an easier concept to understand than what it is really called: a mole fraction.
By the time a mass of air reaches Mauna Loa, it is well mixed and representative of the Northern Hemisphere as a whole, but why is this station located in the Northern Hemisphere? Most of the activities that affect CO2 levels are located in the Northern Hemisphere: manufacturing, automobiles, coal power plants, etc. This makes it a good representation of how much CO2 is increasing overall, but you will also see a similar trend in the growth of CO2 at other measurement locations around the world because the rate at which CO2 increases is fairly steady globally, it is well mixed in the atmosphere, and it stays there for about 100 years.
Note: The Mauna Loa volcano erupted in November 2022, and measurements were disrupted until July 2023. In between, the official Keeling Curve measurements were replaced by a station on the Big Island of Hawai’i called Maunakea.
How do we know this is high?
We actually have a record of atmospheric CO2 concentration going back 800,000 years, thanks to ice cores that have been retrieved from the ice sheets on Greenland and Antarctica.
- This is how it works:
- Fresh snowfall on the ground is actually about 90% air; that’s why it is so fluffy—air is in between the snow crystals.
- As snow continues to fall, the layers on the bottom get squished down, and that seals the air into little pockets.
- Eventually, that pressure will continue to build, and the compressed snow will turn into ice, but the air from when that snow originally fell still remains trapped in air bubbles.
- If we drill down into the ice sheet, we can collect and analyze this record of snow and air from hundreds of thousands of years ago.
How do we know if CO2 comes from fossil fuels?
This all has to do with the three different types (isotopes) of carbon. Since we know how much of each type of carbon should be in the atmosphere, any changes in those numbers will indicate a change in the source of the carbon.
- Carbon-12 (C12): Light carbon. Used by plants. Makes up 98.9% of all carbon on Earth.
- Carbon-13 (C13): Heavy carbon. Comes from volcanoes. 1.1% of all carbon on Earth.
- Carbon-14 (C14): Radioactive carbon (aka radiocarbon). C14 is found in very low amounts, like less than 0.0001% of all carbon.
Think of it like this: Your friend Bob gives you one apple a day, and your friend Sue gives you one pear every day. If you don’t eat anything, you know you will have 50% apples and 50% pears in your fridge. If you end up with more than 50% of either fruit, you know who it came from.
Proof point #1: When we burn fossil fuels, we are increasing the concentration of C12 in the atmosphere
We know what the ratio of C12 to C13 in the atmosphere was before the Industrial Revolution because of the air bubbles in ice cores. Fossil fuels are made up of very old plant material, and plants use C12 when they grow. When we burn fossil fuels, they release carbon dioxide, and that carbon dioxide will be made up of C12. Since we are seeing more CO2 made from C12 in the air, we know it had to come from burning plant material.
- Here is a pretty clear explainer video about how this works if you want to learn more.
Proof point #2: When we burn fossil fuels, we decrease the concentration of C14 in the atmosphere.
The carbon in fossil fuels does not have any C14, so CO2 from fossil fuels does not have any C14. Have you ever heard of radiocarbon dating? This is how we know, for instance, how old the bones are from an archaeological site. Living organisms constantly absorb C14, but after they die, the C14 will begin to change into another material (nitrogen). By measuring how much C14 is left, scientists can estimate how long that organism has been dead.
Radiocarbon dating is only effective going back 60,000 years, because for anything older than that, all of the C14 will have totally decayed away. In other words, anything older than 60,000 years (or material that was never alive, to begin with) will not have any C14, meaning.
Since fossil fuels are millions of years old, there is no C14 in that material, and the CO2 that is released when fossil fuels are burned will not have any C14 in it. So, CO2 coming from fossil fuels will reduce the overall concentration of C14 in the atmosphere.
The biosphere is the area above and below the Earth’s surface that can sustain life. The biosphere doesn’t just refer to the living organisms, but also the material that feeds into sustaining that life.
- Boundary: <10 extinctions/million species-years
- Where we are: >100 extinctions/million species-years
This has to do with how quickly different species can go extinct before it negatively impacts ecosystem function. Extinctions are measured in “extinctions per million species-years” (E/MSY), which really means for every one million species there are on earth, one species will go extinct every year.
