The sixth planetary limit has been exceeded according to many scientists. It has to do with the water cycle and in particular green water, that which is absorbed by plants. In the Amazon, but everywhere else, soil moisture is changing and forests are turning into savannas due to climate change and deforestation. The world contains nine planetary boundaries, so only three have not been crossed yet.
Six out of nine. The world just crossed a new planetary frontier for the second time this year. The sixth limit of the planets relates to the water cycle, more specifically green water, that which is absorbed by plants. They include precipitation as well as soil moisture and evaporation. So far it has not been sufficiently studied. However, according to a new assessment conducted by researchers at the Stockholm Resilience Center, in collaboration with other scientists around the world, green water appears to fall outside the safe zone.
In 2009, scientists invented the concept of planetary boundaries. It identifies nine variables that regulate the stability of the planet and must not be overridden to ensure a “safe and equitable” development for humankind. Among them are climate change, erosion of biodiversity, ocean acidification, chemical pollution and global use of water. The green water cycle is the sixth frontier the world has crossed. Credit: Stockholm Resilience Center
So far, the water limit has been considered within the safety zone. However, the original freshwater limit focused only on extracting water from rivers, lakes, and groundwater – known as blue water. The researchers believe that previous assessments have not adequately defined the role of green water and especially soil moisture in ensuring the resilience of the biosphere, in securing terrestrial carbon sinks and in regulating atmospheric circulation. They suggest that soil moisture in the root zone of plants is a control variable for green water.
The forest is losing soil moisture due to climate change and deforestation
“The Amazon rainforest depends on soil moisture for its survival. But there is evidence that parts of the Amazon are drying up. The forest is losing soil moisture due to climate change and deforestation.Arne Tobian, second author of the study, explains.These changes are likely to bring the Amazon closer to a tipping point where large parts of the rainforest can turn into savannah-like nations.”, he adds. And it’s not just in the Amazon. This phenomenon is global. Everywhere, from boreal forests to the tropics, from farmlands to forests, soil moisture changes. Abnormally dry and wet soils are increasingly common.
Water is one of nine regulators of the state of the Earth system and a sixth boundary that scientists have deemed to have been crossed. “Water is the blood of the biosphere. But we are profoundly changing the water cycle. And that is now affecting the health of the entire planet,” Lan Wang Erlandson, lead author commented. Other limits that have been crossed are: climate change, biosphere integrity, biogeochemical cycles, change in the Earth system and, in January 2022, chemical pollution.
“This new scientific analysis shows how we humans are pushing green water beyond the variability experienced by the Earth several thousand years ago during the Holocene period.Adds co-author Johan Rockström, Director of the Potsdam Institute for Climate Impact Research and Professor at the Stockholm Resilience Center. “Reducing the risks of green water change to the Earth’s system now requires immediate water actions to address climate change, deforestation and land degradation”says Ingo Fetzer, co-author of the study and researcher at the Stockholm Resilience Center.
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