What exactly are these boundaries?
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.
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.
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.
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.