A sixth planetary boundary has just been crossed. This was the announcement made by the study "A planetary boundary for green water", published in Nature on April 26 by researchers from the Stockholm Resilience Center and the Potsdam Institute.
What are the planetary boundaries?
Defined in 2009 by the group of 28 scientists led by Johan Rockstrom of the Stockholm Resilience Centre, the planetary boundary framework defines humanity’s safe operating space. It identifies nine thresholds beyond which civilization could collapse, highlighting the pressure humans place on the biophysical processes that regulate the state and resilience of Earth’s systems. Each boundary is associated with a control variable that monitors our impact on Earth’s systems. Five of the boundaries have already been crossed: climate change, biosphere integrity, biogeochemical cycles, changes in Earth systems, and, as of last January, pollution from plastic and other human-made substances.
The freshwater boundary
Until now, the freshwater boundary has been considered within a safe limit; now, it has officially been assigned to the category of crossed boundaries. The original freshwater boundary only considered “blue water”- rivers, lakes and groundwater- and was based on human consumption of 4000 km^3/year of water not returned as runoff. The Nature study takes on a new perspective, considering the role of “green water”- that is, water from atmospheric precipitation that is absorbed by plants, particularly soil moisture, in maintaining the resilience of the biosphere, strengthening terrestrial carbon sinks, and regulating atmospheric circulation.
The water cycle is directly influenced by man-made pressures and has a direct impact on ecological, climatic, biogeochemical, and hydrological dynamics. Lan Wang-Erlandsson, co-author of the Nature study, said: "Water is the bloodstream of the biosphere. But we are profoundly changing the water cycle. This will affect the health of the entire planet and make it much less resistant to shocks." The study explains that in times of drought, the lack of soil moisture leads to increased plant mortality, particularly among plants such as tropical trees that don’t generally have a drought adaptation strategy. Also, anomalies in soil moisture in the root zone affect the terrestrial carbon cycle. Changes in soil moisture during a high-carbon scenario may shift the land from a net carbon sink to a carbon source by mid-century.
This process has already begun to show in the diminishing resilience of critical ecosystems like the tropical rainforests of the Amazon and Congo, two reservoirs of biodiversity and carbon that are essential to humanity and vital to Earth’s functioning systems. Arne Tobian, co-author of the study and PhD student at the Stockholm Resilience Centre and the Potsdam Institute for Climate Impact research, said, “The Amazon rainforest depends on soil moisture for its survival. But it is evident that parts of the Amazon are drying out. The forest is losing soil moisture due to climate change and deforestation. These changes are potentially bringing the Amazon closer to a tipping point where large parts could go from tropical rainforest to a savanna-like state.”
This situation doesn’t only concern the Amazon and Congo; rather, the study reports a global phenomenon: increasingly, abnormally moist or dry soils are becoming a normality that affects boreal and tropical forests as well as agricultural land. The meteorological consequences of climate change lead to droughts and floods, and changes in land use also contribute to severe soil drying. Ms. Wang-Erlandsson is clear: the continued deterioration of the functioning of earth’s systems increases the risk of changes in the regional environmental regime. Humanity must act to reverse these growing changes and return to a stable zone.