This is more bad news in the infernal process of global warming, and the growing threat of a general collapse of life. On Tuesday April 26, a study by international researchers led by the Stockholm Resilience Center, published in the American journal Nature, explains that a sixth planetary limit has just been crossed: that concerning the cycle of green water. This is the second of the year, after the announcement in January of exceeding the safety zone in the field of chemical pollution. In all, six thresholds out of the nine accepted by the scientific community have already been exceeded.
Green water, a new criterion for analyzing the freshwater cycle
The study focuses on the cycle of fresh water, and more particularly that of green water, hitherto little examined and yet precious since it ensures good humidification of the soil, essential for the development of plants. Unlike blue water, that of atmospheric precipitation, that contained in rivers, lakes and underground and consumed directly by human beings, green water, transpired by plants, is invisible to us. “This study is out of the ordinary insofar as it focuses on the share of fresh water fixed by plants at the root level. The green water cycle is known but we talk about it little because it does not does not concern us directly”, explains Fabienne Lagarde, teacher-researcher in environmental chemistry at the University of Le Mans. “When the concept of planetary limit was created in 2009, the water cycle did not seem to be in danger, but we cannot study fresh water by considering only the consumption we make of it. The proof, with this new criterion, we realize that the limit has been crossed”, she continues. Indeed, the study tells us that the rate of soil imbalance reaches 18%. The limit not to be exceeded was 10%.
If man does not have direct access to green water, he exerts enormous pressure on it. The main culprit: intensive agriculture, already at the origin of 70% of man’s needs for blue water, which modifies the composition of the soil, dries it out and acidifies it. “The risks of non-linear and large-scale changes compromise the ability of our planet to remain in Holocene-like conditions”, specifies Mélanie Mignot, lecturer in analytical chemistry at INSA Rouen and at the COBRA laboratory. The Holocene is the geological era that preceded us: it dates back 11,700 years and constitutes a climatically stable period serving as a point of comparison to our current era, the Anthropocene.
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As an example, and as a warning, the study cites the case of the Amazonian forest, whose survival depends on the humidity of its soil. “There is evidence that parts of the Amazon are drying up due to climate change and deforestation. These changes are bringing it closer to a tipping point where large parts could shift from rainforest to savannah states” , writes Arne Torban, one of the authors. If the soil no longer manages to retain water, aridification and desertification spread. The planet then loses its capacity for resilience.
Water at the heart of planetary interactions
The study therefore promotes the use of the new indicator “green water” – in addition to blue water – to study the freshwater cycle and refine the knowledge of the earth system. “Research in paleoclimatology is progressing thanks to field measurements and offers more data on the humidity of soils, plants, and on the total storage of water”, notes Mélanie Mignot. Long used, the soil aridity index has shown its limits, and Fabienne Lagarde sees in this publication “a desire to make the different environmental domains interact with each other in order to show the seriousness of the situation and propose a more effective global response. .” Because if the green water cycle is in danger today, the blue water cycle will be tomorrow, before other alarming interferences follow.
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To fight against the disruption of the general water cycle, the researcher Ingo Fetzer, co-author of the study, calls for “immediate actions in terms of deforestation and soil degradation”, the activity of man imposing too much variability on the Earth. Much work remains to be carried out, “research still lacking direct measurements concerning the access of plants to water, in particular in areas of great importance for the terrestrial functioning of green water”, explains Mélanie Mignot. But this study does open up a new avenue of reflection and, with it, a final call to action.
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