Freshwater as a planetary boundary: blue water and green water

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.