A planetary limit is an indicator – recognized and adopted at European and international levels, indicates the thresholds not to be exceeded, as this risks causing abrupt environmental changes. Planetary boundaries are the result of an observation: Human activities are dangerously disturbing the balance of planet Earth. Thus, the human footprint now rivals nature, becoming a geological force in its own right.
Crossing a boundary increases the risk that human activities will unintentionally tip the Earth system into a much less hospitable state, which would harm poverty reduction efforts and lead to a deterioration of human well-being in many parts of the world. world, including in rich countries.
1- Climate change
Climate change is characterized by an increase in the concentration of greenhouse gases in the atmosphere, caused by human activities. In other words, the use of fossil fuels, the use of synthetic fertilizers or the production of artificial GHGs disturb the fragile climatic balance.
The most notable changes are an increase in global temperatures, acidification of the oceans, a massive destruction of biodiversity, an increase in food crises, and an increase in extreme climatic phenomena making life on earth more and more difficult, for plants and animals and for humans.
2- Ocean acidification
Main carbon sinks – the oceans have absorbed a quarter of emissions since the industrial revolution – the oceans transform CO2 into carbonic acid. However, absorbing too many carbon emissions increases the acidity level of the ocean, jeopardizing marine diversity, while compromising the oceans’ ability to absorb more CO2. Furthermore, this excess carbon in the atmosphere is – in fact – absorbed by living beings, which affects the reproduction of species, marine ecosystems and the food chain.
This global limit is set according to the aragonite saturation rate, which must not fall below 80% – compared to its level in the pre-industrial era. In 2009, this limit is 84% and has therefore not yet been exceeded. Unfortunately, following the same pace until 2050, saturation should reach the dreaded 80%.
3- Degradation of the ozone layer
Stratospheric ozone is the layer of the atmosphere located between 20 and 50 km altitude which protects living beings by filtering a large part of UV radiation. Its thinning, even its disappearance, is a disaster for terrestrial life (cancers of the skin and modification of the photosynthesis system of plants). For this reason, the concentration of ozone in the atmosphere is set at 275 DU (Dobson Unit), knowing that the average value of the ozone column is 300 DU, but the concentration reached 283 DU in 2019.
Nevertheless, the World Meteorological Organization says the ozone layer has been recovering at the rate of 1-3% per decade since 2000, and is expected to recover completely by 2030.
4- Global disturbances of the nitrogen and phosphorus cycle
Nitrogen and phosphorus are essential nutrients for plant growth. Emitted in too large quantities, however, they become harmful to the environment, contributing to water pollution by nitrates – nitrogen – leading to the eutrophication of fresh waters – phosphorus – and causing anoxia in the oceans and the proliferation of green algae . Thus, the nitrogen limit is between 62 and 82 million tonnes (Mt) per year released globally. At the current rate, natural phosphorus reserves will be depleted within 50 to 100 years.
5- The use of water in the world
Unevenly distributed on the planet – and representing only 3% of the world’s water – fresh water is essential to our survival and that of living beings (plants, animals, etc.). However, water withdrawals accelerated during the 20th century, particularly for agricultural and industrial uses. By way of illustration, the share of the annual renewable freshwater resource withdrawn to serve human activities (or rain-fed agriculture) increased from less than 2% to 10% during the 20th century. This planetary boundary also takes into account green water, which is taken up by plants – and therefore by terrestrial carbon sinks.
This human alteration of the water cycle risks disrupting the entire planet. Worse, in its latest report, the IPCC asserts that the water cycle is already at “a rate greater than anything we have experienced during the geological epoch of the Holocene. For example, many Amazonian regions risk becoming savannahs.
6- Land use changes
Land use change encompasses the transformation of natural and semi-natural environments into agricultural land. In other words, the deployment of human activities on natural surfaces has serious consequences for the environment, through the deforestation of large areas of forest, the loss of biodiversity, soil erosion, the increase in GHG emissions, reduced CO2 storage capacity, carbon depletion and flood risk. Thus, instead of having 75% of formerly forest land still wooded, there is only 62% left.
7- The erosion of biodiversity
The erosion of biodiversity – that is, of living beings, ecosystems and living species – consists of increasing the rate of extinction of species. This implies: the degradation of their habitat, the destruction and fragmentation of natural environments due to human activities, the pollution of these habitats, the overexploitation of wild species (overfishing or deforestation), the introduction of invasive alien species and the climatic.
We are currently witnessing the sixth mass extinction on earth with a speed and intensity never seen before. Faced with these disastrous impacts, the threshold not to be exceeded is 10 species extinctions out of a million per year. Unfortunately, the limit has already been crossed in 2009 with 100 extinctions out of a million species.
8- The increase in aerosols in the atmosphere
Contrary to popular belief, aerosols are mostly fine particles of an organic nature suspended in the air – solid, liquid or mineral (dust, spray, soot or eroded rock). However, human activity has created toxic and transformed aerosols (primary or secondary) which are harmful to our health, as well as to the environment. The accumulation of particles from aerosols increases the rate of opacity of the atmosphere, leading to a reduction of 10 to 15% of solar radiation at the surface of the earth. In addition, the black carbon and the organic carbon present in the composition of the treated aerosols lead to global warming.
Despite its preponderant importance in global warming, it is impossible to quantify a threshold not to be exceeded, due to the complexity of aerosols, the spatio-temporal variability of particles, sources and impacts.
9- The introduction of new substances
This involves quantifying the pollution created or introduced by humans into the biosphere (new chemical substances, nanomaterials and plastic polymers, among others). This introduction is considered a planetary boundary because it has three adverse effects: on the physiological development of man, on the functioning of ecosystems and on other planetary boundaries. Still, we must remember that plastic weighs twice as much as all the animals on earth, and we ingest 2000 microplastic particles every week.
It is time to collectively understand that there will be no profit on a dead planet and that it is time to plan the gradual exit from fossil fuels, the only solution to respect our climate commitments and hope to limit global warming to +1 .5°C.