Assessing our Planet's Limits: The Impact of Human Activity on Planetary Boundaries
Exploring the critical role of planetary boundaries: the nine key processes that maintain Earth's stability and how human activities are impacting these vital limits.
The idea behind the planetary boundaries framework has gained a lot of attention from scientists and policymakers since its first use in 2009 in the reputable scientific publication a safe operating space for humanity. This article will dive into what these boundaries are and how far are we from crossing them now compared to almost 15 years ago.
What are planetary boundaries?
The concept of planetary boundaries is a crucial framework that helps us understand and manage the well-being of our planet. It draws upon the science of Earth systems to identify nine key processes that are essential for keeping our planet stable. Unfortunately, these processes are currently being disrupted by human activities.
Imagine Earth as a delicate, interconnected system that has been in a relatively stable and warm state for the last 10,000 years, a period known as the Holocene. During this time, human civilization evolved, and our environment remained relatively consistent.
The planetary boundaries framework is designed to set limits on how much we can disrupt the Earth's systems without causing irreversible damage. It does this by defining a safe zone in which we should operate to protect both the Holocene-like conditions we've enjoyed and the planet's ability to bounce back from challenges.
The 9 planet boundaries are: climate change, novel entities, stratospheric ozone depletion, atmospheric aerosol loading, ocean acidification, biogeochemical flows, freshwater change, land-system change, and biosphere integrity.
1. Climate change
Climate change is by far the most talked about planetary boundary. The climate change boundary refers to two metrics: the amount of greenhouse gases (GHG) in the atmosphere and the change in radiative forcing. Radiative forcing refers to the difference between the amount of energy that enters the Earth’s atmosphere and the amount that leaves it1 . When more radiations enter the Earth than they leave it, as this is happening today, the atmosphere warms up.
The planetary boundary for CO2 emission concentration is set at 350 ppm (parts per million) and at 1 watt/m2 for radiative forcing. However, we are currently significantly above these limits and have already reached a CO2 concentration of 417 ppm and 2.91 watt/m2 according to the latest planetary boundaries report published in September 2023.
While this planetary boundary is essential, the following other 8 planetary boundaries should not be overlooked.
2. Stratospheric ozone depletion
The stratospheric ozone layer is a critical component of Earth's atmosphere, located approximately 15 to 30 kilometers above the surface2. It functions as a protective shield, absorbing most of the sun's harmful ultraviolet radiation and thereby safeguarding all forms of life from potential damage.
Since the mid-1980s, a substantial global effort has been made to address the depletion of the ozone layer, primarily caused by the use of ozone-depleting substances (ODS). The 1987 Montreal Protocol, under the United Nations Environment Program (UNEP), has been pivotal in achieving significant reductions in the consumption of ODS. Continued compliance with the Montreal Protocol and its subsequent amendments is crucial to ensure the ongoing recovery of the ozone layer.
3. Atmospheric aerosol loading
Atmospheric aerosols are tiny particles made up of solids and liquids, like dust and water, that are suspended in the air3. They may be directly released into the atmosphere as particles, such as ash, or form from gases that react chemically and then condense. These aerosols come from both natural sources—like dust, sea salt, smoke, and biological sources (biogenic aerosols)—and human activities.
These aerosols affect Earth's system, including climate, weather patterns, and ecosystems. The planetary boundary for aerosol loading—a measure of how much sunlight is blocked from reaching Earth's surface—is set at an Aerosol Optical Depth (AOD) of 0.25. However, regions like southern Asia and East China have exceeded this boundary, with current AOD values around 0.3 to 0.4. While the global mean AOD is still below the planetary boundary, the uneven distribution of aerosols has already shifted weather patterns, which shows the profound effects aerosols have on our planet's systems.
4. Ocean acidification
Ocean acidification is a long-term decrease in the pH of ocean waters, primarily caused by the absorption of atmospheric carbon dioxide (CO2)4. In fact; approximately 30% of this CO2 is absorbed by the oceans. This absorption leads to chemical reactions making seawater more acidic and reducing the abundance of carbonate ions.
