Freshwater planetary boundaries

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By PETRO KOTZÉ*

The water situation will get worse before any trend reversal is observed.

Earth's operating systems have remained in relative balance for thousands of years, allowing civilization to flourish. However, humanity's actions have resulted in the transgression of multiple planetary boundaries, resulting in the destabilization of these vital operating systems.

This week scientists announced that humanity has crossed the planetary freshwater boundary. Other frontiers already crossed are climate change, integrity of the biosphere, biogeochemical cycles (pollution by nitrogen and phosphorus), alteration of the Earth system and new entities (pollution by synthetic substances).

In the past, the freshwater frontier was defined only by “blue water” – a measure of humanity's use of lakes, rivers and groundwater. But scientists have now extended that definition to include “green water” – rainfall, evaporation and soil moisture.

Scientists say soil moisture conditions are changing from the boreal forests to the tropics, with unusually dry and wet soils now commonplace, at risk of changes in the biome. The Amazon, for example, is becoming much drier, which could cause a tipping point between rainforest and savannah to be reached, releasing large amounts of stored carbon.

Mankind's modification of the water cycle has taken the world far beyond a safe operating space for life to continue on Earth, scientists say. A reassessment of the planetary frontier for freshwater, which now includes precipitation, soil moisture and evaporation – the so-called “green water” - concluded that the frontier has been “considerably crossed”, with the situation likely to get worse before any reversal. of the trend is observed. Previously, researchers had only considered rivers, lakes and groundwater in their assessments.

"Green water modifications are now causing increased risks to the Earth system on a scale that modern civilizations may never have faced," according to researchers at the Stockholm Resilience Center in collaboration with colleagues from Germany, the Netherlands, Finland, Austria, Australia, the United States and Canada. The results were published recently in Nature Reviews Earth & Environment.

"This is a wake-up call to the fact that we must stop the way we modify green water," says lead author Lan Wang-Erlandsson of the Stockholm Resilience Center at Stockholm University. “We are profoundly modifying the water cycle,” she says, noting that this destabilization of the Earth system is now affecting the health of the entire planet, making it significantly less resilient to environmental shocks.

Based on the results, water is now the sixth frontier to be crossed out of nine identified by the Planetary Boundaries Panel. Published in 2009 and regularly updated, the panel demarcates a safe operating space for humanity, beyond which civilization could collapse, changing life as we know it. The other frontiers already crossed are climate change, integrity of the biosphere, biogeochemical fluxes (nitrogen and phosphorus pollution), alteration of the earth system, and also, since 2022, new entities, which include pollution from plastics and other substances of human origin.

The nine planetary frontiers: climate change, biosphere integrity (functional and genetic), earth system change, freshwater change, biogeochemical fluxes (nitrogen and phosphorus), ocean acidification, atmospheric aerosol pollution, stratospheric ozone depletion, and release of new chemicals.

In 2022, scientists announced the transgression of the limits of both fresh water and new entities.

Until now, the freshwater border was considered to be within the safe zone. The so-called “fresh water use” limit was based on permitted human consumption, and set at 4.000 km3/year of water used and not returned as surface runoff. He evaluated the water taken from rivers, lakes and groundwater, the so-called “blue water”.

The updated assessment uses soil moisture in the root zone of plants to measure the “green water” boundary because it is directly influenced by human pressures, and because it has a direct impact on a range of large-scale ecological, climatic, biogeochemical and hydrological dynamics. .

During droughts, for example, plants can maintain photosynthesis and transpiration by accessing soil moisture, but once these moisture levels drop below a critical threshold, vegetation mortality increases, especially for plants such as tropical trees that do not normally have alternative strategies for coping with drought. The study points out that soil moisture anomalies in the root zone are also key factors in the soil carbon cycle, and that changes in soil moisture in a scenario of high carbon emissions risk transforming land from a net sink of carbon into a carbon source until the middle of this century.

Evidence of this escalation process can already be found in the diminishing resilience of critical ecosystems such as the Amazon and the Congo rainforests, which store large amounts of carbon and boast immense biodiversity. These two biomes are considered vital to Earth's operating systems, but could be pushed beyond the environmental tipping points by transgressions of freshwater boundaries.

“The Amazon rainforest depends on soil moisture for its survival. But there is evidence that parts of the Amazon are drying. The forest is losing soil moisture as a result of climate change and deforestation”, says Arne Tobian, co-author of the new assessment: “These changes are potentially pushing the Amazon closer to a tipping point where large parts could shift from rainforest states to savannah-like states”, he adds.

The new assessment found that the phenomenon is global, with soil moisture shifting from boreal forests to the tropics, from farmland to forests. Increasingly, abnormally wet and dry soils are common. Extreme weather events triggered by climate change cause increases in both severe drought and torrential rains, while changes in land use for agricultural and other purposes can cause soil drying.

“Water is fundamental for any living organism on Earth,” says Wang-Erlandsson. “There are a lot of interconnected things that are now changing in an unprecedented way,” she adds, noting that impacts on the water cycle are driven by multiple human actions, far beyond withdrawal for consumption. “It is massively affected by climate change, land management, land degradation, etc. It is complex and woven into our human activities; in everything we do,” she explains.

"This latest scientific analysis shows how we humans may be pushing green water out of the variability that Earth experienced for several thousand years during the Holocene epoch," concludes evaluation co-author Johan Rockström, director of the Potsdam Institute for Climate. Impact Research and professor at the Stockholm Resilience Centre.

With six of the nine planetary boundaries already crossed, the resilience of the Earth's operating system as a whole is now quite low, warns Wang-Erlandsson. Continued deterioration in the functioning of Earth systems will increase the risk of regional shifts in environmental regimes. Humanity needs to act to reverse these escalating changes and return to a safe zone, she says.

“Reducing the risks of altering the earth system's green water now requires immediate action on water as a whole to address climate change, deforestation and land degradation,” says Ingo Fetzer, co-author of the assessment and researcher at the Stockholm Resilience Centre.

Based on the results, “current worldwide trends and trajectories of increased water use, deforestation, land degradation, soil erosion, air pollution and climate change must be promptly stopped and reversed to increase the chances of staying in the safe operating space ”.

*Petro Kotze is a journalist.

Translation: Fernando Lima das Neves.

Originally published on: Mongabay

 

Reference


Wang-Erlandsson L., Tobian A., van der Ent RJ, andsuch. A planetary boundary for green water. Nat Rev Earth Environ (2022)

 

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