Around the world, the freshwater cycle is under threat of disruption. And according to new international research, green water resources in particular need to be closely monitored. But what exactly is "green water"?
Most of us are familiar with the term freshwater (or fresh water), meaning water from rain, rivers, lakes and groundwater. And that's logical, since we bathe in it and, above all, consume it. This water is commonly referred to as "blue water," a color that we automatically attribute to this natural element. But water is also associated with green, when it comes from soil and plants. This is what we call green water.
Green water comes from the water absorbed and rejected by plants, a phenomenon called evapotranspiration, combining evaporation from the soil and transpiration through the plant leaves. It is precisely this water that allows for the earth's soils to be humid. It is also at the origin of "green mold," which can be observed with some plants.
A study published at the end of April in the journal Nature specifically warns of the threat to the water cycle, which until now has been measured only in terms of blue water. Green water is managed by farmers, foresters and pasture users. However, this intensive use and consumption by humans depletes the resources of this water, essential for maintaining humidity levels of the forests, including the Amazon basin, largely threatened by deforestation. Adding to that is the deterioration of soils and greenhouse gas emissions, which disrupt the evapotranspiration process of plants.
"Provisional estimates of departures from Holocene-like conditions, alongside evidence of widespread deterioration in Earth system functioning, indicate that the green water planetary boundary is already transgressed," outline the study authors.
Six planetary limits (out of nine) already crossed
In 2009, the Stockholm Resilience Centre established a list of nine criteria considered as boundaries that must not be crossed to maintain the balance of the planet's ecosystems and human survival. Of these, six have already been crossed: climate change, freshwater use, biodiversity loss, global disruption of the nitrogen and phosphorus nutrient cycles, deforestation and chemical pollution.
Ocean acidification, stratospheric ozone depletion and atmospheric aerosol loading remain under close scrutiny. And the green water planetary boundary, already transgressed, may well become the tenth criterion on this list.
"Everywhere, from the boreal forests to the tropics, from farmlands to forests, soil moisture is changing," note researchers at the Stockholm Resilience Centre who propose using soil moisture as a control variable and "suggest that the green water boundary variable to be defined as the percentage of land where root-zone soil moisture exceeds the historical variability."
