What do "planetary boundaries" refer to?

The concept of planetary boundaries first emerged in 2009 in an article published in the famous scientific journal Nature. Its authors: a team of 26 researchers, including climate scientists, led by the Swedish specialist Johan Rockström, then director of the Stockholm Resilience Centre.

Their work was based on a systemic analysis of our planet and its dynamics. In practice, they studied the interactions between the different systems that sustain life on Earth: the atmosphere (in other words, air), the hydrosphere (water), the biosphere (different forms of life), the lithosphere (soil), etc.

Their conclusions are as follows:
1️⃣ nine major processes facilitate and regulate the Earth's system as it has existed for approximately 10,000 years;
2️⃣ these processes are today impacted by human activities;
3️⃣ there exists for each of these a limit beyond which the process will no longer produce the usual effects and lead to radical imbalances.

In simple terms, we could say that the planetary boundaries are a series of critical thresholds linked to the current major equilibrium on Earth. Below these thresholds, we remain within what these scientists call a "safe operating space for humanity". If, however, we go beyond these, we have to expect the worst given the overall balance will be disrupted.

Johan Rockström's team identified nine of these boundaries (or "biophysical thresholds"). In each case, scientists specify a measurement unit used and provide a precise value beyond which they estimate there will be a tipping point moving us towards radical change.

Global warming
It is expressed as the level of CO2 in the atmosphere [CO2 being the main man-made greenhouse gas]. The critical threshold set by scientists is 350 ppm (parts per million). It is today at 400 ppm. The limit is, therefore, crossed.

Loss of biosphere integrity
We are referring here to biodiversity, meaning all living things on Earth. It is expressed as the number of extinct species each year. The critical threshold is 10 extinctions per year and million of species. We are currently at 100. The limit is, therefore, crossed.

Land use change
It refers to deforestation in favour of urbanisation or agriculture. Here, we compare (in %) the world's forest surface area to what it was initially. The critical threshold set by scientists is 75%.However, the forest surface area has now fallen to 62% of what it was.The limit is, therefore, crossed. 

Altered biogeochemical cycles (phosphorus and nitrogen)
These are two elements that are necessary to life's equilibrium on Earth, but which humans use so intensively (in agriculture, in particular) that they have become toxic for the environment. Please note that in some reports, these two elements are treated separately: we then refer to 10 planetary boundaries.
For nitrogen, the critical threshold set by scientists is 62 billion tonnes released each year (Gt N/year). Humanity generates today over 150 Gt N/year. For phosphorus, the threshold is 11 million tonnes per year (Mt P/year). Humanity releases over 22 Mt P/year. In both cases, the limit is, therefore, crossed.

Freshwater use
It's the change to freshwater cycles (flow and quality). The limit is calculated as the volume of water taken by humans from surface water and groundwater. The critical threshold set by scientists is 4,000 km3 per year.According to a study published this year by Swedish researchers, we've crossed this threshold.

The introduction of new pollutants
These are all the new substances created and introduced by humans into the biosphere (microplastics, chemical products, etc.). Researchers from the Stockholm Resilience Centre have for a long time found it hard to assess but finally concluded, in a study published in 2022 , that the limit has in actual fact been crossed.

Ocean acidification
Here, we compare seawater saturation with "aragonites" (calcium) compared to the pre-industrial era. As this saturation decreases, the danger to marine ecosystems increases. The critical threshold set by scientists is 80%. We are currently at 84%. The limit has, therefore, yet to be crossed. 

Stratospheric ozone depletion
The ozone layer protects the Earth from the sun's harmful rays. We measure its thickness in "DU" (Dobson unit). The critical threshold set by scientists is 275 DU.The thickness is currently 285 DU. The threshold has not yet been crossed.


Atmospheric aerosol pollution
It refers to the amount of particles suspended in our atmosphere. We do not yet know how to precisely measure it on the scale of the planet. This limit has, therefore, not yet been calculated by scientists.

We had already crossed three planetary boundaries in 2009: global warming, altered nitrogen cycles and biosphere losses. Since then, the situation has worsened, with 4 new limits officially declared as crossed. The most recent being: Land use change (in 2015), the introduction of new pollutants and freshwater use (both in 2022).

Humanity has, therefore, crossed six of the nine planetary boundaries leaving its "safe operating space for humanity", entering an area of risk.

The choice of nine boundaries, the exact values beyond which a threshold is crossed or even the measurement units selected… : the planetary boundaries concept is the subject of debate and discussion within the scientific community. It is nothing new, given the progress science makes by these exchanges of views and other controversies.

That said, the planetary boundaries have gradually established themselves as part of the environmentalist's fight to preserve life on Earth. As soon as 2011, the UN and the European Commission, for example, very quickly adopted this forecasting tool. In France, some, such as the Fondation pour la Nature et l’Homme (Nature and Humans Foundation), even advocate amending the Constitution.