Planetary Boundaries

“One can see from space how the human race has changed the Earth.
Nearly all of the available land has been cleared of forest and is now used for agriculture or urban development.
The polar icecaps are shrinking and the desert areas are increasing.
At night, the Earth is no longer dark, but large areas are lit up.
All of this is evidence that human exploitation of the planet is reaching a critical limit.
But human demands and expectations are ever-increasing.
We cannot continue to pollute the atmosphere, poison the ocean and exhaust the land.
There isn’t any more available.”
Stephen Hawking

Our planet Earth has a carrying capacity which supports myriads of life forms in various ecosystems, including that of humans. The survival of humans depends on the survival of ecosystems and the life forms supported by those ecosystems. If we overshoot the carrying capacity of our ecosystems, then that carrying capacity degrades. Earth would no longer be able to support the same number of life forms, including that of humans.

Figure 1: Overshooting Earth’s carrying capacity

Excessive mining of minerals and use of fossil fuels have resulted in climate change and overshoot of the safe thresholds of many planetary boundaries (Rockstrom et al 2009; Steffen et al. 2015; Wackernagel et al. 2021, Richardson et al. 2023).
Excessive production and consumption are due to a combination of growth in population and growth in per-capita production and consumption. The current world population of 8.2 billion people will continue to increase over the next number of decades due to population momentum and, with a current declining rate of growth, is projected by the United Nations (2024) to reach zero population growth (ZPG) of 10.2 billion people by 2100. Given that our world population continues to grow, the only way that humankind can stay within planetary boundaries is by reducing its per-capita production and consumption.

In 2015, Johan Rockström and colleagues identified 10 planetary boundaries which must not be exceeded to ensure a sustainable future for life on Earth. These boundaries are interlinked, and crossing certain biophysical thresholds can only but have disastrous consequences for humans and other life forms. Human activity has the potential to overshoot the threshold of all these planetary boundaries. Climate change is but only one of many boundaries of overshoot. Other planetary boundaries include ocean acidification, ozone depletion, the nitrogen cycle, the phosphorous cycle, freshwater use, deforestation, biodiversity loss, particle pollution, and chemical pollution.

Figure 2: Planetary boundaries (Rockström et al. 2009)

In 2015, Will Steffen and colleagues identified the extent that planetary boundaries were at risk. In Figure 3 below, the zones of planetary boundaries which were safe in 2015 are coloured green, those subject to uncertain but increasing risk are coloured yellow, and those subject to high risk beyond uncertainty are coloured red. The safe thresholds of three of the planetary boundaries – genetic diversity, flows of phosphorous, and flows of nitrogen – had been already been well exceeded. A critical boundary was the phosphorous cycle which is essential as a nutrient for all life forms. Climate change was in a zone of uncertain increasing risk. Continued climate change would exacerbate the overshoot of several other planetary boundaries.

Figure 3: Planetary boundaries zones of risk (Steffen et al. 2015)

In 2021, Mathis Wackernagel and colleagues researched the ecological footprint of humans and estimated that in 2020, the demand of humans on biological resources exceeded the amount that Earth’s ecosystems produce by at least 56%. Moderate business-as-usual would only but further exceed our demands on biological resources. We needed to rapidly reduce our demands. To do otherwise would result in degradation of the carrying capacity of biological resources upon which we rely on for our survival.

Figure 4: Ecological footprint

In 2023, Katherine Richardson and colleagues updated Will Steffen and colleagues’ 2015 study of planetary boundaries. They evaluated Earth’s current conditions with respect to nine planetary boundaries to better understand how humans have changed our planet since the preindustrial portion of the Holocene epoch, the last 11,700 years of the Earth's history. Their abstract summarises their findings as follows:

“This planetary boundaries framework update finds that six of the nine boundaries are transgressed, suggesting that Earth is now well outside of the safe operating space for humanity. Ocean acidification is close to being breached, while aerosol loading regionally exceeds the boundary. Stratospheric ozone levels have slightly recovered. The transgression level has increased for all boundaries earlier identified as overstepped. As primary production drives Earth system biosphere functions, human appropriation of net primary production is proposed as a control variable for functional biosphere integrity. This boundary is also transgressed. Earth system modeling of different levels of the transgression of the climate and land system change boundaries illustrates that these anthropogenic impacts on Earth system must be considered in a systemic context.”

