9 Planetary Boundaries we must not cross — or the Earth may not be able to support Human Life
A summary of: John Rockstrom et al. 2009.
Planetary Boundaries: Exploring Safe Operating Space for Humanity Ecology and Society, Vol. 14, No. 2, Article 32
By Alice Friedemann
Due to global human alteration of the environment, we’ve reached the point where abrupt environmental changes could occur on a global scale that could threaten humankind. We have made rough estimates of when thresholds being crossed might trigger harmful, irreversible, and catastrophic results.
Each boundary crossed decreases the planet’s ability to support human life.
We have already crossed 3 of these boundaries:
Climate change Biodiversity loss Global nitrogen cycle
And all the others are moving towards dangerous thresholds:
Potential consequences of crossing boundaries:
- Loss of freshwater [from glaciers melting, lakes evaporating, etc ]
- Regional climate disruptions [drought, floods, severe storms, etc]
- Potential melting of the Greenland and Antarctic ice sheets resulting in rapid sea-level rise — difficult for society to cope with
- Severe and irreversible UV-B radiation
- Thermohaline circulation
- A major oceanic anoxic event (extinction of most marine life globally) and eutrophication of coastal & freshwater systems (loss of fish and shellfish regionally).
- Deforestation of the Amazon could reduce rainfall in Asia, especially Tibet, a source of freshwater for about 750 million people in India and China.
- Destruction of too much of the Amazon rainforest could cause it to irreversibly convert to a semi-arid savanna
We have strong evidence of the danger of crossing thresholds already
- Arctic sea ice melting
- Mountain glaciers retreating globally
- Accelerated rate of melting of Greenland and West Antarctic ice sheets
- Increased rate of sea-level rise in the past 10-15 years
- Subtropical regions have expanded 4 degrees toward the poles
- Bleaching and death of coral reefs
- A rise in the number of large floods
- Oceans becoming less able to absorb CO2
- Antarctic ozone hole
New Challenges require new thinking on Global Sustainability
Nothing is being done because at both social and economic levels, society is oblivious to the risks we are incurring. We hope that nations will use our definitions of a safe operating space to set new policies and prevent disastrous changes from occurring.
Although we’ve altered the earth for 10,000 years since the last glaciers receded (agriculture and hunting megafauna to extinction), it’s only recently that we’ve affected earth systems at a global scale.
Climate change and stratospheric ozone have received most of the attention, but there are 7 other significant areas that need to be watched as well.
Introducing the concept of planetary boundaries
Boundaries are set individually without consideration of other boundaries that may have already been crossed. They extend limits-to-growth model, precautionary principle, and other models.
We know a lot about abrupt triggering of destruction from local and regional scale ecosystems by seeing what’s happened to lakes, forests, and coral reefs that have been pushed past sustainability. This information has allowed us to consider what might tip similar catastrophes on a global scale. This can happen either “top down” or local and regional disasters that escalate up globally when occurring in multiple locations at the same time.
The international recommendation to try to stay below a 2 degrees Celsius “guardrail” is problematic on several scores. First, even if this is achieved, there will still be severe consequences to the environment. Secondly, this “limit” was set with political rather than scientific reasoning.
We know from a great deal of evidence [check out the references cited at the bottom] that CO2 levels greatly affect cooling and warming trends, that the earth has been ice-free once CO2 reaches 450 ppm, which is why we set the threshold at 350 ppm – once you go above that range you risk continuing on to 450 ppm and above from global feedbacks beyond our control [a.k.a. runaway greenhouse -- melting of permafrost, release of methane hydrates, etc]. Even 350 ppm may be too high because the aerosols [released by industry and fossil fuel burning] have a cooling effect [a global depression and/or energy shortages will lower the amount of aerosols in the atmosphere]
CO2 concentration (parts per million) – Preindustrial value: 280, Current: 387, Boundary: 350.
This is happening 100 times faster than any time in the last 20 million years. Too fast for organisms to adapt.
It’s due to CO2, which increases the acidity (lowering the pH) of seawater. The first creatures to be affected are the ones that use carbon ions to build protective shells (with Aragonite). This will lead to the death of coral reefs [which protect and nurture the fisheries we depend on], and the death of organisms at the bottom of the food chain, as well as reduce the ability of the ocean to absorb more CO2, and increase the hypoxic areas of the ocean [the “dead” areas with no oxygen, i.e. Gulf of Mexico].
Aragonite saturation state in surface water (Omega Units) – Preindustrial value: 3.44, Current: 2.90, Boundary: 2.75.
Stratospheric Ozone Depletion (SOD)
The ozone in the stratospheric layer protects land and marine life from harmful ultraviolet radiation. The sudden Antarctic ozone hole is a good example of a boundary we didn’t even know about being crossed — completely unexpectedly – from chlorofluorcarbons (CFCs) and polar stratospheric clouds – which are likely to increase in the Arctic from global warming, perhaps creating an ozone hole there as well. Fortunately we’ve taken actions to reduce ozone-depleting gases, so we’re capable of making wise decisions.
Ozone concentration (Dobson Units) – Preindustrial value: 290, Current: 283, Boundary: 276.
