A sixth planetary boundary has been crossed (and it sucks)

After climate change, the erosion of biodiversity, the global disturbances of the nitrogen and phosphorus cycle or even land use and chemical pollution, it is the turn of fresh water to reach its boundaries. In any case, this is what a study carried out by several Swedish scientists, published this Thursday in the Nature magazine. What is a “planetary boundary” and what does the depletion of a resource such as fresh water mean? Answers with Mélanie Mignot, researcher in analytical chemistry and lecturer at theNational Institute of Applied Sciences (INSA) Rouen.

What is a planetary boundary?

This concept was defined by an international team of 26 researchers in a study published in 2009 in scientific journals Nature and Ecology and Society. Planetary boundaries correspond to thresholds, on a global scale, that humanity should not exceed in order to continue to live in conditions that are favorable to it, and to preserve a safe ecosystem, i.e. a certain stability of the planet. Exceeding these limits [il en existe neuf : le changement climatique, les pertes de biodiversité, les perturbations globales du cycle de l’azote et du phosphore, l’usage des sols, l’acidification des océans, la déplétion de la couche d’ozone
les aérosols atmosphériques, l’usage de l’eau douce et les pollutions chimiques] could lead to sudden, difficult to predict changes in humans and their environment.

What does the study published on April 26 say about the 6th planetary limit crossed?

This article is about a new assessment by researchers from the Stockholm Resilience Center with other scientists around the world about green water. They highlighted that the fresh water assessed so far was mostly blue water, ie water in rivers, lakes and groundwater. The researchers believe that the role of green water has not been sufficiently taken into account in previous studies.

They suggest also considering soil moisture in the root part of plants, which contributes to ensuring the resilience of the biosphere, preserving carbon sinks and regulating atmospheric circulation. In their study, the researchers showed that the forest loses moisture as a result of climate change and deforestation. They note that abnormally wet and dry soils are increasingly common.

By taking into account this new component (blue water AND green water), the researchers estimated that the limit had also been crossed for the freshwater cycle.

And what is this green water the study is talking about?

Green water corresponds to water adsorbed by plants and comes from precipitation and soil moisture and evaporation. The sixth planetary limit therefore concerns the water cycle.

Should we be worried?

Two planetary limits crossed in 2022, we arrive at six planetary limits exceeded on the nine defined. It is difficult to remain impassive in the face of these alarming findings. These studies complement the work of the last IPCC report, and particularly the chapters dealing with the water cycle, which has been modified at a rate greater than anything we have known during the geological era of the Holocene.

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