Six out of the nine “planetary boundaries” that track how close humanity is coming to environmental disaster had already been crossed by the end of 2023. And in July scientists said there may be a tenth: the oceans are running out of oxygen. 

The idea of boundaries was first proposed by the climatologist Johan Rockström. A team of 28 renowned scientists then defined a set of nine planetary boundaries – processes essential for maintaining a stable climate suitable for human life – in 2009 to set a benchmark for measuring the unfolding Climate Crisis.

When the original report was written only three boundaries had been crossed: Climate Change (CO₂), biodiversity (extinction rate) and biogeochemical flows (nitrogen cycles). There was plenty of wiggle room left over. 

By the time the Paris Summit came around six years later in 2015 it was already clear the world was in trouble, as not only had two more boundary been crossed – land change system (expansion of agriculture, growth of cities), biogeochemical flows (phosphorous cycles) – but the wiggle room of three more was rapidly being used up.

According to the last update report in September 2023, six of these boundaries have been exceeded, and there is very little wiggle room left. The acidification of the oceans is going to happen very soon, which will have catastrophic effects on global marine life and its connected ecosystem chains.  

Since 2009 the number of planetary boundaries that have been crossed has increased from three to six

The latest update not only measures each boundary but also underscores the critical consequences of transgressing them. "Crossing boundaries increases the risk of generating large-scale abrupt or irreversible environmental changes," the report states.

The planetary boundaries encompass interrelated processes within the Earth's complex biophysical system. This interconnectivity implies that a singular focus on climate change is insufficient for sustainability. Instead, the interplay of boundaries, particularly between climate change and biodiversity loss, is crucial for both scientific research and practical applications.

Interest in the planetary boundaries framework has surged across scientific, policy and practical domains since its introduction. The concept highlights the need for a holistic approach to environmental sustainability, recognising that the Earth's stability depends on maintaining these crucial thresholds.

Here are the nine planetary boundaries, with those that have been crossed already marked in red:

1. Climate Change: This boundary focuses on the concentration of greenhouse gases (GHGs) in the atmosphere, primarily carbon dioxide, which influences global climate patterns and temperatures.
2. Biosphere Integrity (Biodiversity Loss): This boundary addresses the rate of species extinction and the loss of ecosystems, both of which are critical for maintaining ecological balance and resilience.
3. Land-System Change: This boundary pertains to the conversion of natural ecosystems into agricultural or urban land, affecting biodiversity, water cycles and climate regulation.
4. Biogeochemical Flows (Nitrogen and Phosphorus Cycles): This boundary involves the disruption of the natural cycles of nitrogen and phosphorus, which are essential nutrients for plant growth but can cause environmental issues when overused in agriculture.
5. Freshwater Use: This boundary considers the consumption and management of freshwater resources, which are crucial for drinking water, agriculture and ecosystem health.
6. Ocean Acidification: This boundary examines the increase in acidity of the world's oceans due to the absorption of excess atmospheric carbon dioxide, which can harm marine life and ecosystems.
7. Atmospheric Aerosol Loading: This boundary deals with the concentration of aerosols (tiny particles) in the atmosphere, which can have an impact on climate and human health.
8. Stratospheric Ozone Depletion: This boundary focuses on the thinning of the ozone layer in the stratosphere, which protects life on Earth from harmful ultraviolet radiation.
9. Introduction of Novel Entities: This boundary includes the release of synthetic chemicals, heavy metals and radioactive materials into the environment, which can have unpredictable and potentially harmful effects on ecosystems and human health.

More recently, scientists argued in a paper published on July 15 that there’s potentially a tenth boundary that’s gone unrecognised, which concerns worldwide aquatic deoxygenation in lakes, reservoirs, oceans and other bodies of water.

Some bodies of water in the world (such as basins in the Black Sea, the Baltic Sea, and various fjords) are naturally anoxic, meaning that they contain little or no oxygen. But widespread deoxygenation is different, as it affects previously oxygenated bodies of water globally and to varying degrees.