Under “normal” extinction rates (how many species went extinct before humans started messing with the planet), up to 2 species across all of the animal groups can be expected to go extinct every million years (and that is a generous estimate; others put the number between 0.1 and 1). So, how many extinctions are we experiencing now? More than 100 E/MSY. That means that humans are causing species to go extinct 50 to 1000 times faster than they should, and we’ve lost more than 10% of our planet's plant and animal diversity over the last 150 years because of it.
- Boundary: 10% human appropriation of net primary productivity (HANPP)
- Where we are: 30% HANPP
Here, they are looking at net primary productivity (NPP) and specifically how much NPP humans have taken away from the biosphere. NPP is the amount of biomass (carbon) that goes into the biosphere minus the energy it uses to consume that carbon, so you can think of it as a measure of carbon sequestration.
A natural forest has a higher NPP than cropland, so it sequesters more carbon. As people have been converting forests and grasslands over to agriculture, we have reduced the planet's NPP, and that means we have reduced the amount of carbon that can be drawn out of the atmosphere. The biosphere should have an NPP of 71.4 gigatonnes (Gt) of carbon/year (based on what it was before us and how much CO2 is in the atmosphere now), but it is currently at 65.8 Gt C/year.
How do we measure it?
First, there are two terms to know:
- Photosynthesis: Plants consume nutrients from the soil and CO2 from the air to grow, and energy from the sun powers this process.
- Respiration: If there is not enough sunlight to power this process, plants release CO2.
NPP is the difference between photosynthesis and respiration. To get this value on a global scale, we can divide the planet into grids and look at the ecosystems contained within each grid. Based on the type and coverage of vegetation there, we can calculate the NPP for each grid, and then we add them all together. Here is a video showing how it’s done if you’re curious.
Land system change
- Global: 75% original forest cover
- Tropical: 85% original forest cover
- Temperate: 50% original forest cover
- Boreal: 85% original forest cover
- Where we are:
- Global: 54% original forest cover
- Tropical: 60% original forest cover
- Temperate: 30% original forest cover
- Boreal: 60% original forest cover
This boundary looks at how much natural forests have been lost since people began to change the world around them. Since people started settling down, we have been cutting down forests and converting that land into agriculture, buildings, roads, parking lots, shopping malls, really the list is almost endless. Forest fires are another cause of rapid land system change.
The reason they are looking at old, diverse, natural forests and not just tree cover is because these ecosystems are much more biologically active and sequester more carbon than most reforestation projects (i.e. planting new trees to replace what has been cut town). More than half of all reforestation is in the form of intensively managed monoculture - meaning one type of tree planted in the same way we grow any crop. These tree plantations trap about 40 times less CO2 than a natural forest would have if left alone and can even end up being a source of emissions.
We can’t afford to waste fresh water. Of all the water on Earth, less than 3% is fresh, and most of that (about 2.5%) isn’t accessible to us—it’s locked up in ice, too polluted, or too far underground to extract affordably.
The previous planetary boundaries assessments only looked at water consumption as a measure of total freshwater change. Splitting this boundary into two components was done to give a better representation of the true extent and impacts on the water cycle, and to allow more consistent comparisons over time.
Blue water = surface and groundwater.
- Boundary: 10.2% disturbance of blue water flow
- Where we are: 18.2% disturbance of blue water flow
How was blue water measured?
Streamflow, literally the flow of water through streams and rivers, is used to represent blue water in this new assessment. Streamflow is a good indicator of human impacts on blue water because it is impacted by factors like water withdrawals, groundwater depletion, and human-caused climate change. While it is possible to directly measure streamflow, those measurements aren’t widespread enough to make a proper assessment, so hydrological models were used.
- The models were run for two time periods: pre-industrial (1661 to 1860) and industrial (1861 to 2005). The results show stable streamflow conditions in the pre-industrial period, and that conditions began to change steadily after industrialization.
Why is this important?
