Our planet also has limits. And we’ve crossed another one.

That is what an international team of researchers states in the magazine today Nature Reviews Earth & Environment† Perhaps a (guilt-conscious) bell will not immediately ring. But we are really wrong. And this time in the field of ‘green water’, in other words water that is available for plants. “Water is the circulatory system of the biosphere,” said researcher Lan Wang-Erlandsson. “But we are seriously changing the water cycle.” And an analysis of the water cycle now shows that we have really broken it. “This is now affecting the state of the entire planet and making it less resilient to disturbances.”

The limits of our planet

The idea that our planet also has its limits is certainly not new. In 2009, researchers published a study in which they set limits on nine processes and systems on Earth that together dictate the stability of the Earth system as a whole. Within those limits lies our leeway: as long as we stay below the established limits, the earth as a whole will not undergo major man-made changes and we can continue to make sustainable use of everything the earth has to offer us.

Fresh water

And one of those so-called planetary boundaries is freshwater. Until recently, it was thought that we were on the right track when it came to that planetary boundary. But an international team of researchers – including scientists from Utrecht University – have now taken a closer look at the water boundary and – unfortunately – have come to a different conclusion.

green water

In the past, when it came to freshwater, only so-called ‘blue water’ was considered. You have to think of freshwater in rivers, lakes and streams. But in the new study, scientists argue in favor of also looking at “green water,” or water that is available to plants, such as rainfall and soil moisture. “Previous evaluations focused mainly on river water discharge and missed important information about the complete water cycle,” explains researcher Ruud van der Ent. “Now we focused in particular on soil moisture in the root zone as the key indicator of the health of our water cycle. This supports healthy and resilient ecosystems that are able to sequester carbon and, in turn, those ecosystems ensure that the atmospheric water cycle remains intact.”

Changes

It is clear that this ‘green water’ is undergoing rapid changes. Globally and in diverse ecosystems – from boreal forests to the tropics and agricultural areas – rainfall changes and with it soil moisture. “Take the Amazon rainforest, for example,” says researcher Arie Staal. “That depends on soil moisture to survive. But there is evidence that parts of the Amazon are drying up due to climate change and deforestation.”

Across borders

In their study, the researchers state that soil moisture is therefore a factor to take into account. And to determine the state of the soil moisture, they propose to see to what extent the amount of soil moisture in the root zone (the soil layer containing roots of living vegetation) at a given moment deviates from what is normal (of course taking into account with the fact that the amount of soil moisture varies naturally during the year and also differs from ecosystem to ecosystem). In their study, they put their money where their mouth is in this regard and they then have to conclude that we have crossed the ‘green water’ limit.

Other boundaries exceeded

Unfortunately, it is also not the first time that we are working across borders in this way. In fact, earlier this year we also crossed another planetary boundary, that of chemical pollution. “At the rate at which societies produce new chemicals and other new materials and release them into the environment, we are not staying within safe limits for humanity,” said study researcher Patricia Villarubia-Gómez in January. And before that, we had already exceeded the limits set for global warming, loss of biodiversity, the nitrogen cycle and land use.

The fact that the exceeded limits are piling up so quickly can be explained well. After all, all planetary boundaries are part of one system. And if one is exceeded, this also has direct consequences for other system components. You also see this happening in the case of green water. For example, climate change leads to changes in precipitation, which in turn affect the water cycle. And a disrupted water cycle can in turn affect global warming, if trees and plants – which absorb carbon – are less able to grow as a result. In a similar way, other planetary boundaries are also intertwined. For example, the disruption of the nitrogen cycle and global warming also contribute to the loss of biodiversity.