Six of 9 planetary boundaries have been breached because of human activities: Study

This does not imply immediate disaster but raises the risk of setting processes in motion that will likely irreversibly change overall environmental conditions on Earth

By Rohini Krishnamurthy
Published: Thursday 14 September 2023
The Earth. Photo: iStock_

The world has breached six of the nine planetary boundaries necessary to maintain Earth’s stability and resilience, according to a new study. 

The six boundaries include climate change, biosphere integrity (genetic diversity and energy available to ecosystems), land system change, freshwater change (changes across the entire water cycle over land), biogeochemical flows (nutrient cycles), and novel entities (microplastics, endocrine disruptors, and organic pollutants), the study published in Science Advances warned.

Katherine Richardson from the University of Copenhagen and the study’s author likens planetary boundaries to blood pressure (BP).

“If your BP is over 120/80, it is not a guarantee of a heart attack but it raises the risk. The same is true here — the breaching of individual boundaries does not imply immediate disaster but raises the risk of setting processes in motion that are likely to dramatically and irreversibly change the overall environmental conditions on Earth to one that no longer supports civilisation as we know it,” Richardson told Down To Earth.

The findings are an update to the planetary boundaries framework, which was first launched in 2009, to define the environmental limits within which humanity can safely operate.

“To continue to be relevant, the framework needs to be updated as our understanding of the Earth as a system and the human effects on it evolve,” Richardson explained.

This is the third iteration of the framework carried out by 29 scientists from eight different countries.

The researchers first identified the processes in the Earth’s ecosystem that have been important for maintaining favourable conditions for humans in the last 12,000 years. This period is known for its stable and warm planetary conditions.

Next, they assessed how much humans are changing them and identified the level at which human activities raise the risk of potentially dramatic and irreversible changes in the overall conditions on Earth. They used performed computer simulations in their research.

Their results showed humans caused a breach in our planet’s safe climate and land system in 1988 and are now facing a risk of approaching systemic disruption.

The researchers set the planetary boundary for atmospheric carbon dioxide concentration and radiative forcing (represents the size of the energy imbalance in the atmosphere) at 350 parts per million (ppm) and 1 Watts per square meter (Wm−2), respectively. Currently, this has reached 417 ppm and is 2.91 W m−2.

As for land system change, the team looked at the global area of forested land as the percentage of the original forest cover boundary. This was estimated at 75 per cent.

The current value is beyond the safe limits. The global value was found to have dropped to 60 per cent. 

For biosphere integrity, the researchers kept a limit of less than 10 extinctions per million species-years. But their calculations, which were done conservatively, estimate the extinction rate was greater than 100 extinctions per million species-years. This boundary has also been violated.

Currently, it is estimated around one million of the 8 million plant and animal species are threatened with extinction, and over 10 per cent of the genetic diversity of plants and animals may have been wiped out over the last 150 years. 

The second aspect of biosphere integrity is the energy available to the ecosystem or the net primary production (NPP). It is equal to the difference between the amount of carbon produced through photosynthesis and the amount of energy that is used for respiration.

Humans are appropriating roughly 30 per cent of the energy that was available to support biodiversity before the Industrial Revolution, the study showed. This, it added, could drive biodiversity loss.

Richardson determined the planetary boundary for freshwater, which includes blue water (surface and groundwater) and green water (available water for plants).

Human impacts on blue and green water were calculated to be 18.2 per cent and 15.8 per cent, respectively, which is higher than the boundary of 10.2 per cent and 11.1 per cent, respectively.

The analysis showed that violations of blue and green water boundaries occurred in 1905 and 1929, respectively.

The paper also looked at the flows of nutrients such as nitrogen and phosphorus. “Changing nutrient availability has huge consequences for biodiversity and water quality. In addition, reactive nitrogen [includes oxides of nitrogen (NOx), ammonia (NH3), and nitrous oxide (N2O)] can lead to the production of potent greenhouse gases that worsen climate change,” Richardson explained.

The boundary was fixed at 11 teragrams (Tg) for Phosphorus and 62 Tg for Nitrogen. This is now 22.6 Tg and 190 Tg, respectively. 

The planetary boundary of novel entities was calculated to be zero. This means humans have transgressed this limit as well.

Stratospheric ozone depletion, aerosol loading and ocean acidification were found to be within the planetary boundary.  Aerosols are minute particles from combustion processes, biomass burning, and plant/microbial materials suspended in the air. They are known to impact climate.

However, the risk that aerosol loading and ocean acidification exiting their boundaries is increasing, the researchers warned.

“It is interesting to note that the ozone layer planetary boundary was transgressed in the 1990s. today, it is back in the safe operating space thanks to global negotiations, including the Montreal Protocol,” Richardson noted.

 She suggests that the world needs to negotiate and respect the limits to the amount of waste that is dumped into the environment.

In short, we need a circular economy (just like the one nature has!) and we need to be moving in that direction, she added.

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