According to the researchers, lakes and reservoirs have experienced oxygen losses of 5.5% and 18.6% respectively in the past 45 years, and the oceans have dropped by a massive 2%, considering their size. One of the most dramatic examples of deoxygenation is in midwaters off the coast of California, where oxygen levels have dropped by a staggering 40% since 1960.

Six out of nine “planetary boundaries” have been crossed

Six of the nine boundaries have been transgressed. In addition, ocean acidification is approaching its planetary boundary. The green zone is the safe operating space (below the boundary). Yellow to red represents the zone of increasing risk. Purple indicates the high-risk zone where interglacial Earth system conditions are transgressed with high confidence.

The wedge lengths are scaled logarithmically. The upper edges of the wedges for the novel entities and the genetic diversity component of the biosphere integrity boundaries are blurred either because the upper end of the zone of increasing risk has not yet been quantitatively defined (novel entities) or because the current value is known only with great uncertainty (loss of genetic diversity). Both, however, are well outside of the safe operating space.

“Transgression of these boundaries reflects unprecedented human disruption of Earth system but is associated with large scientific uncertainties,” the 2023 planetary boundaries update paper said.

The nine planetary boundaries are measured using specific control variables that serve as indicators for the respective processes. These control variables help scientists quantify the current state and track changes over time. Here's how each boundary is measured:

1. Climate Change:
   • Control Variables:
     • Atmospheric CO2 concentration (measured in parts per million, ppm).
     • Radiative forcing (measured in watts per square metre, W/m²).
   • Thresholds:
     • Pre-industrial CO2 levels were about 280 ppm, while the boundary is set at 350 ppm to avoid dangerous interference with the climate system.
2. Biosphere Integrity (Biodiversity Loss):
   • Control Variables:
     • Genetic diversity: Rate of species extinction (measured as extinctions per million species-years, E/MSY).
     • Functional diversity: Biodiversity intactness index.
   • Thresholds:
     • The boundary is considered transgressed if the extinction rate exceeds 10 E/MSY.
3. Land-System Change:
   • Control Variable:
     • Percentage of global land cover converted to cropland.
   • Thresholds:
     • The boundary is set at 15% of the global ice-free land surface being converted to cropland.
4. Biogeochemical Flows (Nitrogen and Phosphorus Cycles):
   • Control Variables:
     • Nitrogen: Amount of nitrogen removed from the atmosphere for human use (measured in million tonnes per year).
     • Phosphorus: Flow of phosphorus into oceans (measured in million tonnes per year).
   • Thresholds:
     • The boundary for nitrogen is 62mn tpy.
     • The boundary for phosphorus is 6.2mn tpy.
5. Freshwater Use:
   • Control Variable:
     • Global consumption of freshwater (measured in cubic kilometres per year).
   • Thresholds:
     • The boundary is set at 4,000 cubic kilometres per year.
6. Ocean Acidification:
   • Control Variable:
     • Mean surface seawater saturation state with respect to aragonite (a form of calcium carbonate).
   • Thresholds:
     • The boundary is set at 80% of the pre-industrial aragonite saturation level.
7. Atmospheric Aerosol Loading:
   • Control Variable:
     • Aerosol Optical Depth (AOD) over land areas.
   • Thresholds:
     • No global threshold has been established, but regional thresholds are considered.
8. Stratospheric Ozone Depletion:
   • Control Variable:
     • Concentration of ozone in the stratosphere (measured in Dobson Units, DU).
   • Thresholds:
     • The boundary is set at a minimum of 275 DU over Antarctica in October.
9. Introduction of Novel Entities:
   • Control Variables:
     • Currently, there are no specific control variables defined due to the complexity and variety of these substances.
   • Thresholds:
     • This boundary is still being developed, with ongoing research to determine safe levels for synthetic chemicals, heavy metals, and radioactive materials.

Image: The 2023 update to the Planetary boundaries. Licensed under CC BY-NC-ND 3.0. Credit: "Azote for Stockholm Resilience Centre, based on analysis in Richardson et al 2023".