Life on Earth cannot exist without water. The ecosystems on our planet, both land-based and aquatic, have developed around specific amounts of water, and the timing in which they receive that water. If the amount and timing of water changes, it can have severe effects on the health and biodiversity of an ecosystem.
- The Aral Sea is an extreme example. The Aral Sea was the world’s 4th largest salt water lake, but heavy irrigation using water from the rivers that would have fed the Aral led to it drying out: ”As the Aral Sea has dried up, fisheries and the communities that depended on them collapsed. The increasingly salty water became polluted with fertilizer and pesticides. The blowing dust from the exposed lakebed, contaminated with agricultural chemicals, became a public health hazard. The salty dust blew off the lakebed and settled onto fields, degrading the soil. Croplands had to be flushed with larger and larger volumes of river water. The loss of the moderating influence of such a large body of water made winters colder and summers hotter and drier.” - NASA
Green water = water that is available to plants.
- Boundary: 11.1% human-induced disturbance of water available to plants
- Where we are: 15.8% human-induced disturbance of water available to plants
How was green water measured?
This boundary looks at all of the ice-free land areas on Earth and how much water is available to plants in the soil (specifically the soil depth where the roots of the plants are located).
Soil moisture was selected as the variable to measure because it was found to best represent human disturbances on green water, and could account for regional shifts. Since we don’t have widespread direct measurements of root-zone soil moisture, the values here are also estimated by hydrological models that analyze records like precipitation, evaporation, surface soil moisture, temperature, etc.
- The models were run for two time periods: pre-industrial (1661 to 1860) and industrial (1861 to 2005). The results show stable root-zone soil moisture conditions in the pre-industrial period, and that conditions began to change steadily after industrialization.
Why is green water important?
The management of green water is very important to land-based ecosystems as well as global water and climate cycles. Let’s look at the Amazon rainforest as an example. The soils of the Amazon rainforest are drying out, which means the trees are stressed and dying. A rainforest is called a rainforest because the trees there literally make their own rain, so when there are less trees, the forest loses this rain generation ability and becomes even dryer. Also, since the trees are struggling, they are absorbing less CO2, and parts of the rainforest are even becoming a source of emissions.
- When there is less soil moisture, that can support less vegetation, there is also less material to hold the soils together so reduced green water also increases soil erosion.
There are certain elements that are fundamental to life on Earth. Carbon is a big one, but we’ve already covered that. Phosphorous (P) and nitrogen (N) are two more essential elements because these are nutrients that plants need to grow.
P and N are naturally available, but we also add a lot of them to the environment through a range of activities. One of the major sources is commercial fertilizer for agriculture, but they also come from livestock production, cleaning products, burning fossil fuels, and wastewater, among others.
What can go wrong? Here are a couple of examples:
- Eutrophication: There aren’t just plants on land; there are also plants that grow in water. When we use too much P and N in agriculture, the extra will wash away with the rain and eventually end up in our lakes, rivers, oceans, etc. This excess in nutrients causes those plants, including microscopic algae, to grow out of control, setting off a chain reaction that ends up with all of the oxygen in the water getting used up and making it impossible for life to exist there.
- Watch this video to learn more about eutrophication.
- Nitrous Oxide (N2O): N2O is a potent GHG that absorbs about 300x as much heat as CO2 in the atmosphere. Microbes in the soil break down excess nitrogen fertilizer and release N2O into the atmosphere. About 75% of all N2O emissions come from agriculture.
- Global: 11 teragrams* (Tg) P flowing into the ocean/year
- Regional: 6.2 Tg P applied to soils/year
- Where we are:
- Global: 22.6 Tg P flowing into the ocean/year
- Regional: 17.5 P applied to soils/year
P was originally set as a global boundary, but the regional component was added because freshwater systems are so sensitive to too much of this nutrient, and this effect is very regional.
- Boundary: 62 Tg N applied to soil/year
- Where we are: 190 Tg N applied to soil//year
*A teragram is one trillion grams. One teragram = one million tonnes.
