Climate: what is the green water cycle, whose planetary limit would be crossed?

This is more bad news in the hellish process of global warming, and the growing threat of a total collapse of life. On Tuesday, April 26, a study by international researchers led by the Stockholm Center for Sustainability, published in the US journal Nature, explains that a sixth planetary limit has just been exceeded: the green water cycle. This is the second year since the announcement in January of exceeding the safety zone in the field of chemical pollution. In total, six of the nine thresholds adopted by the scientific community have already been exceeded.

Green water – a new criterion for analyzing the fresh water cycle

The research focuses on the fresh water cycle, and in particular the green water cycle, still little understood but still valuable because it provides good soil moisture for plant development. Unlike blue water, precipitation contained in rivers, lakes and underground and directly consumed by people, green water evaporated by plants is invisible to us. “This study is unusual because it focuses on the proportion of fresh water fixed by plants at the root level. The green water cycle is known, but we don’t talk much about it because it doesn’t concern us directly,” explains Fabien Lagarde, Research Lecturer in Environmental Chemistry at the University of Le Mans. “When the planetary limit concept was created in 2009, the water cycle didn’t seem to be in danger, but we can’t study fresh water just by looking at how we consume it. 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 is as high as 18%. The limit that could not be exceeded was 10%.

If a person does not have direct access to green water, he puts enormous pressure on it. The main culprit: intensive farming, already underpinning 70% of the human need for blue water, which changes the composition of the soil, dries it and acidifies it. “The risks of non-linear and large-scale changes threaten our planet’s ability to remain in a Holocene-like environment,” said Melanie Mignot, professor of analytical chemistry at INSA Rouen and the COBRA laboratory. The Holocene is a geological epoch that preceded us: it is 11,700 years old and is a climatically stable period that serves as a point of comparison with our current epoch, the Anthropocene.

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As an example and as a warning, the study cites the case of the Amazonian forest, which depends on the moisture content of its soil for survival. “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 may move from rainforest to savannah states,” writes Arne Torban, one of the authors. If the soil can no longer hold water, aridity and desertification spread. Then the planet loses its ability to be stable.

Water at the heart of planetary interactions

In this way, the study promotes the use of the new indicator “green water” – in addition to blue water – to study the fresh water cycle and refine knowledge of the earth system. “Research in paleoclimatology is advancing through field measurements and offering more data on soil moisture, plant moisture, and total water storage,” says Melanie Mignot. The long-used soil aridity index has shown its limits, and Fabienne Lagarde sees this publication as “an effort to make different areas of the environment interact with each other to show the gravity of the situation and suggest a more effective global response.” Because if the green water cycle is in danger today, the blue water cycle will be tomorrow, before other disturbing interventions follow.

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To combat the disruption of the overall water cycle, researcher Ingo Fetzer, co-author of the study, calls for “immediate action in the face of deforestation and soil degradation,” human activities that impose too much variability on the Earth. There is still a lot of work to be done, “there is still a lack of research regarding plant access to water, in particular in areas of great importance for the terrestrial functioning of green water,” Melanie Mignot explains. But this research does provide a new opportunity for reflection, and with it a final call to action.



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