Carbonate ions are crucial for the formation of structures like sea shells and coral skeletons. The scarcity of these ions makes it harder for calcifying organisms like oysters, clams, sea urchins, corals, and some plankton to build and maintain their calcium carbonate structures.
While ocean acidification is still within the planetary boundary at the moment, it still has significant economic implications, especially for communities dependent on fish and shellfish, and affects global food security.
5. Biogeochemical flows
Biogeochemical flows refer to anthropogenic perturbation of global element cycles5. The two metrics used are the quantity of nitrogen (N) and phosphorus (P). Their large quantity (above the planetary boundary) shows significant human-induced changes in global element cycles due to agriculture and industry, impacting ecosystem composition and Earth system effects over various time scales.
Agriculture's use of fertilizers contributes to this by releasing reactive forms of nitrogen and phosphorus into land and oceans, altering nutrient flows and element ratios, which in turn affects climate, biosphere integrity, and ecosystem health.
6. Freshwater change
In revising the planetary boundary for freshwater, a new measurement approach includes both green water (plant-available water in soil) and blue water (surface and groundwater), with green water previously not considered. Green water accounts for hydrological regulation of terrestrial ecosystems, while blue water reflects river regulation and aquatic ecosystem health.
Currently, approximately 18% of global land area for blue water and 16% for green water are experiencing deviations from preindustrial conditions, indicating a substantial transgression of the freshwater change boundary (5). This revised approach, contrasting with earlier assessments that only considered blue water, shows that the freshwater boundary was already transgressed around a century ago.
7. Land system change
The boundary analysis focuses on the three major forest biomes: tropical, temperate, and boreal, which are vital for global biogeophysical processes, using forest cover compared to potential Holocene levels (5).
Recent data indicates an increase in deforestation, especially in the Amazon, leading to a breach of the planetary boundary for tropical forests. The global trend of decreasing forest area led by deforestation and wildfires is clear and concerning.
8. Novel entities
The planetary boundary for novel entities now focuses on synthetic chemicals, radioactive materials, and genetic modifications (5). The framework suggests a zero-tolerance approach, proposing that the boundary be set at zero release of synthetic compounds unless certified as harmless. Current analysis reveals a significant breach of this boundary, with about 80% of chemicals under EU regulation used for over a decade without safety assessments, indicating a substantial overshoot of the safe operating space (5).
9. Biosphere integrity
The planetary boundary for biosphere integrity centers on genetic diversity, essential for maintaining the biosphere's dynamic and adaptive character within the Earth system. This boundary is quantified by the maximum extinction rate (less than 10 extinctions per million species-years) that preserves the biosphere's genetic and ecological complexity (5). Currently, the extinction rate is conservatively estimated to be over 100 extinctions per million species-years, significantly exceeding the set boundary and indicating a marked breach in the genetic component of biosphere integrity.
According to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES):
Of an estimated 8 million plant and animal species, around 1 million are threatened with extinction, and over 10% of genetic diversity of plants and animals may have been lost over the past 150 years.
Conclusion: Pathways to Planetary Preservation
In light of the evident breaches in most planetary boundaries, it is imperative to intensify global efforts towards sustainable practices, particularly in areas like chemical regulation, deforestation, carbon emissions, and biodiversity loss.
The insights from the planetary boundaries framework should guide policies and actions, fostering a deeper commitment to preserving the Earth's resilience and ensuring a stable environment for future generations.
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https://climate.mit.edu/explainers/radiative-forcing
https://www.eea.europa.eu/en/topicchange-mitigation-reducing-emissions/current-state-of-the-ozone-layer
https://www.pnnl.gov/atmospheric-aerosols
https://oceanservice.noaa.gov/facts/acidification.html#:~:text=Ocean%20acidification%20refers%20to%20a,CO2)%20from%20the%20atmosphere.
https://www.science.org/doi/10.1126/sciadv.adh2458