- Boundary: 0% of untested synthetic material
- Where we are: unknown, but definitely higher than 0%
Novel entities mean anything that has been introduced into Earth’s systems that does not come from nature. So this can be microplastics, antibiotics, nuclear waste, pesticides, and much more.
It has been difficult to quantify the amount of novel entities going into the environment because we are producing them faster than we can actually measure them—especially synthetic chemicals where there are hundreds of thousands already out there. In fact, 80% of the chemicals in use over the past 10 years, according to those registered under the EU REACH regulation (a small sample of everything out there), haven’t undergone a safety assessment.
- We also don’t know how these chemicals are interacting, meaning there could be entirely new substances out there that we aren’t even aware of.
One of the largest contributors to novel entities in the environment is the plastics industry. The total mass of plastic in our world is more than double that of all living mammals on the planet. Only about 9% of plastic produced actually gets recycled to make new plastic.
Boundaries at immediate risk
Ocean acidification (WIP)
- Boundary: 3.44 the average global surface ocean saturation state with respect to aragonite (Ωarag)
- Where we are: 2.8 Ωarag
Ocean acidification doesn’t mean the oceans are becoming acidic, but rather that they are becoming less basic. Try to remember back to the pH scale you learned about in science class, one end has a low pH (acids), and the other has a high pH (bases). Ocean water is naturally basic. When CO2 dissolves in the ocean, it combines with becomes carbonic acid, and this decreases the pH of the water, making it less basic.
Aragonite is a mineral (calcium carbonate) that ocean creatures like snails and mollusks use to produce their shells. If there isn’t enough aragonite dissolved in the water, these creatures will not be able to build the shells and skeletons that they need to survive. Ocean acidification decreases the amount of aragonite that is available to ocean life, so measuring aragonite saturation is a good indicator for ocean acidification.
Why is this important?
Ocean acidification is threatening the balance of our oceans. It is changing marine food chains, which will have an effect on the food supply for humans, but there are other less obvious effects to consider. Coral reefs, for example, are important for protecting coastlines from waves and storms. Corals, are made of calcium carbonate, so acidification slows down the growth of corals and at a certain point, they will start to erode away faster than they can grow.
Boundaries in the safe zone
Atmospheric aerosol loading
- Boundary: 0.1 mean annual interhemispheric difference
- Where we are: 0.076 mean annual interhemispheric difference
Let’s start off by defining the terms here.
- Aerosol: Solid particles or liquid droplets that are hanging out in the air. These are small, so you usually can’t see them with the naked eye. Aerosols can be natural, like soot from a wildfire, dust that gets blown up during a drought, or salt and minerals from the ocean when waves crash against rocks. Aerosols can also be man-made, like exhaust fumes from cars and smog from industry.
- Aerosol loading refers to the amount of aerosols that are suspended in the air at a given time and place.
- Interhemispheric: The Earth is divided into four hemispheres, which are lines that divide our planet into two equal halves. Inter means “between,” so interhemispheric is literally “between two hemispheres.”
- The interhemispheric difference is just the difference between two hemispheres for whatever value you are counting.
If we bring it all together, the mean annual interhemispheric difference of atmospheric aerosol loading is the difference in the amount of aerosols in the atmosphere between the hemispheres, in this case, the Northern and Southern Hemispheres.
How is this measured?
The variable being measured here is aerosol optical depth (AOD). Aerosols can block sunlight from getting to the surface of the earth, and AOD shows how much the sunlight getting through has been reduced.
Why is it important?
Aerosols can have a cooling or warming effect, depending on the type, size, and location of the aerosol. They are also very important for cloud formation and precipitation. Water needs something to cling to in order to form clouds and eventually raindrops, so it uses the aerosols in the air. Changes in the concentration of aerosols between the Northern and Southern hemispheres can have a lot of consequences on the health of our planet and everything that lives on it.
There is something called the “Intertropical Convergence Zone” (ITCZ), which is a region in the tropics where the winds of the Northern and Southern Hemispheres come together. When these winds meet, the air is forced up, creating thunderstorm clouds. The ITCZ moves around seasonally, following the movement of the sun, and this creates the rainy and dry seasons in tropical areas. If there are many more aerosols in the Northern Hemisphere, this will cause a cooling effect and push the ITCZ further south, leading to droughts in the areas where those clouds should have been and pushing other weather patterns (like El Nino) out of whack.
