Anja Krieger

Freshwater as a planetary boundary: blue water and green water

The availability of freshwater and moist, temperate air is crucially important for humans, plants and many other life forms. Until now, researchers assumed that we have not yet exceeded the planetary boundary for the earth’s hydrological balance. But now it appears that the ‘green water’ is falling to ever-lower levels. This is in contrast to the situation with rivers, lakes and groundwater, which is less concerning for now.

The nine planetary boundaries according to Rockström, Stockholm Resilience Center
The nine planetary boundaries according to Rockström, Stockholm Resilience Center
Julia Blenn / Helmholtz-Klima-Initiative

From a scientific perspective, there are two types of freshwater on earth: blue water and green water. 'Blue water' is water in rivers and lakes, groundwater and the water frozen in glaciers and the polar ice caps. Freshwater is also found in plants, the soil and rain – experts call this ‘green water’.

In 2022, an international team of researchers under the direction of the Stockholm Resilience Center found that the planetary boundary for green water from rain, ground moisture and evaporation that is available to plants have already been exceeded. This means that global ground moisture levels are now very different from the conditions typical of the Holocene – the approximately 11,000-year epoch of earth history characterized by the rise of human civilization – and lie outside the safe zone.

In addition to unusually dry conditions, unusually moist soil is being observed more frequently as well. However, the situation with ‘blue water’, i.e. the freshwater found in rivers, lakes and groundwater, is better and remains within the safe range, according to findings so far. This is even though many rivers are in a poor ecological condition, due to elevated levels of fertilizer nutrients and other pollutants, for example.

Radical changes in the water cycle

As the earth’s largest rainforest, the Amazon is often called our planet’s ‘green lung’. It helps regulate the climate, storing huge amounts of CO2 in the form of carbon. But the rainforest requires sufficient moisture if it is to survive. Climate change and deforestation are causing the soil to dry, and the regional water cycle of evaporation and precipitation is being irreversibly damaged through the loss of rainforest vegetation.

If the Amazon reaches its tipping point, the rainforest will turn into a savanna, changing from a CO2 sink into a source of additional greenhouse gases as the CO2 stored in plants and the soil is released. The planetary boundary for freshwater plays a key role in the climate system because of this feedback loop.

Hydrological balance and climate crisis inextricably linked

The hydrological balance is also crucial regarding the ability of us humans to adapt to climate change on both a regional and global scale. Cities store too little water, having insufficient parks, green spaces and water bodies. Rainwater simply evaporates off sealed ground surfaces or flows rapidly away from cities, exacerbating summer warming and creating urban heat islands more rapidly. Excessive heat in the cities impairs residents’ quality of life tremendously and can cause fatalities, especially among infants, the elderly and people with health conditions.

How can we return to the safe zone?

We can take the following steps to meet the planetary water boundary:

  • Stop deforestation, because plants maintain the water cycle.
  • Use water more prudently and efficiently, in alignment with the sustainable, renewable resources found regionally.
  • Protect our hydrological balance to ensure that evaporation, seepage and runoff remain in balance sustainably.
  • Stop the polluting of surface water and groundwater.
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