6th planetary limit: the limit of green water has just been breached

The international researchers gathered by the Stockholm Resilience Centre stated in a study published on 28 April in the review Nature that green water was insufficiently taken into account in previous studies. Indeed, green water is invisible to the naked eye: it is rainwater absorbed by plants. On the contrary, blue water is easier to measure: it’s the water that flows in rivers and streams, fills lakes, aquifers, water tables, and the seas. Only underground and surface waters were taken into account in previous studies.

To assess this green water, scientists measured the variance between the humidity observed in soils and the normal variations expected in the Holocene period. They propose to integrate this new criterion in the assessment of the change of the freshwater cycle, and possibly make it the tenth planetary limit. Above all, on the basis of their analysis and observation of the generalised deterioration of the terrestrial system by human activities, they conclude that the planetary limit of green water has already been exceeded by human beings.

A disturbance with domino effects

Green water is nonetheless essential for the functioning of the water cycle: it plays a crucial role in maintaining the moisture of soils, the resilience of the biosphere, securing carbon sinks and regulating the circulation of water in the atmosphere. More generally, disturbances of the water cycle in general are threats that weigh on the resilience of ecosystems. Indeed, the disturbance of the water cycle contributes to two dangerous phenomena: water can accumulate in the atmosphere and increase GHG effects, or no longer be absorbed by soil and thus accelerate rising water levels.

Water is the blood circulating in the biosphere. However, we’re considerably modifying the water cycle. It will affect the health of the entire planet and make it much less resistant to shocks – Lan Wang-Erlandsson, , the main author of the study.

The disruption of the water cycle was also raised by the hydrologist Emma Haziza, in the conference she gave for IFGR (link to article).Ce bouleversement du cycle de l’eau également évoqué par l’hydrologue Emma Haziza, dans la conférence qu’elle a donnée pour IAGF (voir l’article).

More than a limit, a frontier

This 6^th limit should be considered like a frontier beyond which humanity crosses into the totally unknown regarding the functioning of the biosphere. Aurélien Boutaud, researcher at CNRS and author of the book The planetary limits, explained on the radio station France Culture on 4 April. He said that these limits are not a tipping point, but a threshold beyond which nothing is certain: “Imagine that scientists tell you that you can advance 10 m onto the pack ice, but that afterwards the ice could crack at any moment. The human species has now exceeded that limit, it is 15 metres. If the ice has not cracked yet, it could occur very soon.”

The green water limit is the second limit breached since the beginning of 2022, after that of chemical pollution in February. Four other limits have already been exceeded: climate change, the erosion of biodiversity, soil-use and global disruptions of the nitrogen and phosphorous cycles. However, it should be borne in mind that these limits are interdependent: exceeding one leads to irreversible modifications of the others. The last three limits of the model could also be quickly exceeded: that of the acidification of the oceans, the degradation of the ozone layer and the increase of aerosols in the atmosphere.