Each planetary boundary reflects one or two quantitative measures that are informed by, but not limited to Holocene conditions, Figure 5 shows the percentage of how far above or below each planetary boundary falls as of 2023. A higher percentage represents a higher risk of exceeding Holocene-like conditions.

Figure 5: Percent of Planetary Boundary Value

Ocean acidification (2023)

Oceans have absorbed approximately 25% of anthropogenic CO₂ emissions since the Industrial Revolution, serving as a crucial carbon sink. Ocean acidification has negatively impacted marine life, particularly coral reefs, which serve as vital fish habitats. While the aragonite saturation state, a measure of ocean acidification, is presently safe, unchecked CO₂ emissions may cause it to surpass safe thresholds by 2050.

Figure 6: Ocean acidification

Stratospheric ozone depletion (2023)

Ozone in the upper stratosphere layer of our atmosphere protects life on Earth from harmful ultraviolet radiation. Release of industrial chemicals such as chlorofluorocarbons (CFCs) used in air conditioners and refrigerators have depleted and thinned this ozone layer, especially at the poles. Depletion of the ozone layer is responsible for skin cancers and changes in plant photosynthesis. The Montreal Protocol of 1987, a global agreement to end the production and use of ozone-harming chemicals, has led to an 80% reduction in these chemicals. The ozone layer is expected to fully recover by 2030 and 2050 over the Northern and Southern hemispheres, and by 2100 over the polar regions.

Figure 7: Ozone depletion

Nitrogen cycle

Natural nutrients, including the key element nitrogen (N), are crucial for supporting life and maintaining ecosystems. The natural nitrogen cycle is a biogeochemical process through which nitrogen is converted into multiple chemical forms, and consecutively circulates from the atmosphere to the soil to organisms and back into the atmosphere as shown in Figure 8.

Although legumes can fix nitrogen in the soil, nitrogen fixation isn't a characteristic of all plants. Many ecosystems, including industrial farms, have limited usable nitrogen in their soil. Although nitrogen forms 78% of our Earth’s atmosphere, atmospheric nitrogen has limited availability for biological use. The problem of limited nitrogen has been addressed through industrial processes that extract nitrogen from the air to create nitrates. Nitrate runoff near waterways has caused eutrophication in freshwater. Primary production and decomposition, vital ecosystem functions, are affected by nitrogen availability; human intervention via nitrogen fixation has disrupted the nitrogen cycle by more than 300%, exceeding safe limits.

Figure 8: The natural nitrogen cycle

Phosphorous cycle

The Phosphorus Cycle details how phosphorus changes and moves through soil, water, and organic matter (both living and dead). As a key component of natural nutrients, phosphorous is critical for supporting life, maintaining ecosystems, and is frequently a limiting factor for aquatic life.

Human activities involving phosphorus—from mining to distribution in products—are changing the global phosphorus cycle, resulting in excess phosphorus in certain soils. Elevated soil phosphorus increases the potential for phosphorus to run off into aquatic environments. The death and decay of these blooms cause eutrophication in freshwater. This is exemplified by the Canadian Experimental Lakes Area. Sewage-borne phosphorous leads to ocean anoxia and excessive green algae growth.

Natural phosphorus reserves will be exhausted in 50 to 100 years.

Figure 9: The Phosphorous Cycle

Freshwater change – freshwater use (2023)

The uneven distribution of freshwater, essential for life on Earth, is highlighted in Figure 10, showing it makes up just 2.5% of the planet's total water. Research indicates almost 500 million people will suffer from water scarcity by 2050. Freshwater resources are suffering from overuse by industries. Changes to water bodies by humans have devastating effects, globally impacting river flows and water vapor distribution. To maintain Earth's overall resilience, scientists suggest a water limit tied to freshwater consumption and environmental flow requirements.