The mechanisms behind the influence of aerosols on the ITCZ is still being studied, so we will hopefully learn more about how and why these effects occur in the near future.
Stratospheric ozone depletion
- Boundary: 276 Dobson units (DU)
- Where we are: 284.6 DU
Ozone is a molecule made up of three oxygen atoms. We have a concentrated layer of ozone in our stratosphere that acts like a shield against solar radiation. In particular, the ozone layer filters out and reflects the most damaging radiation (the kind that causes cancer). The ozone layer is measured in Dobson units (DU), which represents the thickness of the layer in a vertical slice of the atmosphere. A higher DU means a thicker ozone layer.
In the 1980s, we discovered that our behavior (like using hairspray and refrigerants) had created a hole in the ozone layer over Antarctica. In preindustrial times, the ozone layer was 290 DU, and this hole we discovered was down to 109 DU in 1987. The truly amazing thing here is that the world came together to phase out the chemical compounds that were damaging the ozone layer by enacting The Montreal Protocol. It took some time, but now the ozone layer is regenerating itself.
This is a great example of how people can come together to solve a large environmental challenge in the face of a looming disaster. We did it once, and we can do it again with climate change. For the ozone layer, we needed to ban the chemicals that were causing the damage, and we did. To save our climate, we know what needs to be done: stop burning fossil fuels and stop chopping down natural forests.
Note: Refrigerant compounds (like the ones that were creating the ozone hole) also play a large role in global warming and each molecule can absorb thousands times more heat than CO2. Learn more in our research into cold storage.
We published this Notion because the planetary boundaries influence our approach to investing, and we want other people to understand what they mean for our lives and our planet. Investing in climate means so much more than just tons of CO2. All of these boundaries are interconnected, and we can’t address one without the others.
- Planetary boundaries (Stockholm Resilience Center)
- Radiative Forcing (MIT Climate Portal)
- Global water cycle shifts far beyond pre-industrial conditions – planetary boundary for freshwater change transgressed [pre-print] (Porkka et al., 2023)
- Earth beyond six of nine planetary boundaries (Richardson et al., 2023)
- Sensitivity of tropical monsoon precipitation to the latitude of stratospheric aerosol injections (Krishnamohan & Bala, 2022)
- Outside the Safe Operating Space of the Planetary Boundary for Novel Entities (Persson et al., 2022)
- A planetary boundary for green water (Wang-Erlandsson et al., 2022)
- [VIDEO] Evidence for human generated increases in atmospheric carbon dioxide (Crash Chemistry Academy, Jul 2022)
- Safe planetary boundary for pollutants, including plastics, exceeded, say researchers (SEI, Jan 2022)
- Climate Response to Latitudinal and Altitudinal Distribution of Stratospheric Sulfate Aerosols (Zhao et al., 2021)
- Rebuilding the ozone layer: how the world came together for the ultimate repair job (UNEP, Sep 2021)
- Changes to Carbon Isotopes in Atmospheric CO2 Over the Industrial Era and Into the Future (Graven et al., 2020)
- Tracking fossil fuel emissions with carbon-14 (NOAA Research, Jun 2020)
- Planting non-native trees accelerates the release of carbon back into the atmosphere (The Conversation, Jun 2020)
- Climate Change Indicators: Climate Forcing (EPA, Nov 2020)
- Restoring natural forests is the best way to remove atmospheric carbon (Lewis et al., 2019)
- Accelerated modern human–induced species losses: Entering the sixth mass extinction (Ceballos et al., 2015)
- The effectiveness of coral reefs for coastal hazard risk reduction and adaptation (Ferrario et al., 2014)
- Management of nitrogen fertilizer to reduce N2O emissions from field crops (Michigan State University, Nov 2014)
- Reconsideration of the planetary boundary for phosphorus (Carpenter & Bennett, 2011)
- A safe operating space for humanity (Rockström et al., 2009)
Last updated: Sep 2023