Global Phosphorus and Nitrogen Cycles
Fossil fuels, biomass burning, and industrial agriculture create excessive flows of Phosphorus and nitrogen into waterways, resulting in oxygen-free water (anoxic, eutrophication). This kills off marine life, including the fish and shellfish we depend on for survival.
In the past, some of the largest extinction events in earth’s history were associated with global oxygen depletion, known as ocean anoxic events (OAE).
P and N also increase atmospheric pollution. N contributes to global warming (ghg nitrous oxide is over 20 times stronger than CO2).
Phosphorous is essential to food production, increasing nitrogen could lead to a widespread condition of chronic phosphorus limitation.
Phosphorous rate of flow into oceans (million tons per year) – Preindustrial value: 1, Current: 10, Boundary: 12.
Nitrogen rate of human removal from the atmosphere (million tons per year) – Preindustrial: 0, Current: 133, Boundary: 9.
Rate of Biodiversity Loss
This is a boundary that affects all other boundaries. Increasing loss of species makes both land and water ecosystems more vulnerable to other stressors such as climate change and acidity.
Humans have increased the destruction of species somewhere between 100 and 1,000 times the normal rate of extinction. This rate is expected to increase 10-fold this century, to between 1,000 and 10,000 times the previous extinction rate.
Already humans are responsible for such an enormous rate of biodiversity loss that scientists consider this the 6th major extinction event in the history of life on Earth. Past extinctions resulted in permanent and irreversible consequences.
The current extinction is especially serious because biodiversity is essential to sustaining the ecosystems we depend on for survival. Less biodiversity means ecosystems will have less resilience or ability to adapt to the new conditions the other 8 boundaries are imposing.
Extinction rate (species per million per year) – Preindustrial: 0.1-1.0, Current: > 100, Boundary: 10
Global Freshwater Use (depletion of fresh water)
We are radically affecting water flows on earth – 25% of the worlds rivers run dry before reaching oceans. We take almost 1500 cubic miles (2,400 cubic km) of fresh water every year to use in irrigation (70%), industry (20%) and domestically (10%). This in turn affects many of the other boundaries.
Threats to human life from deterioration of fresh water include
- Loss of agricultural land to grow crops due to deforestation and erosion, which in turn reduces the ability of the soil to store carbon and lower CO2 levels
- The melting of mountain glaciers will greatly reduce fresh water for billions of people
- Less water means less moisture leading to less rainfall and fewer monsoons
Rate of human consumption (cubic km per year) – Pre-industrial: 415, Current: 2400, Boundary: 4000.
Land System Change
As fossil fuel based agriculture increased the number of humans that could be supported on the earth from roughly 1 billion to 7 billion, we’ve increasingly expanded agriculture by destroying forests, filling in wetlands, and destroying other ecosystems. This has greatly affected other boundaries as well – it’s a major cause of biodiversity loss, and the Nitrogen and phosphorus pollution (to intensify the amount of crops grown with fertilizers). This also increases climate change and decreases fresh water.
We shouldn’t use more than 15% of the land on the planet, yet we already have taken over 35% of Earth’s land surface to grow crops (12%) and herds of domesticated animals (23%). The earth is very close to the point where there is no more arable land available – indeed, arable land may be declining faster than we can clear new land from desertification, topsoil loss, erosion, etc.
To reduce our land use, we need to reduce our population, change our diets, stop wasting so much food, and stop growing crops in monocultures.
Aerosol Loading – boundary unknown
Airborne Ozone, soot, and chemicals are a boundary because they affect climate change, human health, degrade forests, kill fish, and damage crops.
It’s hard to set a boundary because there are so many kinds of aerosols, they interact in unknown ways, we don’t fully know all of their impacts, and so on.
Chemical pollution – boundary unknown
Chemical pollution affects human and ecosystem health at local, regional, and global scales. We chose this as a boundary because:
- chemicals affect the development of organisms
- chemicals affect other boundaries, such as biodiversity (by killing organisms) or making creatures more vulnerable to other stresses
- chemicals like mercury released from burning coal affect life negatively
- chemicals from burning fossil fuels release CO2 and increase climate change
- Climate change in turn could increase the numbers and distribution of pests, resulting in more pesticides, resulting in more climate change, biodiversity loss and so on
There are between 80,000 and 100,000 chemicals traded globally, making it impossible to measure all the chemicals in the environment or assess the impact of how these thousands interact to produce harmful effects individually or in combination.
But we can try to measure some of the most harmful chemicals that hinder development, disrupt endocrine systems, hamper reproduction, or likely cause cancer. Some of these chemicals include mercury, arsenic, lead, toluene, DDT, PCBs, dioxins, etc.
Humanity risks long-term social and environmental disruption if we don’t become an active steward of all the planetary boundaries.
We’ve already gone past 3 boundaries. We don’t know how far or how long we can do this before there’s no possibility of retreating back to safe levels. Some of these processes are slow, some are fast, and most of them affect the others. If we do nothing, we risk unexpectedly crossing thresholds that can suddenly destroy the ecosystems we depend on for survival.