Figure 10: Freshwater use

Land system change – deforestation (2023)

To accommodate human needs and feed a growing population, forests, wetlands, and grasslands are being replaced by farmland. The shift in land use, from reforestation to deforestation for farming, is overwhelming the Earth's ability to meet humanity's growing needs.

Land-system changes have influenced CO2 concentrations, water flow patterns, biodiversity, and ecosystem health. Deforestation for agriculture moves us closer to the irreversible tipping point. To set limits on human impacts to land systems, we must consider total land area, its use, condition, and location.

Figure 11: Deforestation

Biosphere integrity - biodiversity loss (2023)

The decline in biodiversity—which includes all living things, ecosystems, and species—leads to a faster rate of species extinction. This involves:

the degradation of their habitat and ecological systems ;
the destruction and fragmentation of natural environments due to human activities preventing global sustainability
the pollution of these habitats;
the overexploitation of wild species (overfishing or deforestation);
the introduction of invasive exotic species;
climate change.

Numerous species within our rich biodiversity face not just endangerment, but extinction. The sixth mass extinction is now upon us, occurring at an unprecedented rate and ferocity. According to the Zoological Society of London, mammal, bird, and reptile populations dropped 68% on average from 1970-2016. Studies indicate that about 30% of mammals, birds, and amphibians are predicted to face extinction this century.

This is happening due to the constant demand for food, water, and natural resources. Biodiversity is being disturbed both by an increase in global temperatures and by civilizations excessively and irresponsibly clearing out the land. Humanity needs to enhance habitats and improve the connectivity between ecosystems to ensure the survival of the next generations. To achieve this target, we must develop policies that promote reforestation so that natural habitats are replenished.

Figure 12: Biodiversity loss

Atmospheric aerosol loading - particle pollution (2023)

Air contains a mixture of solid and liquid particles, known as particulate matter or particle pollution. Dust, dirt, soot, and smoke are some examples of particles large or dark enough for naked-eye visibility.
Both primary and secondary sources contribute to particle pollution. Primary particulate matter comes directly from construction, wildfires, wood burning, gravel pits, farms, and dusty roads. Secondary particulate matter results from intricate atmospheric chemical reactions.

Changes in airborne particles, from both human activities and natural events, affect climate by changing temperatures and rainfall. Atmospheric aerosols influence global atmospheric circulation patterns. Aerosols can create clouds impacting Earth's temperature beyond typical natural patterns. Tropical monsoons are particularly susceptible to aerosol influence. Aerosols also influence how the environment reflects and absorbs solar rays.

Prolonged exposure to air pollution, specifically particle pollution and ozone, during pregnancy and early childhood, has been associated with impaired lung development and a greater asthma risk.
The developing brain and heart are vulnerable; damage can lead to lifelong problems. Inhaling air polluted by harmful aerosols results in almost 800,000 annual deaths. This, and other warning signs, has made it necessary to create a planetary boundary specifically for controlling aerosol use.

Figure 13: Particle pollution

Novel entities - chemical pollution (2023)

New chemicals, new forms of existing substances, and altered life forms—defined as novel entities—pose the risk of undesirable geophysical and biological consequences. Human activities have introduced many of these into the environment. These entities have long-term consequences: decreased fertility and a higher risk of permanent genetic damage in future generations. The decline in bird populations and hampered marine animal development have already been connected to them. Although there's no single major chemical pollution boundary, scientists are still concerned enough to prioritize action and research.

Figure 14: Chemical pollution

Figure 15 shows the changes in exceeding Planetary Boundaries over time.

The impact of further rises in the global average temperature will grow in frequency and severity if unchecked causing further decreases in biodiversity, water shortages, and greater mass migration of climate change refugees due to rising sea levels, flooding, and desertification impacting on their homelands. We are all affected by climate change, but some are affected more than others. The poor and underdeveloped countries of the South with less access to healthcare, adequate housing, and financial resources are less able to respond

Figure 15: Planetary Boundaries over Time